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-comment @# @;
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-1.13
-date 2011.03.01.11.46.54; author rusty; state Exp;
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-desc
-@First draft
-@
-
-
-1.13
-log
-@Thread-safe API
-@
-text
-@#LyX 1.6.7 created this file. For more info see http://www.lyx.org/
-\lyxformat 345
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-\textclass article
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-\language english
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-\quotes_language english
-\papercolumns 1
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-\output_changes true
-\author "Rusty Russell,,,"
-\author ""
-\end_header
-
-\begin_body
-
-\begin_layout Title
-TDB2: A Redesigning The Trivial DataBase
-\end_layout
-
-\begin_layout Author
-Rusty Russell, IBM Corporation
-\end_layout
-
-\begin_layout Date
-1-December-2010
-\end_layout
-
-\begin_layout Abstract
-The Trivial DataBase on-disk format is 32 bits; with usage cases heading
- towards the 4G limit, that must change.
- This required breakage provides an opportunity to revisit TDB's other design
- decisions and reassess them.
-\end_layout
-
-\begin_layout Section
-Introduction
-\end_layout
-
-\begin_layout Standard
-The Trivial DataBase was originally written by Andrew Tridgell as a simple
- key/data pair storage system with the same API as dbm, but allowing multiple
- readers and writers while being small enough (< 1000 lines of C) to include
- in SAMBA.
- The simple design created in 1999 has proven surprisingly robust and performant
-, used in Samba versions 3 and 4 as well as numerous other projects.
- Its useful life was greatly increased by the (backwards-compatible!) addition
- of transaction support in 2005.
-\end_layout
-
-\begin_layout Standard
-The wider variety and greater demands of TDB-using code has lead to some
- organic growth of the API, as well as some compromises on the implementation.
- None of these, by themselves, are seen as show-stoppers, but the cumulative
- effect is to a loss of elegance over the initial, simple TDB implementation.
- Here is a table of the approximate number of lines of implementation code
- and number of API functions at the end of each year:
-\end_layout
-
-\begin_layout Standard
-\begin_inset Tabular
-<lyxtabular version="3" rows="12" columns="3">
-<features>
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-Year End
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-
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-\begin_inset Text
-
-\begin_layout Plain Layout
-API Functions
-\end_layout
-
-\end_inset
-</cell>
-<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-Lines of C Code Implementation
-\end_layout
-
-\end_inset
-</cell>
-</row>
-<row>
-<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-1999
-\end_layout
-
-\end_inset
-</cell>
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-\begin_inset Text
-
-\begin_layout Plain Layout
-13
-\end_layout
-
-\end_inset
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-\begin_inset Text
-
-\begin_layout Plain Layout
-1195
-\end_layout
-
-\end_inset
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-</row>
-<row>
-<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-2000
-\end_layout
-
-\end_inset
-</cell>
-<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-24
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-2001
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-\begin_inset Text
-
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-2007
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-<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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-
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-
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-4398
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-\begin_inset Text
-
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-2008
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-
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-</cell>
-<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
-\begin_inset Text
-
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-71
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-
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-</cell>
-<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-4768
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-</cell>
-</row>
-<row>
-<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-2009
-\end_layout
-
-\end_inset
-</cell>
-<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-73
-\end_layout
-
-\end_inset
-</cell>
-<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none">
-\begin_inset Text
-
-\begin_layout Plain Layout
-5715
-\end_layout
-
-\end_inset
-</cell>
-</row>
-</lyxtabular>
-
-\end_inset
-
-
-\end_layout
-
-\begin_layout Standard
-This review is an attempt to catalog and address all the known issues with
- TDB and create solutions which address the problems without significantly
- increasing complexity; all involved are far too aware of the dangers of
- second system syndrome in rewriting a successful project like this.
-\end_layout
-
-\begin_layout Section
-API Issues
-\end_layout
-
-\begin_layout Subsection
-tdb_open_ex Is Not Expandable
-\end_layout
-
-\begin_layout Standard
-The tdb_open() call was expanded to tdb_open_ex(), which added an optional
- hashing function and an optional logging function argument.
- Additional arguments to open would require the introduction of a tdb_open_ex2
- call etc.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\begin_inset CommandInset label
-LatexCommand label
-name "attributes"
-
-\end_inset
-
-
-\end_layout
-
-\begin_layout Standard
-tdb_open() will take a linked-list of attributes:
-\end_layout
-
-\begin_layout LyX-Code
-enum tdb_attribute {
-\end_layout
-
-\begin_layout LyX-Code
- TDB_ATTRIBUTE_LOG = 0,
-\end_layout
-
-\begin_layout LyX-Code
- TDB_ATTRIBUTE_HASH = 1
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout LyX-Code
-struct tdb_attribute_base {
-\end_layout
-
-\begin_layout LyX-Code
- enum tdb_attribute attr;
-\end_layout
-
-\begin_layout LyX-Code
- union tdb_attribute *next;
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout LyX-Code
-struct tdb_attribute_log {
-\end_layout
-
-\begin_layout LyX-Code
- struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_LOG */
-\end_layout
-
-\begin_layout LyX-Code
- tdb_log_func log_fn;
-\end_layout
-
-\begin_layout LyX-Code
- void *log_private;
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout LyX-Code
-struct tdb_attribute_hash {
-\end_layout
-
-\begin_layout LyX-Code
- struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_HASH */
-\end_layout
-
-\begin_layout LyX-Code
- tdb_hash_func hash_fn;
-\end_layout
-
-\begin_layout LyX-Code
- void *hash_private;
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout LyX-Code
-union tdb_attribute {
-\end_layout
-
-\begin_layout LyX-Code
- struct tdb_attribute_base base;
-\end_layout
-
-\begin_layout LyX-Code
- struct tdb_attribute_log log;
-\end_layout
-
-\begin_layout LyX-Code
- struct tdb_attribute_hash hash;
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout Standard
-This allows future attributes to be added, even if this expands the size
- of the union.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-tdb_traverse Makes Impossible Guarantees
-\end_layout
-
-\begin_layout Standard
-tdb_traverse (and tdb_firstkey/tdb_nextkey) predate transactions, and it
- was thought that it was important to guarantee that all records which exist
- at the start and end of the traversal would be included, and no record
- would be included twice.
-\end_layout
-
-\begin_layout Standard
-This adds complexity (see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "Reliable-Traversal-Adds"
-
-\end_inset
-
-) and does not work anyway for records which are altered (in particular,
- those which are expanded may be effectively deleted and re-added behind
- the traversal).
-\end_layout
-
-\begin_layout Subsubsection
-\begin_inset CommandInset label
-LatexCommand label
-name "traverse-Proposed-Solution"
-
-\end_inset
-
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Abandon the guarantee.
- You will see every record if no changes occur during your traversal, otherwise
- you will see some subset.
- You can prevent changes by using a transaction or the locking API.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
- Delete-during-traverse will still delete every record, too (assuming no
- other changes).
-\end_layout
-
-\begin_layout Subsection
-Nesting of Transactions Is Fraught
-\end_layout
-
-\begin_layout Standard
-TDB has alternated between allowing nested transactions and not allowing
- them.
- Various paths in the Samba codebase assume that transactions will nest,
- and in a sense they can: the operation is only committed to disk when the
- outer transaction is committed.
- There are two problems, however:
-\end_layout
-
-\begin_layout Enumerate
-Canceling the inner transaction will cause the outer transaction commit
- to fail, and will not undo any operations since the inner transaction began.
- This problem is soluble with some additional internal code.
-\end_layout
-
-\begin_layout Enumerate
-An inner transaction commit can be cancelled by the outer transaction.
- This is desirable in the way which Samba's database initialization code
- uses transactions, but could be a surprise to any users expecting a successful
- transaction commit to expose changes to others.
-\end_layout
-
-\begin_layout Standard
-The current solution is to specify the behavior at tdb_open(), with the
- default currently that nested transactions are allowed.
- This flag can also be changed at runtime.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Given the usage patterns, it seems that the
-\begin_inset Quotes eld
-\end_inset
-
-least-surprise
-\begin_inset Quotes erd
-\end_inset
-
- behavior of disallowing nested transactions should become the default.
- Additionally, it seems the outer transaction is the only code which knows
- whether inner transactions should be allowed, so a flag to indicate this
- could be added to tdb_transaction_start.
- However, this behavior can be simulated with a wrapper which uses tdb_add_flags
-() and tdb_remove_flags(), so the API should not be expanded for this relatively
--obscure case.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1298979572
-Incomplete; nesting flag is still defined as per tdb1.
-\change_inserted 0 1298979584
-Complete; the nesting flag has been removed.
-\change_unchanged
-
-\end_layout
-
-\begin_layout Subsection
-Incorrect Hash Function is Not Detected
-\end_layout
-
-\begin_layout Standard
-tdb_open_ex() allows the calling code to specify a different hash function
- to use, but does not check that all other processes accessing this tdb
- are using the same hash function.
- The result is that records are missing from tdb_fetch().
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-The header should contain an example hash result (eg.
- the hash of 0xdeadbeef), and tdb_open_ex() should check that the given
- hash function produces the same answer, or fail the tdb_open call.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-tdb_set_max_dead/TDB_VOLATILE Expose Implementation
-\end_layout
-
-\begin_layout Standard
-In response to scalability issues with the free list (
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "TDB-Freelist-Is"
-
-\end_inset
-
-) two API workarounds have been incorporated in TDB: tdb_set_max_dead()
- and the TDB_VOLATILE flag to tdb_open.
- The latter actually calls the former with an argument of
-\begin_inset Quotes eld
-\end_inset
-
-5
-\begin_inset Quotes erd
-\end_inset
-
-.
-\end_layout
-
-\begin_layout Standard
-This code allows deleted records to accumulate without putting them in the
- free list.
- On delete we iterate through each chain and free them in a batch if there
- are more than max_dead entries.
- These are never otherwise recycled except as a side-effect of a tdb_repack.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-With the scalability problems of the freelist solved, this API can be removed.
- The TDB_VOLATILE flag may still be useful as a hint that store and delete
- of records will be at least as common as fetch in order to allow some internal
- tuning, but initially will become a no-op.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete.
- TDB_VOLATILE still defined, but implementation should fail on unknown flags
- to be future-proof.
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "TDB-Files-Cannot"
-
-\end_inset
-
-TDB Files Cannot Be Opened Multiple Times In The Same Process
-\end_layout
-
-\begin_layout Standard
-No process can open the same TDB twice; we check and disallow it.
- This is an unfortunate side-effect of fcntl locks, which operate on a per-file
- rather than per-file-descriptor basis, and do not nest.
- Thus, closing any file descriptor on a file clears all the locks obtained
- by this process, even if they were placed using a different file descriptor!
-\end_layout
-
-\begin_layout Standard
-Note that even if this were solved, deadlock could occur if operations were
- nested: this is a more manageable programming error in most cases.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-We could lobby POSIX to fix the perverse rules, or at least lobby Linux
- to violate them so that the most common implementation does not have this
- restriction.
- This would be a generally good idea for other fcntl lock users.
-\end_layout
-
-\begin_layout Standard
-Samba uses a wrapper which hands out the same tdb_context to multiple callers
- if this happens, and does simple reference counting.
- We should do this inside the tdb library, which already emulates lock nesting
- internally; it would need to recognize when deadlock occurs within a single
- process.
- This would create a new failure mode for tdb operations (while we currently
- handle locking failures, they are impossible in normal use and a process
- encountering them can do little but give up).
-\end_layout
-
-\begin_layout Standard
-I do not see benefit in an additional tdb_open flag to indicate whether
- re-opening is allowed, as though there may be some benefit to adding a
- call to detect when a tdb_context is shared, to allow other to create such
- an API.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete.
-\end_layout
-
-\begin_layout Subsection
-TDB API Is Not POSIX Thread-safe
-\end_layout
-
-\begin_layout Standard
-The TDB API uses an error code which can be queried after an operation to
- determine what went wrong.
- This programming model does not work with threads, unless specific additional
- guarantees are given by the implementation.
- In addition, even otherwise-independent threads cannot open the same TDB
- (as in
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "TDB-Files-Cannot"
-
-\end_inset
-
-).
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Reachitecting the API to include a tdb_errcode pointer would be a great
- deal of churn
-\change_inserted 0 1298979557
-, but fortunately most functions return 0 on success and -1 on error: we
- can change these to return 0 on success and a negative error code on error,
- and the API remains similar to previous.
- The tdb_fetch, tdb_firstkey and tdb_nextkey functions need to take a TDB_DATA
- pointer and return an error code.
- It is also simpler to have tdb_nextkey replace its key argument in place,
- freeing up any old .dptr.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1298979438
-; we are better to guarantee that the tdb_errcode is per-thread so the current
- programming model can be maintained.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1298979438
-This requires dynamic per-thread allocations, which is awkward with POSIX
- threads (pthread_key_create space is limited and we cannot simply allocate
- a key for every TDB).
-\change_unchanged
-
-\end_layout
-
-\begin_layout Standard
-Internal locking is required to make sure that fcntl locks do not overlap
- between threads, and also that the global list of tdbs is maintained.
-\end_layout
-
-\begin_layout Standard
-The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe
- version of the library, and otherwise no overhead will exist.
- Alternatively, a hooking mechanism similar to that proposed for
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "Proposed-Solution-locking-hook"
-
-\end_inset
-
- could be used to enable pthread locking at runtime.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete
-\change_inserted 0 1298979681
-; API has been changed but thread safety has not been implemented.
-\change_deleted 0 1298979669
-.
-\change_unchanged
-
-\end_layout
-
-\begin_layout Subsection
-*_nonblock Functions And *_mark Functions Expose Implementation
-\end_layout
-
-\begin_layout Standard
-CTDB
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-Clustered TDB, see http://ctdb.samba.org
-\end_layout
-
-\end_inset
-
- wishes to operate on TDB in a non-blocking manner.
- This is currently done as follows:
-\end_layout
-
-\begin_layout Enumerate
-Call the _nonblock variant of an API function (eg.
- tdb_lockall_nonblock).
- If this fails:
-\end_layout
-
-\begin_layout Enumerate
-Fork a child process, and wait for it to call the normal variant (eg.
- tdb_lockall).
-\end_layout
-
-\begin_layout Enumerate
-If the child succeeds, call the _mark variant to indicate we already have
- the locks (eg.
- tdb_lockall_mark).
-\end_layout
-
-\begin_layout Enumerate
-Upon completion, tell the child to release the locks (eg.
- tdb_unlockall).
-\end_layout
-
-\begin_layout Enumerate
-Indicate to tdb that it should consider the locks removed (eg.
- tdb_unlockall_mark).
-\end_layout
-
-\begin_layout Standard
-There are several issues with this approach.
- Firstly, adding two new variants of each function clutters the API for
- an obscure use, and so not all functions have three variants.
- Secondly, it assumes that all paths of the functions ask for the same locks,
- otherwise the parent process will have to get a lock which the child doesn't
- have under some circumstances.
- I don't believe this is currently the case, but it constrains the implementatio
-n.
-
-\end_layout
-
-\begin_layout Subsubsection
-\begin_inset CommandInset label
-LatexCommand label
-name "Proposed-Solution-locking-hook"
-
-\end_inset
-
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Implement a hook for locking methods, so that the caller can control the
- calls to create and remove fcntl locks.
- In this scenario, ctdbd would operate as follows:
-\end_layout
-
-\begin_layout Enumerate
-Call the normal API function, eg tdb_lockall().
-\end_layout
-
-\begin_layout Enumerate
-When the lock callback comes in, check if the child has the lock.
- Initially, this is always false.
- If so, return 0.
- Otherwise, try to obtain it in non-blocking mode.
- If that fails, return EWOULDBLOCK.
-\end_layout
-
-\begin_layout Enumerate
-Release locks in the unlock callback as normal.
-\end_layout
-
-\begin_layout Enumerate
-If tdb_lockall() fails, see if we recorded a lock failure; if so, call the
- child to repeat the operation.
-\end_layout
-
-\begin_layout Enumerate
-The child records what locks it obtains, and returns that information to
- the parent.
-\end_layout
-
-\begin_layout Enumerate
-When the child has succeeded, goto 1.
-\end_layout
-
-\begin_layout Standard
-This is flexible enough to handle any potential locking scenario, even when
- lock requirements change.
- It can be optimized so that the parent does not release locks, just tells
- the child which locks it doesn't need to obtain.
-\end_layout
-
-\begin_layout Standard
-It also keeps the complexity out of the API, and in ctdbd where it is needed.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete.
-\end_layout
-
-\begin_layout Subsection
-tdb_chainlock Functions Expose Implementation
-\end_layout
-
-\begin_layout Standard
-tdb_chainlock locks some number of records, including the record indicated
- by the given key.
- This gave atomicity guarantees; no-one can start a transaction, alter,
- read or delete that key while the lock is held.
-\end_layout
-
-\begin_layout Standard
-It also makes the same guarantee for any other key in the chain, which is
- an internal implementation detail and potentially a cause for deadlock.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-None.
- It would be nice to have an explicit single entry lock which effected no
- other keys.
- Unfortunately, this won't work for an entry which doesn't exist.
- Thus while chainlock may be implemented more efficiently for the existing
- case, it will still have overlap issues with the non-existing case.
- So it is best to keep the current (lack of) guarantee about which records
- will be effected to avoid constraining our implementation.
-\end_layout
-
-\begin_layout Subsection
-Signal Handling is Not Race-Free
-\end_layout
-
-\begin_layout Standard
-The tdb_setalarm_sigptr() call allows the caller's signal handler to indicate
- that the tdb locking code should return with a failure, rather than trying
- again when a signal is received (and errno == EAGAIN).
- This is usually used to implement timeouts.
-\end_layout
-
-\begin_layout Standard
-Unfortunately, this does not work in the case where the signal is received
- before the tdb code enters the fcntl() call to place the lock: the code
- will sleep within the fcntl() code, unaware that the signal wants it to
- exit.
- In the case of long timeouts, this does not happen in practice.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-The locking hooks proposed in
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "Proposed-Solution-locking-hook"
-
-\end_inset
-
- would allow the user to decide on whether to fail the lock acquisition
- on a signal.
- This allows the caller to choose their own compromise: they could narrow
- the race by checking immediately before the fcntl call.
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-It may be possible to make this race-free in some implementations by having
- the signal handler alter the struct flock to make it invalid.
- This will cause the fcntl() lock call to fail with EINVAL if the signal
- occurs before the kernel is entered, otherwise EAGAIN.
-\end_layout
-
-\end_inset
-
-
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete.
-\end_layout
-
-\begin_layout Subsection
-The API Uses Gratuitous Typedefs, Capitals
-\end_layout
-
-\begin_layout Standard
-typedefs are useful for providing source compatibility when types can differ
- across implementations, or arguably in the case of function pointer definitions
- which are hard for humans to parse.
- Otherwise it is simply obfuscation and pollutes the namespace.
-\end_layout
-
-\begin_layout Standard
-Capitalization is usually reserved for compile-time constants and macros.
-\end_layout
-
-\begin_layout Description
-TDB_CONTEXT There is no reason to use this over 'struct tdb_context'; the
- definition isn't visible to the API user anyway.
-\end_layout
-
-\begin_layout Description
-TDB_DATA There is no reason to use this over struct TDB_DATA; the struct
- needs to be understood by the API user.
-\end_layout
-
-\begin_layout Description
-struct
-\begin_inset space ~
-\end_inset
-
-TDB_DATA This would normally be called 'struct tdb_data'.
-\end_layout
-
-\begin_layout Description
-enum
-\begin_inset space ~
-\end_inset
-
-TDB_ERROR Similarly, this would normally be enum tdb_error.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-None.
- Introducing lower case variants would please pedants like myself, but if
- it were done the existing ones should be kept.
- There is little point forcing a purely cosmetic change upon tdb users.
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "tdb_log_func-Doesnt-Take"
-
-\end_inset
-
-tdb_log_func Doesn't Take The Private Pointer
-\end_layout
-
-\begin_layout Standard
-For API compatibility reasons, the logging function needs to call tdb_get_loggin
-g_private() to retrieve the pointer registered by the tdb_open_ex for logging.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-It should simply take an extra argument, since we are prepared to break
- the API/ABI.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-Various Callback Functions Are Not Typesafe
-\end_layout
-
-\begin_layout Standard
-The callback functions in tdb_set_logging_function (after
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "tdb_log_func-Doesnt-Take"
-
-\end_inset
-
- is resolved), tdb_parse_record, tdb_traverse, tdb_traverse_read and tdb_check
- all take void * and must internally convert it to the argument type they
- were expecting.
-\end_layout
-
-\begin_layout Standard
-If this type changes, the compiler will not produce warnings on the callers,
- since it only sees void *.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-With careful use of macros, we can create callback functions which give
- a warning when used on gcc and the types of the callback and its private
- argument differ.
- Unsupported compilers will not give a warning, which is no worse than now.
- In addition, the callbacks become clearer, as they need not use void *
- for their parameter.
-\end_layout
-
-\begin_layout Standard
-See CCAN's typesafe_cb module at http://ccan.ozlabs.org/info/typesafe_cb.html
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete.
-\end_layout
-
-\begin_layout Subsection
-TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, tdb_reopen_all Problematic
-\end_layout
-
-\begin_layout Standard
-The TDB_CLEAR_IF_FIRST flag to tdb_open indicates that the TDB file should
- be cleared if the caller discovers it is the only process with the TDB
- open.
- However, if any caller does not specify TDB_CLEAR_IF_FIRST it will not
- be detected, so will have the TDB erased underneath them (usually resulting
- in a crash).
-\end_layout
-
-\begin_layout Standard
-There is a similar issue on fork(); if the parent exits (or otherwise closes
- the tdb) before the child calls tdb_reopen_all() to establish the lock
- used to indicate the TDB is opened by someone, a TDB_CLEAR_IF_FIRST opener
- at that moment will believe it alone has opened the TDB and will erase
- it.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Remove TDB_CLEAR_IF_FIRST.
- Other workarounds are possible, but see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "TDB_CLEAR_IF_FIRST-Imposes-Performance"
-
-\end_inset
-
-.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1298979699
-Incomplete, TDB_CLEAR_IF_FIRST still defined, but not implemented.
-\change_inserted 0 1298979700
-Complete.
-\change_unchanged
-
-\end_layout
-
-\begin_layout Subsection
-Extending The Header Is Difficult
-\end_layout
-
-\begin_layout Standard
-We have reserved (zeroed) words in the TDB header, which can be used for
- future features.
- If the future features are compulsory, the version number must be updated
- to prevent old code from accessing the database.
- But if the future feature is optional, we have no way of telling if older
- code is accessing the database or not.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-The header should contain a
-\begin_inset Quotes eld
-\end_inset
-
-format variant
-\begin_inset Quotes erd
-\end_inset
-
- value (64-bit).
- This is divided into two 32-bit parts:
-\end_layout
-
-\begin_layout Enumerate
-The lower part reflects the format variant understood by code accessing
- the database.
-\end_layout
-
-\begin_layout Enumerate
-The upper part reflects the format variant you must understand to write
- to the database (otherwise you can only open for reading).
-\end_layout
-
-\begin_layout Standard
-The latter field can only be written at creation time, the former should
- be written under the OPEN_LOCK when opening the database for writing, if
- the variant of the code is lower than the current lowest variant.
-\end_layout
-
-\begin_layout Standard
-This should allow backwards-compatible features to be added, and detection
- if older code (which doesn't understand the feature) writes to the database.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete.
-\end_layout
-
-\begin_layout Subsection
-Record Headers Are Not Expandible
-\end_layout
-
-\begin_layout Standard
-If we later want to add (say) checksums on keys and data, it would require
- another format change, which we'd like to avoid.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-We often have extra padding at the tail of a record.
- If we ensure that the first byte (if any) of this padding is zero, we will
- have a way for future changes to detect code which doesn't understand a
- new format: the new code would write (say) a 1 at the tail, and thus if
- there is no tail or the first byte is 0, we would know the extension is
- not present on that record.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Incomplete.
-\end_layout
-
-\begin_layout Subsection
-TDB Does Not Use Talloc
-\end_layout
-
-\begin_layout Standard
-Many users of TDB (particularly Samba) use the talloc allocator, and thus
- have to wrap TDB in a talloc context to use it conveniently.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-The allocation within TDB is not complicated enough to justify the use of
- talloc, and I am reluctant to force another (excellent) library on TDB
- users.
- Nonetheless a compromise is possible.
- An attribute (see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "attributes"
-
-\end_inset
-
-) can be added later to tdb_open() to provide an alternate allocation mechanism,
- specifically for talloc but usable by any other allocator (which would
- ignore the
-\begin_inset Quotes eld
-\end_inset
-
-context
-\begin_inset Quotes erd
-\end_inset
-
- argument).
-\end_layout
-
-\begin_layout Standard
-This would form a talloc heirarchy as expected, but the caller would still
- have to attach a destructor to the tdb context returned from tdb_open to
- close it.
- All TDB_DATA fields would be children of the tdb_context, and the caller
- would still have to manage them (using talloc_free() or talloc_steal()).
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Deferred.
-\end_layout
-
-\begin_layout Section
-Performance And Scalability Issues
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "TDB_CLEAR_IF_FIRST-Imposes-Performance"
-
-\end_inset
-
-TDB_CLEAR_IF_FIRST Imposes Performance Penalty
-\end_layout
-
-\begin_layout Standard
-When TDB_CLEAR_IF_FIRST is specified, a 1-byte read lock is placed at offset
- 4 (aka.
- the ACTIVE_LOCK).
- While these locks never conflict in normal tdb usage, they do add substantial
- overhead for most fcntl lock implementations when the kernel scans to detect
- if a lock conflict exists.
- This is often a single linked list, making the time to acquire and release
- a fcntl lock O(N) where N is the number of processes with the TDB open,
- not the number actually doing work.
-\end_layout
-
-\begin_layout Standard
-In a Samba server it is common to have huge numbers of clients sitting idle,
- and thus they have weaned themselves off the TDB_CLEAR_IF_FIRST flag.
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-There is a flag to tdb_reopen_all() which is used for this optimization:
- if the parent process will outlive the child, the child does not need the
- ACTIVE_LOCK.
- This is a workaround for this very performance issue.
-\end_layout
-
-\end_inset
-
-
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Remove the flag.
- It was a neat idea, but even trivial servers tend to know when they are
- initializing for the first time and can simply unlink the old tdb at that
- point.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1298979837
-Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing.
-\change_inserted 0 1298979837
-Complete.
-\change_unchanged
-
-\end_layout
-
-\begin_layout Subsection
-TDB Files Have a 4G Limit
-\end_layout
-
-\begin_layout Standard
-This seems to be becoming an issue (so much for
-\begin_inset Quotes eld
-\end_inset
-
-trivial
-\begin_inset Quotes erd
-\end_inset
-
-!), particularly for ldb.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-A new, incompatible TDB format which uses 64 bit offsets internally rather
- than 32 bit as now.
- For simplicity of endian conversion (which TDB does on the fly if required),
- all values will be 64 bit on disk.
- In practice, some upper bits may be used for other purposes, but at least
- 56 bits will be available for file offsets.
-\end_layout
-
-\begin_layout Standard
-tdb_open() will automatically detect the old version, and even create them
- if TDB_VERSION6 is specified to tdb_open.
-\end_layout
-
-\begin_layout Standard
-32 bit processes will still be able to access TDBs larger than 4G (assuming
- that their off_t allows them to seek to 64 bits), they will gracefully
- fall back as they fail to mmap.
- This can happen already with large TDBs.
-\end_layout
-
-\begin_layout Standard
-Old versions of tdb will fail to open the new TDB files (since 28 August
- 2009, commit 398d0c29290: prior to that any unrecognized file format would
- be erased and initialized as a fresh tdb!)
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-TDB Records Have a 4G Limit
-\end_layout
-
-\begin_layout Standard
-This has not been a reported problem, and the API uses size_t which can
- be 64 bit on 64 bit platforms.
- However, other limits may have made such an issue moot.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Record sizes will be 64 bit, with an error returned on 32 bit platforms
- which try to access such records (the current implementation would return
- TDB_ERR_OOM in a similar case).
- It seems unlikely that 32 bit keys will be a limitation, so the implementation
- may not support this (see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "sub:Records-Incur-A"
-
-\end_inset
-
-).
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-Hash Size Is Determined At TDB Creation Time
-\end_layout
-
-\begin_layout Standard
-TDB contains a number of hash chains in the header; the number is specified
- at creation time, and defaults to 131.
- This is such a bottleneck on large databases (as each hash chain gets quite
- long), that LDB uses 10,000 for this hash.
- In general it is impossible to know what the 'right' answer is at database
- creation time.
-\end_layout
-
-\begin_layout Subsubsection
-\begin_inset CommandInset label
-LatexCommand label
-name "sub:Hash-Size-Solution"
-
-\end_inset
-
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-After comprehensive performance testing on various scalable hash variants
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-http://rusty.ozlabs.org/?p=89 and http://rusty.ozlabs.org/?p=94 This was annoying
- because I was previously convinced that an expanding tree of hashes would
- be very close to optimal.
-\end_layout
-
-\end_inset
-
-, it became clear that it is hard to beat a straight linear hash table which
- doubles in size when it reaches saturation.
- Unfortunately, altering the hash table introduces serious locking complications
-: the entire hash table needs to be locked to enlarge the hash table, and
- others might be holding locks.
- Particularly insidious are insertions done under tdb_chainlock.
-\end_layout
-
-\begin_layout Standard
-Thus an expanding layered hash will be used: an array of hash groups, with
- each hash group exploding into pointers to lower hash groups once it fills,
- turning into a hash tree.
- This has implications for locking: we must lock the entire group in case
- we need to expand it, yet we don't know how deep the tree is at that point.
-\end_layout
-
-\begin_layout Standard
-Note that bits from the hash table entries should be stolen to hold more
- hash bits to reduce the penalty of collisions.
- We can use the otherwise-unused lower 3 bits.
- If we limit the size of the database to 64 exabytes, we can use the top
- 8 bits of the hash entry as well.
- These 11 bits would reduce false positives down to 1 in 2000 which is more
- than we need: we can use one of the bits to indicate that the extra hash
- bits are valid.
- This means we can choose not to re-hash all entries when we expand a hash
- group; simply use the next bits we need and mark them invalid.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "TDB-Freelist-Is"
-
-\end_inset
-
-TDB Freelist Is Highly Contended
-\end_layout
-
-\begin_layout Standard
-TDB uses a single linked list for the free list.
- Allocation occurs as follows, using heuristics which have evolved over
- time:
-\end_layout
-
-\begin_layout Enumerate
-Get the free list lock for this whole operation.
-\end_layout
-
-\begin_layout Enumerate
-Multiply length by 1.25, so we always over-allocate by 25%.
-\end_layout
-
-\begin_layout Enumerate
-Set the slack multiplier to 1.
-\end_layout
-
-\begin_layout Enumerate
-Examine the current freelist entry: if it is > length but < the current
- best case, remember it as the best case.
-\end_layout
-
-\begin_layout Enumerate
-Multiply the slack multiplier by 1.05.
-\end_layout
-
-\begin_layout Enumerate
-If our best fit so far is less than length * slack multiplier, return it.
- The slack will be turned into a new free record if it's large enough.
-\end_layout
-
-\begin_layout Enumerate
-Otherwise, go onto the next freelist entry.
-\end_layout
-
-\begin_layout Standard
-Deleting a record occurs as follows:
-\end_layout
-
-\begin_layout Enumerate
-Lock the hash chain for this whole operation.
-\end_layout
-
-\begin_layout Enumerate
-Walk the chain to find the record, keeping the prev pointer offset.
-\end_layout
-
-\begin_layout Enumerate
-If max_dead is non-zero:
-\end_layout
-
-\begin_deeper
-\begin_layout Enumerate
-Walk the hash chain again and count the dead records.
-\end_layout
-
-\begin_layout Enumerate
-If it's more than max_dead, bulk free all the dead ones (similar to steps
- 4 and below, but the lock is only obtained once).
-\end_layout
-
-\begin_layout Enumerate
-Simply mark this record as dead and return.
-
-\end_layout
-
-\end_deeper
-\begin_layout Enumerate
-Get the free list lock for the remainder of this operation.
-\end_layout
-
-\begin_layout Enumerate
-\begin_inset CommandInset label
-LatexCommand label
-name "right-merging"
-
-\end_inset
-
-Examine the following block to see if it is free; if so, enlarge the current
- block and remove that block from the free list.
- This was disabled, as removal from the free list was O(entries-in-free-list).
-\end_layout
-
-\begin_layout Enumerate
-Examine the preceeding block to see if it is free: for this reason, each
- block has a 32-bit tailer which indicates its length.
- If it is free, expand it to cover our new block and return.
-\end_layout
-
-\begin_layout Enumerate
-Otherwise, prepend ourselves to the free list.
-\end_layout
-
-\begin_layout Standard
-Disabling right-merging (step
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "right-merging"
-
-\end_inset
-
-) causes fragmentation; the other heuristics proved insufficient to address
- this, so the final answer to this was that when we expand the TDB file
- inside a transaction commit, we repack the entire tdb.
-\end_layout
-
-\begin_layout Standard
-The single list lock limits our allocation rate; due to the other issues
- this is not currently seen as a bottleneck.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-The first step is to remove all the current heuristics, as they obviously
- interact, then examine them once the lock contention is addressed.
-\end_layout
-
-\begin_layout Standard
-The free list must be split to reduce contention.
- Assuming perfect free merging, we can at most have 1 free list entry for
- each entry.
- This implies that the number of free lists is related to the size of the
- hash table, but as it is rare to walk a large number of free list entries
- we can use far fewer, say 1/32 of the number of hash buckets.
-\end_layout
-
-\begin_layout Standard
-It seems tempting to try to reuse the hash implementation which we use for
- records here, but we have two ways of searching for free entries: for allocatio
-n we search by size (and possibly zone) which produces too many clashes
- for our hash table to handle well, and for coalescing we search by address.
- Thus an array of doubly-linked free lists seems preferable.
-\end_layout
-
-\begin_layout Standard
-There are various benefits in using per-size free lists (see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "sub:TDB-Becomes-Fragmented"
-
-\end_inset
-
-) but it's not clear this would reduce contention in the common case where
- all processes are allocating/freeing the same size.
- Thus we almost certainly need to divide in other ways: the most obvious
- is to divide the file into zones, and using a free list (or table of free
- lists) for each.
- This approximates address ordering.
-\end_layout
-
-\begin_layout Standard
-Unfortunately it is difficult to know what heuristics should be used to
- determine zone sizes, and our transaction code relies on being able to
- create a
-\begin_inset Quotes eld
-\end_inset
-
-recovery area
-\begin_inset Quotes erd
-\end_inset
-
- by simply appending to the file (difficult if it would need to create a
- new zone header).
- Thus we use a linked-list of free tables; currently we only ever create
- one, but if there is more than one we choose one at random to use.
- In future we may use heuristics to add new free tables on contention.
- We only expand the file when all free tables are exhausted.
-\end_layout
-
-\begin_layout Standard
-The basic algorithm is as follows.
- Freeing is simple:
-\end_layout
-
-\begin_layout Enumerate
-Identify the correct free list.
-\end_layout
-
-\begin_layout Enumerate
-Lock the corresponding list.
-\end_layout
-
-\begin_layout Enumerate
-Re-check the list (we didn't have a lock, sizes could have changed): relock
- if necessary.
-\end_layout
-
-\begin_layout Enumerate
-Place the freed entry in the list.
-\end_layout
-
-\begin_layout Standard
-Allocation is a little more complicated, as we perform delayed coalescing
- at this point:
-\end_layout
-
-\begin_layout Enumerate
-Pick a free table; usually the previous one.
-\end_layout
-
-\begin_layout Enumerate
-Lock the corresponding list.
-\end_layout
-
-\begin_layout Enumerate
-If the top entry is -large enough, remove it from the list and return it.
-\end_layout
-
-\begin_layout Enumerate
-Otherwise, coalesce entries in the list.If there was no entry large enough,
- unlock the list and try the next largest list
-\end_layout
-
-\begin_layout Enumerate
-If no list has an entry which meets our needs, try the next free table.
-\end_layout
-
-\begin_layout Enumerate
-If no zone satisfies, expand the file.
-\end_layout
-
-\begin_layout Standard
-This optimizes rapid insert/delete of free list entries by not coalescing
- them all the time..
- First-fit address ordering ordering seems to be fairly good for keeping
- fragmentation low (see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "sub:TDB-Becomes-Fragmented"
-
-\end_inset
-
-).
- Note that address ordering does not need a tailer to coalesce, though if
- we needed one we could have one cheaply: see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "sub:Records-Incur-A"
-
-\end_inset
-
-.
-
-\end_layout
-
-\begin_layout Standard
-Each free entry has the free table number in the header: less than 255.
- It also contains a doubly-linked list for easy deletion.
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "sub:TDB-Becomes-Fragmented"
-
-\end_inset
-
-TDB Becomes Fragmented
-\end_layout
-
-\begin_layout Standard
-Much of this is a result of allocation strategy
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-The Memory Fragmentation Problem: Solved? Johnstone & Wilson 1995 ftp://ftp.cs.ute
-xas.edu/pub/garbage/malloc/ismm98.ps
-\end_layout
-
-\end_inset
-
- and deliberate hobbling of coalescing; internal fragmentation (aka overallocati
-on) is deliberately set at 25%, and external fragmentation is only cured
- by the decision to repack the entire db when a transaction commit needs
- to enlarge the file.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-The 25% overhead on allocation works in practice for ldb because indexes
- tend to expand by one record at a time.
- This internal fragmentation can be resolved by having an
-\begin_inset Quotes eld
-\end_inset
-
-expanded
-\begin_inset Quotes erd
-\end_inset
-
- bit in the header to note entries that have previously expanded, and allocating
- more space for them.
-\end_layout
-
-\begin_layout Standard
-There are is a spectrum of possible solutions for external fragmentation:
- one is to use a fragmentation-avoiding allocation strategy such as best-fit
- address-order allocator.
- The other end of the spectrum would be to use a bump allocator (very fast
- and simple) and simply repack the file when we reach the end.
-\end_layout
-
-\begin_layout Standard
-There are three problems with efficient fragmentation-avoiding allocators:
- they are non-trivial, they tend to use a single free list for each size,
- and there's no evidence that tdb allocation patterns will match those recorded
- for general allocators (though it seems likely).
-\end_layout
-
-\begin_layout Standard
-Thus we don't spend too much effort on external fragmentation; we will be
- no worse than the current code if we need to repack on occasion.
- More effort is spent on reducing freelist contention, and reducing overhead.
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "sub:Records-Incur-A"
-
-\end_inset
-
-Records Incur A 28-Byte Overhead
-\end_layout
-
-\begin_layout Standard
-Each TDB record has a header as follows:
-\end_layout
-
-\begin_layout LyX-Code
-struct tdb_record {
-\end_layout
-
-\begin_layout LyX-Code
- tdb_off_t next; /* offset of the next record in the list */
-\end_layout
-
-\begin_layout LyX-Code
- tdb_len_t rec_len; /* total byte length of record */
-\end_layout
-
-\begin_layout LyX-Code
- tdb_len_t key_len; /* byte length of key */
-\end_layout
-
-\begin_layout LyX-Code
- tdb_len_t data_len; /* byte length of data */
-\end_layout
-
-\begin_layout LyX-Code
- uint32_t full_hash; /* the full 32 bit hash of the key */
-\end_layout
-
-\begin_layout LyX-Code
- uint32_t magic; /* try to catch errors */
-\end_layout
-
-\begin_layout LyX-Code
- /* the following union is implied:
-\end_layout
-
-\begin_layout LyX-Code
- union {
-\end_layout
-
-\begin_layout LyX-Code
- char record[rec_len];
-\end_layout
-
-\begin_layout LyX-Code
- struct {
-\end_layout
-
-\begin_layout LyX-Code
- char key[key_len];
-\end_layout
-
-\begin_layout LyX-Code
- char data[data_len];
-\end_layout
-
-\begin_layout LyX-Code
- }
-\end_layout
-
-\begin_layout LyX-Code
- uint32_t totalsize; (tailer)
-\end_layout
-
-\begin_layout LyX-Code
- }
-\end_layout
-
-\begin_layout LyX-Code
- */
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout Standard
-Naively, this would double to a 56-byte overhead on a 64 bit implementation.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-We can use various techniques to reduce this for an allocated block:
-\end_layout
-
-\begin_layout Enumerate
-The 'next' pointer is not required, as we are using a flat hash table.
-\end_layout
-
-\begin_layout Enumerate
-'rec_len' can instead be expressed as an addition to key_len and data_len
- (it accounts for wasted or overallocated length in the record).
- Since the record length is always a multiple of 8, we can conveniently
- fit it in 32 bits (representing up to 35 bits).
-\end_layout
-
-\begin_layout Enumerate
-'key_len' and 'data_len' can be reduced.
- I'm unwilling to restrict 'data_len' to 32 bits, but instead we can combine
- the two into one 64-bit field and using a 5 bit value which indicates at
- what bit to divide the two.
- Keys are unlikely to scale as fast as data, so I'm assuming a maximum key
- size of 32 bits.
-\end_layout
-
-\begin_layout Enumerate
-'full_hash' is used to avoid a memcmp on the
-\begin_inset Quotes eld
-\end_inset
-
-miss
-\begin_inset Quotes erd
-\end_inset
-
- case, but this is diminishing returns after a handful of bits (at 10 bits,
- it reduces 99.9% of false memcmp).
- As an aside, as the lower bits are already incorporated in the hash table
- resolution, the upper bits should be used here.
- Note that it's not clear that these bits will be a win, given the extra
- bits in the hash table itself (see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "sub:Hash-Size-Solution"
-
-\end_inset
-
-).
-\end_layout
-
-\begin_layout Enumerate
-'magic' does not need to be enlarged: it currently reflects one of 5 values
- (used, free, dead, recovery, and unused_recovery).
- It is useful for quick sanity checking however, and should not be eliminated.
-\end_layout
-
-\begin_layout Enumerate
-'tailer' is only used to coalesce free blocks (so a block to the right can
- find the header to check if this block is free).
- This can be replaced by a single 'free' bit in the header of the following
- block (and the tailer only exists in free blocks).
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-This technique from Thomas Standish.
- Data Structure Techniques.
- Addison-Wesley, Reading, Massachusetts, 1980.
-\end_layout
-
-\end_inset
-
- The current proposed coalescing algorithm doesn't need this, however.
-\end_layout
-
-\begin_layout Standard
-This produces a 16 byte used header like this:
-\end_layout
-
-\begin_layout LyX-Code
-struct tdb_used_record {
-\end_layout
-
-\begin_layout LyX-Code
- uint32_t used_magic : 16,
-\end_layout
-
-\begin_layout LyX-Code
-
-\end_layout
-
-\begin_layout LyX-Code
- key_data_divide: 5,
-\end_layout
-
-\begin_layout LyX-Code
- top_hash: 11;
-\end_layout
-
-\begin_layout LyX-Code
- uint32_t extra_octets;
-\end_layout
-
-\begin_layout LyX-Code
- uint64_t key_and_data_len;
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout Standard
-And a free record like this:
-\end_layout
-
-\begin_layout LyX-Code
-struct tdb_free_record {
-\end_layout
-
-\begin_layout LyX-Code
- uint64_t free_magic: 8,
-\end_layout
-
-\begin_layout LyX-Code
- prev : 56;
-\end_layout
-
-\begin_layout LyX-Code
-
-\end_layout
-
-\begin_layout LyX-Code
- uint64_t free_table: 8,
-\end_layout
-
-\begin_layout LyX-Code
- total_length : 56
-\end_layout
-
-\begin_layout LyX-Code
- uint64_t next;;
-\end_layout
-
-\begin_layout LyX-Code
-};
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1291206079
-
-\change_unchanged
-Note that by limiting valid offsets to 56 bits, we can pack everything we
- need into 3 64-byte words, meaning our minimum record size is 8 bytes.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-Transaction Commit Requires 4 fdatasync
-\end_layout
-
-\begin_layout Standard
-The current transaction algorithm is:
-\end_layout
-
-\begin_layout Enumerate
-write_recovery_data();
-\end_layout
-
-\begin_layout Enumerate
-sync();
-\end_layout
-
-\begin_layout Enumerate
-write_recovery_header();
-\end_layout
-
-\begin_layout Enumerate
-sync();
-\end_layout
-
-\begin_layout Enumerate
-overwrite_with_new_data();
-\end_layout
-
-\begin_layout Enumerate
-sync();
-\end_layout
-
-\begin_layout Enumerate
-remove_recovery_header();
-\end_layout
-
-\begin_layout Enumerate
-sync();
-\end_layout
-
-\begin_layout Standard
-On current ext3, each sync flushes all data to disk, so the next 3 syncs
- are relatively expensive.
- But this could become a performance bottleneck on other filesystems such
- as ext4.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Neil Brown points out that this is overzealous, and only one sync is needed:
-\end_layout
-
-\begin_layout Enumerate
-Bundle the recovery data, a transaction counter and a strong checksum of
- the new data.
-\end_layout
-
-\begin_layout Enumerate
-Strong checksum that whole bundle.
-\end_layout
-
-\begin_layout Enumerate
-Store the bundle in the database.
-\end_layout
-
-\begin_layout Enumerate
-Overwrite the oldest of the two recovery pointers in the header (identified
- using the transaction counter) with the offset of this bundle.
-\end_layout
-
-\begin_layout Enumerate
-sync.
-\end_layout
-
-\begin_layout Enumerate
-Write the new data to the file.
-\end_layout
-
-\begin_layout Standard
-Checking for recovery means identifying the latest bundle with a valid checksum
- and using the new data checksum to ensure that it has been applied.
- This is more expensive than the current check, but need only be done at
- open.
- For running databases, a separate header field can be used to indicate
- a transaction in progress; we need only check for recovery if this is set.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Deferred.
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "sub:TDB-Does-Not"
-
-\end_inset
-
-TDB Does Not Have Snapshot Support
-\end_layout
-
-\begin_layout Subsubsection
-Proposed SolutionNone.
- At some point you say
-\begin_inset Quotes eld
-\end_inset
-
-use a real database
-\begin_inset Quotes erd
-\end_inset
-
- (but see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "replay-attribute"
-
-\end_inset
-
-).
-\end_layout
-
-\begin_layout Standard
-But as a thought experiment, if we implemented transactions to only overwrite
- free entries (this is tricky: there must not be a header in each entry
- which indicates whether it is free, but use of presence in metadata elsewhere),
- and a pointer to the hash table, we could create an entirely new commit
- without destroying existing data.
- Then it would be easy to implement snapshots in a similar way.
-\end_layout
-
-\begin_layout Standard
-This would not allow arbitrary changes to the database, such as tdb_repack
- does, and would require more space (since we have to preserve the current
- and future entries at once).
- If we used hash trees rather than one big hash table, we might only have
- to rewrite some sections of the hash, too.
-\end_layout
-
-\begin_layout Standard
-We could then implement snapshots using a similar method, using multiple
- different hash tables/free tables.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Deferred.
-\end_layout
-
-\begin_layout Subsection
-Transactions Cannot Operate in Parallel
-\end_layout
-
-\begin_layout Standard
-This would be useless for ldb, as it hits the index records with just about
- every update.
- It would add significant complexity in resolving clashes, and cause the
- all transaction callers to write their code to loop in the case where the
- transactions spuriously failed.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-None (but see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "replay-attribute"
-
-\end_inset
-
-).
- We could solve a small part of the problem by providing read-only transactions.
- These would allow one write transaction to begin, but it could not commit
- until all r/o transactions are done.
- This would require a new RO_TRANSACTION_LOCK, which would be upgraded on
- commit.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Deferred.
-\end_layout
-
-\begin_layout Subsection
-Default Hash Function Is Suboptimal
-\end_layout
-
-\begin_layout Standard
-The Knuth-inspired multiplicative hash used by tdb is fairly slow (especially
- if we expand it to 64 bits), and works best when the hash bucket size is
- a prime number (which also means a slow modulus).
- In addition, it is highly predictable which could potentially lead to a
- Denial of Service attack in some TDB uses.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-The Jenkins lookup3 hash
-\begin_inset Foot
-status open
-
-\begin_layout Plain Layout
-http://burtleburtle.net/bob/c/lookup3.c
-\end_layout
-
-\end_inset
-
- is a fast and superbly-mixing hash.
- It's used by the Linux kernel and almost everything else.
- This has the particular properties that it takes an initial seed, and produces
- two 32 bit hash numbers, which we can combine into a 64-bit hash.
-\end_layout
-
-\begin_layout Standard
-The seed should be created at tdb-creation time from some random source,
- and placed in the header.
- This is far from foolproof, but adds a little bit of protection against
- hash bombing.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-\begin_inset CommandInset label
-LatexCommand label
-name "Reliable-Traversal-Adds"
-
-\end_inset
-
-Reliable Traversal Adds Complexity
-\end_layout
-
-\begin_layout Standard
-We lock a record during traversal iteration, and try to grab that lock in
- the delete code.
- If that grab on delete fails, we simply mark it deleted and continue onwards;
- traversal checks for this condition and does the delete when it moves off
- the record.
-\end_layout
-
-\begin_layout Standard
-If traversal terminates, the dead record may be left indefinitely.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-Remove reliability guarantees; see
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "traverse-Proposed-Solution"
-
-\end_inset
-
-.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Complete.
-\end_layout
-
-\begin_layout Subsection
-Fcntl Locking Adds Overhead
-\end_layout
-
-\begin_layout Standard
-Placing a fcntl lock means a system call, as does removing one.
- This is actually one reason why transactions can be faster (everything
- is locked once at transaction start).
- In the uncontended case, this overhead can theoretically be eliminated.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-None.
-\end_layout
-
-\begin_layout Standard
-We tried this before with spinlock support, in the early days of TDB, and
- it didn't make much difference except in manufactured benchmarks.
-\end_layout
-
-\begin_layout Standard
-We could use spinlocks (with futex kernel support under Linux), but it means
- that we lose automatic cleanup when a process dies with a lock.
- There is a method of auto-cleanup under Linux, but it's not supported by
- other operating systems.
- We could reintroduce a clear-if-first-style lock and sweep for dead futexes
- on open, but that wouldn't help the normal case of one concurrent opener
- dying.
- Increasingly elaborate repair schemes could be considered, but they require
- an ABI change (everyone must use them) anyway, so there's no need to do
- this at the same time as everything else.
-\end_layout
-
-\begin_layout Subsection
-Some Transactions Don't Require Durability
-\end_layout
-
-\begin_layout Standard
-Volker points out that gencache uses a CLEAR_IF_FIRST tdb for normal (fast)
- usage, and occasionally empties the results into a transactional TDB.
- This kind of usage prioritizes performance over durability: as long as
- we are consistent, data can be lost.
-\end_layout
-
-\begin_layout Standard
-This would be more neatly implemented inside tdb: a
-\begin_inset Quotes eld
-\end_inset
-
-soft
-\begin_inset Quotes erd
-\end_inset
-
- transaction commit (ie.
- syncless) which meant that data may be reverted on a crash.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\end_layout
-
-\begin_layout Standard
-None.
-\end_layout
-
-\begin_layout Standard
-Unfortunately any transaction scheme which overwrites old data requires
- a sync before that overwrite to avoid the possibility of corruption.
-\end_layout
-
-\begin_layout Standard
-It seems possible to use a scheme similar to that described in
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "sub:TDB-Does-Not"
-
-\end_inset
-
-,where transactions are committed without overwriting existing data, and
- an array of top-level pointers were available in the header.
- If the transaction is
-\begin_inset Quotes eld
-\end_inset
-
-soft
-\begin_inset Quotes erd
-\end_inset
-
- then we would not need a sync at all: existing processes would pick up
- the new hash table and free list and work with that.
-\end_layout
-
-\begin_layout Standard
-At some later point, a sync would allow recovery of the old data into the
- free lists (perhaps when the array of top-level pointers filled).
- On crash, tdb_open() would examine the array of top levels, and apply the
- transactions until it encountered an invalid checksum.
-\end_layout
-
-\begin_layout Subsection
-Tracing Is Fragile, Replay Is External
-\end_layout
-
-\begin_layout Standard
-The current TDB has compile-time-enabled tracing code, but it often breaks
- as it is not enabled by default.
- In a similar way, the ctdb code has an external wrapper which does replay
- tracing so it can coordinate cluster-wide transactions.
-\end_layout
-
-\begin_layout Subsubsection
-Proposed Solution
-\begin_inset CommandInset label
-LatexCommand label
-name "replay-attribute"
-
-\end_inset
-
-
-\end_layout
-
-\begin_layout Standard
-Tridge points out that an attribute can be later added to tdb_open (see
-
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "attributes"
-
-\end_inset
-
-) to provide replay/trace hooks, which could become the basis for this and
- future parallel transactions and snapshot support.
-\end_layout
-
-\begin_layout Subsubsection
-Status
-\end_layout
-
-\begin_layout Standard
-Deferred.
-\end_layout
-
-\end_body
-\end_document
-@
-
-
-1.12
-log
-@Add status, some fixes, linked freelists.
-@
-text
-@d53 1
-a53 7
-
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-\change_unchanged
--2010
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-d702 1
-a702 1
-\change_inserted 0 1291204665
-d704 2
-a728 2
-\change_inserted 0 1291204671
-
-a731 2
-
-\change_inserted 0 1291204671
-a735 2
-
-\change_inserted 0 1291204673
-a736 2
-\change_unchanged
-
-a780 2
-\change_inserted 0 1291204731
-
-a783 2
-
-\change_inserted 0 1291204732
-a787 2
-
-\change_inserted 0 1291204779
-a790 2
-\change_unchanged
-
-a842 2
-\change_inserted 0 1291204830
-
-a845 2
-
-\change_inserted 0 1291204831
-a849 2
-
-\change_inserted 0 1291204834
-a850 2
-\change_unchanged
-
-d879 9
-a887 2
- deal of churn; we are better to guarantee that the tdb_errcode is per-thread
- so the current programming model can be maintained.
-d891 9
-d903 2
-a922 2
-\change_inserted 0 1291204847
-
-a925 2
-
-\change_inserted 0 1291204847
-d930 5
-a934 3
-
-\change_inserted 0 1291204852
-Incomplete.
-a1051 2
-\change_inserted 0 1291204881
-
-a1054 2
-
-\change_inserted 0 1291204881
-a1058 2
-
-\change_inserted 0 1291204885
-a1059 2
-\change_unchanged
-
-a1140 2
-\change_inserted 0 1291204898
-
-a1143 2
-
-\change_inserted 0 1291204898
-a1147 2
-
-\change_inserted 0 1291204901
-a1148 2
-\change_unchanged
-
-a1224 2
-\change_inserted 0 1291204908
-
-a1227 2
-
-\change_inserted 0 1291204908
-a1231 2
-
-\change_inserted 0 1291204908
-a1232 2
-\change_unchanged
-
-a1271 2
-\change_inserted 0 1291204917
-
-a1274 2
-
-\change_inserted 0 1291204917
-a1278 2
-
-\change_inserted 0 1291204920
-a1279 2
-\change_unchanged
-
-a1316 2
-\change_inserted 0 1291204927
-
-a1319 2
-
-\change_inserted 0 1291204928
-d1325 1
-a1325 1
-\change_inserted 0 1291204942
-d1327 2
-a1381 2
-\change_inserted 0 1291205003
-
-a1384 2
-
-\change_inserted 0 1291205004
-a1388 2
-
-\change_inserted 0 1291205007
-a1411 2
-\change_inserted 0 1291205019
-
-a1414 2
-
-\change_inserted 0 1291205019
-a1418 2
-
-\change_inserted 0 1291205023
-a1419 2
-\change_unchanged
-
-a1465 2
-\change_inserted 0 1291205029
-
-a1468 2
-
-\change_inserted 0 1291205029
-a1472 2
-
-\change_inserted 0 1291206020
-a1473 2
-\change_unchanged
-
-a1528 2
-\change_inserted 0 1291205043
-
-a1531 2
-
-\change_inserted 0 1291205043
-d1537 1
-a1537 1
-\change_inserted 0 1291205057
-d1539 2
-a1589 2
-\change_inserted 0 1291205062
-
-a1592 2
-
-\change_inserted 0 1291205062
-a1596 2
-
-\change_inserted 0 1291205062
-a1597 2
-\change_unchanged
-
-a1626 2
-\change_inserted 0 1291205072
-
-a1629 2
-
-\change_inserted 0 1291205073
-a1633 2
-
-\change_inserted 0 1291205073
-a1634 2
-\change_unchanged
-
-a1674 4
-
-\change_deleted 0 1291204504
-
-\change_unchanged
-a1699 2
-\change_inserted 0 1291205079
-
-a1702 2
-
-\change_inserted 0 1291205080
-a1706 2
-
-\change_inserted 0 1291205080
-a1707 2
-\change_unchanged
-
-a1833 2
-\change_inserted 0 1291205090
-
-d1869 2
-a1870 7
- is to divide the file into zones, and using a free list (or
-\change_inserted 0 1291205498
-table
-\change_deleted 0 1291205497
-set
-\change_unchanged
- of free lists) for each.
-a1871 2
-\change_inserted 0 1291205203
-
-a1874 2
-
-\change_inserted 0 1291205358
-a1890 21
-\change_unchanged
-
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1291205198
-Note that this means we need to split the free lists when we expand the
- file; this is probably acceptable when we double the hash table size, since
- that is such an expensive operation already.
- In the case of increasing the file size, there is an optimization we can
- use: if we use M in the formula above as the file size rounded up to the
- next power of 2, we only need reshuffle free lists when the file size crosses
- a power of 2 boundary,
-\emph on
-and
-\emph default
-reshuffling the free lists is trivial: we simply merge every consecutive
- pair of free lists.
-\change_unchanged
-
-d1899 1
-a1899 7
-Identify the correct
-\change_inserted 0 1291205366
-free list
-\change_deleted 0 1291205364
-zone
-\change_unchanged
-.
-d1907 2
-a1908 7
-Re-check the
-\change_inserted 0 1291205372
-list
-\change_deleted 0 1291205371
-zone
-\change_unchanged
- (we didn't have a lock, sizes could have changed): relock if necessary.
-d1912 1
-a1912 5
-Place the freed entry in the list
-\change_deleted 0 1291205382
- for that zone
-\change_unchanged
-.
-d1921 1
-a1921 15
-Pick a
-\change_deleted 0 1291205403
-zone either the zone we last freed into, or based on a
-\begin_inset Quotes eld
-\end_inset
-
-random
-\begin_inset Quotes erd
-\end_inset
-
- number.
-\change_inserted 0 1291205411
-free table; usually the previous one.
-\change_unchanged
-
-a1925 10
-\change_deleted 0 1291205432
-
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1291205428
-Re-check the zone: relock if necessary.
-\change_unchanged
-
-d1934 1
-a1934 7
- unlock the list and try the next
-\change_inserted 0 1291205455
-largest list
-\change_deleted 0 1291205452
-zone.
-\change_inserted 0 1291205457
-
-a1937 2
-
-\change_inserted 0 1291205476
-a1938 2
-\change_unchanged
-
-a1966 2
-\change_inserted 0 1291205542
-
-a1969 2
-
-\change_inserted 0 1291205591
-a1971 70
-\change_unchanged
-
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1291205539
-I anticipate that the number of entries in each free zone would be small,
- but it might be worth using one free entry to hold pointers to the others
- for cache efficiency.
-\change_unchanged
-
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1291205534
-\begin_inset CommandInset label
-LatexCommand label
-name "freelist-in-zone"
-
-\end_inset
-
-If we want to avoid locking complexity (enlarging the free lists when we
- enlarge the file) we could place the array of free lists at the beginning
- of each zone.
- This means existing array lists never move, but means that a record cannot
- be larger than a zone.
- That in turn implies that zones should be variable sized (say, power of
- 2), which makes the question
-\begin_inset Quotes eld
-\end_inset
-
-what zone is this record in?
-\begin_inset Quotes erd
-\end_inset
-
- much harder (and
-\begin_inset Quotes eld
-\end_inset
-
-pick a random zone
-\begin_inset Quotes erd
-\end_inset
-
-, but that's less common).
- It could be done with as few as 4 bits from the record header.
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-Using
-\begin_inset Formula $2^{16+N*3}$
-\end_inset
-
-means 0 gives a minimal 65536-byte zone, 15 gives the maximal
-\begin_inset Formula $2^{61}$
-\end_inset
-
- byte zone.
- Zones range in factor of 8 steps.
- Given the zone size for the zone the current record is in, we can determine
- the start of the zone.
-\end_layout
-
-\end_inset
-
-
-\change_inserted 0 1291205139
-
-d2218 1
-a2218 5
- uint32_t
-\change_inserted 0 1291205758
-used_
-\change_unchanged
-magic : 16,
-a2222 4
-\change_deleted 0 1291205693
- prev_is_free: 1,
-\change_unchanged
-
-d2230 1
-a2230 7
- top_hash: 1
-\change_inserted 0 1291205704
-1
-\change_deleted 0 1291205704
-0
-\change_unchanged
-;
-d2254 1
-a2254 9
- uint
-\change_inserted 0 1291205725
-64
-\change_deleted 0 1291205723
-32
-\change_unchanged
-_t
-\change_inserted 0 1291205753
-free_magic: 8,
-a2257 2
-
-\change_inserted 0 1291205746
-a2262 24
-\change_deleted 0 1291205749
-free_magic;
-\change_unchanged
-
-\end_layout
-
-\begin_layout LyX-Code
- uint64_t
-\change_inserted 0 1291205786
-free_table: 8,
-\end_layout
-
-\begin_layout LyX-Code
-
-\change_inserted 0 1291205788
-
-\change_unchanged
-total_length
-\change_inserted 0 1291205792
- : 56
-\change_deleted 0 1291205790
-;
-\change_unchanged
-
-d2266 1
-a2266 7
- uint64_t
-\change_deleted 0 1291205801
-prev,
-\change_unchanged
-next;
-\change_deleted 0 1291205811
-
-d2270 1
-a2270 3
-
-\change_deleted 0 1291205811
- ...
-d2274 1
-a2274 5
-
-\change_deleted 0 1291205808
- uint64_t tailer
-\change_unchanged
-;
-d2283 5
-a2287 16
-\change_deleted 0 1291205827
-We might want to take some bits from the used record's top_hash (and the
- free record which has 32 bits of padding to spare anyway) if we use variable
- sized zones.
- See
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "freelist-in-zone"
-
-\end_inset
-
-.
-
-\change_inserted 0 1291205885
- Note that by limiting valid offsets to 56 bits, we can pack everything
- we need into 3 64-byte words, meaning our minimum record size is 8 bytes.
-a2290 2
-
-\change_inserted 0 1291205886
-a2294 2
-
-\change_inserted 0 1291205886
-a2295 2
-\change_unchanged
-
-a2385 2
-\change_inserted 0 1291205894
-
-a2388 2
-
-\change_inserted 0 1291205894
-a2392 2
-
-\change_inserted 0 1291205902
-a2393 2
-\change_unchanged
-
-a2415 4
-
-\change_deleted 0 1291204504
-
-\change_unchanged
-a2445 2
-\change_inserted 0 1291205910
-
-a2448 2
-
-\change_inserted 0 1291205910
-a2452 2
-
-\change_inserted 0 1291205914
-a2453 2
-\change_unchanged
-
-a2485 2
-\change_inserted 0 1291205919
-
-a2488 2
-
-\change_inserted 0 1291205919
-a2492 2
-
-\change_inserted 0 1291205922
-a2493 2
-\change_unchanged
-
-a2533 2
-\change_inserted 0 1291205929
-
-a2536 2
-
-\change_inserted 0 1291205929
-a2540 2
-
-\change_inserted 0 1291205929
-a2541 2
-\change_unchanged
-
-a2578 2
-\change_inserted 0 1291205932
-
-a2581 2
-
-\change_inserted 0 1291205933
-a2585 2
-
-\change_inserted 0 1291205933
-a2586 2
-\change_unchanged
-
-a2724 2
-\change_inserted 0 1291205944
-
-a2727 2
-
-\change_inserted 0 1291205945
-a2731 2
-
-\change_inserted 0 1291205948
-a2732 2
-\change_unchanged
-
-@
-
-
-1.11
-log
-@Merge changes
-@
-text
-@d53 7
-a59 1
-14-September-2010
-d587 16
-d644 18
-d716 16
-d753 16
-d813 18
-d883 16
-d953 16
-d1084 16
-d1181 16
-d1273 16
-d1328 16
-d1381 16
-d1447 19
-a1465 2
- if older code (which doesn't understand the feature) writes to the database.Reco
-rd Headers Are Not Expandible
-d1484 16
-d1546 16
-d1617 16
-d1680 16
-d1725 16
-d1810 16
-d1951 8
-a1958 3
-Proposed SolutionThe first step is to remove all the current heuristics,
- as they obviously interact, then examine them once the lock contention
- is addressed.
-d1989 7
-a1995 2
- is to divide the file into zones, and using a free list (or set of free
- lists) for each.
-d1997 2
-d2002 25
-d2039 2
-d2049 7
-a2055 1
-Identify the correct zone.
-d2063 7
-a2069 2
-Re-check the zone (we didn't have a lock, sizes could have changed): relock
- if necessary.
-d2073 5
-a2077 1
-Place the freed entry in the list for that zone.
-d2086 3
-a2088 1
-Pick a zone either the zone we last freed into, or based on a
-d2097 4
-d2105 2
-d2110 2
-d2113 2
-d2123 15
-a2137 1
- unlock the list and try the next zone.
-d2166 11
-d2180 2
-d2185 2
-d2190 2
-d2223 1
-a2223 1
-status open
-d2243 2
-d2491 5
-a2495 1
- uint32_t magic : 16,
-d2499 2
-d2502 2
-d2511 7
-a2517 1
- top_hash: 10;
-d2541 29
-a2569 1
- uint32_t free_magic;
-d2573 11
-a2583 1
- uint64_t total_length;
-d2587 7
-a2593 1
- uint64_t prev, next;
-d2597 2
-d2603 5
-a2607 1
- uint64_t tailer;
-d2615 2
-d2628 18
-d2736 16
-d2808 16
-d2856 16
-d2912 16
-d2965 16
-d3119 16
-@
-
-
-1.10
-log
-@Tracing attribute, talloc support.
-@
-text
-@d1 1
-a1 1
-#LyX 1.6.5 created this file. For more info see http://www.lyx.org/
-d53 1
-a53 7
-
-\change_deleted 0 1283307542
-26-July
-\change_inserted 0 1284423485
-14-September
-\change_unchanged
--2010
-a472 2
-\change_inserted 0 1284422789
-
-a479 2
-\change_unchanged
-
-a838 2
-
-\change_inserted 0 1284016998
-a846 2
-\change_unchanged
-
-a1194 2
-\change_inserted 0 1284015637
-
-a1197 2
-
-\change_inserted 0 1284015716
-a1201 2
-
-\change_inserted 0 1284015906
-a1210 2
-
-\change_inserted 0 1284015637
-a1214 2
-
-\change_inserted 0 1284016114
-a1227 2
-
-\change_inserted 0 1284016149
-a1232 2
-
-\change_inserted 0 1284016639
-a1237 2
-
-\change_inserted 0 1284016821
-a1243 2
-
-\change_inserted 0 1284016803
-d1245 2
-a1246 9
- if older code (which doesn't understand the feature) writes to the database.
-\change_deleted 0 1284016101
-
-\end_layout
-
-\begin_layout Subsection
-
-\change_inserted 0 1284015634
-Record Headers Are Not Expandible
-a1249 2
-
-\change_inserted 0 1284015634
-a1254 2
-
-\change_inserted 0 1284015634
-a1258 2
-
-\change_inserted 0 1284422552
-a1267 2
-
-\change_inserted 0 1284422568
-a1271 2
-
-\change_inserted 0 1284422646
-a1276 2
-
-\change_inserted 0 1284422656
-a1280 2
-
-\change_inserted 0 1284423065
-a1305 2
-
-\change_inserted 0 1284423042
-a1310 2
-\change_unchanged
-
-a1457 2
-
-\change_inserted 0 1283336713
-a1463 2
-
-\change_unchanged
-d1482 2
-d1485 1
-a1485 51
-\change_deleted 0 1283307675
-There are three details which become important:
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-On encountering a full bucket, we use the next bucket.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-Extra hash bits are stored with the offset, to reduce comparisons.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-A marker entry is used on deleting an entry.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-The doubling of the table must be done under a transaction; we will not
- reduce it on deletion, so it will be an unusual case.
- It will either be placed at the head (other entries will be moved out the
- way so we can expand).
- We could have a pointer in the header to the current hashtable location,
- but that pointer would have to be read frequently to check for hashtable
- moves.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-The locking for this is slightly more complex than the chained case; we
- currently have one lock per bucket, and that means we would need to expand
- the lock if we overflow to the next bucket.
- The frequency of such collisions will effect our locking heuristics: we
- can always lock more buckets than we need.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-One possible optimization is to only re-check the hash size on an insert
- or a lookup miss.
-
-\change_inserted 0 1283307770
-a1492 2
-
-\change_inserted 0 1283336187
-a1500 2
-
-\change_inserted 0 1283336586
-a1510 2
-\change_unchanged
-
-d1636 3
-a1638 8
-Proposed Solution
-\change_deleted 0 1283336858
-
-\end_layout
-
-\begin_layout Standard
-The first step is to remove all the current heuristics, as they obviously
- interact, then examine them once the lock contention is addressed.
-a1647 2
-\change_inserted 0 1283336910
-
-a1650 2
-
-\change_inserted 0 1283337052
-a1655 2
-\change_unchanged
-
-a1776 2
-\change_inserted 0 1283309850
-
-a1779 2
-
-\change_inserted 0 1283337216
-a1813 2
-
-\change_inserted 0 1284424151
-a1825 2
-\change_unchanged
-
-a1830 2
-\change_unchanged
-
-a2031 2
-
-\change_inserted 0 1283336739
-a2040 2
-\change_unchanged
-
-a2117 2
-\change_inserted 0 1283337133
-
-a2120 2
-
-\change_inserted 0 1283337139
-a2121 2
-\change_unchanged
-
-a2136 2
-
-\change_inserted 0 1283337235
-a2147 2
-\change_unchanged
-
-d2251 1
-a2251 7
-Proposed Solution
-\change_deleted 0 1284423472
-
-\end_layout
-
-\begin_layout Standard
-None.
-d2261 1
-a2261 1
-\change_inserted 0 1284423891
-d2263 1
-a2263 4
-\change_deleted 0 1284423891
-.
-
-\change_inserted 0 1284423901
-a2271 2
-\change_unchanged
-
-a2293 2
-\change_inserted 0 1284423495
-
-a2312 2
-
-\change_inserted 0 1284424201
-d2321 1
-a2321 3
-
-\change_unchanged
-We could solve a small part of the problem by providing read-only transactions.
-a2505 2
-\change_inserted 0 1284423555
-
-a2508 2
-
-\change_inserted 0 1284423617
-a2512 2
-
-\change_inserted 0 1284423719
-a2519 2
-
-\change_inserted 0 1284423864
-a2530 2
-
-\change_inserted 0 1284423850
-a2540 2
-\change_unchanged
-
-@
-
-
-1.9
-log
-@Extension mechanism.
-@
-text
-@d56 2
-a57 2
-\change_inserted 0 1284016854
-9-September
-d479 11
-d1303 1
-a1303 1
-\change_inserted 0 1284016847
-d1310 56
-d1945 1
-a1945 1
-\change_inserted 0 1283310945
-d1956 2
-d2402 2
-d2416 4
-d2421 12
-d2455 2
-d2476 12
-d2673 47
-@
-
-
-1.8
-log
-@Remove bogus footnote
-@
-text
-@d56 2
-a57 2
-\change_inserted 0 1283307544
-1-September
-d838 12
-d1198 103
-@
-
-
-1.7
-log
-@Moving hash table does not work.
-@
-text
-@a1436 12
-\begin_inset Foot
-status collapsed
-
-\begin_layout Plain Layout
-
-\change_inserted 0 1283336450
-If we make the hash offsets zone-relative, then this only restricts the
- zone size, not the overall database size.
-\end_layout
-
-\end_inset
-
-@
-
-
-1.6
-log
-@Commit changes
-@
-text
-@d38 1
-a38 1
-\author ""
-d53 7
-a59 1
-26-July-2010
-d1333 10
-d1361 3
-a1363 1
- There are three details which become important:
-d1367 2
-d1373 2
-d1379 2
-d1385 2
-d1397 2
-d1407 2
-d1411 45
-d1582 2
-d1598 14
-d1733 62
-d1996 13
-d2086 10
-d2110 15
-a2124 1
-\begin_layout LyX-Code
-@
-
-
-1.5
-log
-@Soft transaction commit
-@
-text
-@d38 1
-a38 1
-\author "Rusty Russell,,,"
-a52 4
-
-\change_deleted 0 1280141199
-10-May-2010
-\change_inserted 0 1280141202
-a53 2
-\change_unchanged
-
-a2028 2
-
-\change_inserted 0 1280140902
-a2034 2
-
-\change_unchanged
-a2212 2
-\change_inserted 0 1280140661
-
-a2215 2
-
-\change_inserted 0 1280140703
-a2219 2
-
-\change_inserted 0 1280708312
-a2226 2
-
-\change_inserted 0 1280708400
-a2239 2
-
-\change_inserted 0 1280140836
-a2243 2
-
-\change_inserted 0 1280708255
-a2247 2
-
-\change_inserted 0 1280708374
-a2252 2
-
-\change_inserted 0 1280141181
-a2274 2
-
-\change_inserted 0 1280141345
-@
-
-
-1.4
-log
-@Merge changes
-@
-text
-@d38 1
-a38 1
-\author ""
-d53 2
-d56 4
-d2035 10
-d2223 84
-@
-
-
-1.3
-log
-@Transaction and freelist rethink.
-@
-text
-@d38 1
-a38 1
-\author "Rusty Russell,,,"
-d53 1
-a53 1
-27-April-2010
-d662 1
-a662 5
- behavior of disallowing
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-nested
-\change_unchanged
-transactions should become the default.
-a1210 2
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-
-a1214 2
-
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-a1218 2
-\change_unchanged
-
-a1223 2
-\change_unchanged
-
-a1301 2
-
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-a1310 2
-\change_unchanged
-
-d1515 1
-a1515 11
-The free list
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-should
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-must
-\change_unchanged
- be split
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-into multiple lists
-\change_unchanged
-to reduce contention.
-a1520 2
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-
-a1523 2
-
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-a1539 2
-
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-a1551 2
-\change_unchanged
-
-a1554 8
-
-\change_deleted 0 1272942055
-There are various ways to organize these lisys, but because we want to be
- able to quickly identify which free list an entry is in, and reduce the
- number of locks required for merging, we will use zoning (eg.
- each free list covers some fixed fraction of the file).
-
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-
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-The algorithm for f
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-F
-\change_unchanged
-reeing is simple:
-d1560 1
-a1560 7
-Identify the correct
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-free list
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-zone
-\change_unchanged
-.
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-a1564 7
-Lock the
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-corresponding
-\change_unchanged
-list
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-.
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-
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-d1573 1
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-
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-, and p
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-P
-\change_unchanged
-lace the freed entry
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-at the head
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-in the list for that zone
-\change_unchanged
-.
-d1577 2
-a1578 7
-Allocation is a little more complicated, as we
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-merge entries as we walk the list:
-\change_inserted 0 1273484250
-perform delayed coalescing at this point:
-\change_unchanged
-
-d1582 1
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-Pick a
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-free list;
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-zone
-\change_unchanged
- either the
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-list
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-zone
-\change_unchanged
- we last freed
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-o
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-i
-\change_unchanged
-nto, or based on a
-d1594 1
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-Lock th
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-e corresponding
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-at
-\change_unchanged
- list.
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-
-a1597 2
-
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-a1598 53
-\change_unchanged
-
-\end_layout
-
-\begin_layout Enumerate
-If the top entry is
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-well-sized,
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--large enough,
-\change_unchanged
-remove it from the list and return it.
-\end_layout
-
-\begin_layout Enumerate
-Otherwise,
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-coalesce entries in the list.
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-examine the entry to the right of it in the file.
- If it is free:
-\end_layout
-
-\begin_deeper
-\begin_layout Enumerate
-
-\change_deleted 0 1273492200
-If that entry is in a different list, lock that list too.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1273492200
-If we had to place a new lock, re-check that the entry is free.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1273492200
-Remove that entry from its free list and expand this entry to cover it.
-\end_layout
-
-\begin_layout Enumerate
-
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-Goto step 3.
-\end_layout
-
-\end_deeper
-\begin_layout Enumerate
-
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-If there was no entry large enough, unlock the list and try the next zone.
-d1602 1
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-
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-Repeat step 3 with each entry in the list.
-\change_unchanged
-
-d1606 2
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-
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-Unlock the list and repeat step 2 with the next list.
-\change_unchanged
-
-d1611 1
-a1611 7
-If no
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-list
-\change_inserted 0 1273483671
-zone
-\change_unchanged
- satisfies, expand the file.
-d1615 2
-a1616 9
-This optimizes rapid insert/delete of free list entries
-\change_inserted 0 1273485794
- by not coalescing them all the time.
-\change_deleted 0 1273483685
-, and allows us to get rid of the tailer altogether
-\change_unchanged
-.
-
-\change_inserted 0 1273492299
-a1638 39
-
-\change_deleted 0 1273476840
-The question of
-\begin_inset Quotes eld
-\end_inset
-
-well-sized
-\begin_inset Quotes erd
-\end_inset
-
- free entries is more difficult: the 25% overhead works in practice for
- ldb because indexes tend to expand by one record at a time.
- This can be resolved by having an
-\begin_inset Quotes eld
-\end_inset
-
-expanded
-\begin_inset Quotes erd
-\end_inset
-
- bit in the header to note entries that have previously expanded, and allocating
- more space for them.
- Whether the
-\begin_inset Quotes eld
-\end_inset
-
-increasing slack
-\begin_inset Quotes erd
-\end_inset
-
- algorithm should be implemented or first-fit used is still unknown: we
- will determine this once these other ideas are implemented.
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-
-\end_layout
-
-\begin_layout Standard
-
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-
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-
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-In theory, we could get away with 2: one after we write the new data, and
- one to somehow atomically change over to it.
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-
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-
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-
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-a1989 50
-
-\change_deleted 0 1273481848
-None.
- Trying to rewrite the transaction code is a separate experiment, which
- I encourage someone else to do.
- At some point you say
-\begin_inset Quotes eld
-\end_inset
-
-use a real database
-\begin_inset Quotes erd
-\end_inset
-
-.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1273481848
-But as a thought experiment:
-\change_unchanged
-
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1273481788
-Say there was a pointer in the header which said where the hash table and
- free list tables were, and that no blocks were labeled with whether they
- were free or not (it had to be derived from what list they were in).
- We could create new hash table and free list in some free space, and populate
- it as we want the post-committed state to look.
- Then we sync, then we switch the offset in the header, then we sync again.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1273481788
-This would not allow arbitrary changes to the database, such as tdb_repack
- does, and would require more space (since we have to preserve the current
- and future entries at once).
- If we used hash trees rather than one big hash table, we might only have
- to rewrite some sections of the hash, too.
-\change_inserted 0 1273481854
-
-\end_layout
-
-\begin_layout Standard
-
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-
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-
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-
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-\change_unchanged
-
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-
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-
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-d2067 2
-a2068 11
-We could
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-then
-\change_unchanged
-implement snapshots using a similar method
-\change_deleted 0 1273481838
- to the above, only
-\change_inserted 0 1273481840
-,
-\change_unchanged
- using multiple different hash tables/free tables.
-@
-
-
-1.2
-log
-@After first feedback (Ronnie & Volker)
-@
-text
-@d1314 13
-d1531 11
-a1541 1
-The free list should be split into multiple lists to reduce contention.
-d1547 39
-d1596 7
-d1604 1
-a1604 1
-The algorithm for freeing is simple:
-d1608 7
-a1614 1
-Identify the correct free list.
-d1618 30
-a1647 1
-Lock the list, and place the freed entry at the head.
-d1651 7
-a1657 2
-Allocation is a little more complicated, as we merge entries as we walk
- the list:
-d1661 19
-a1679 1
-Pick a free list; either the list we last freed onto, or based on a
-d1691 17
-a1707 1
-Lock that list.
-d1711 7
-a1717 1
-If the top entry is well-sized, remove it from the list and return it.
-d1721 5
-a1725 1
-Otherwise, examine the entry to the right of it in the file.
-d1731 2
-d1737 2
-d1743 2
-d1749 2
-d1756 8
-d1765 2
-d1770 2
-d1773 2
-d1778 7
-a1784 1
-If no list satisfies, expand the file.
-d1788 28
-a1815 2
-This optimizes rapid insert/delete of free list entries, and allows us to
- get rid of the tailer altogether.
-d1819 2
-d1851 1
-a1851 1
-\change_inserted 0 1272941474
-d1857 303
-a2159 18
-\change_inserted 0 1272942759
-There are various ways to organize these lists, but because we want to be
- able to quickly identify which free list an entry is in, and reduce the
- number of locks required for merging, we will use zoning (eg.
- each of the N free lists in a tdb file of size M covers a fixed fraction
- M/N).
- Note that this means we need to reshuffle the free lists when we expand
- the file; this is probably acceptable when we double the hash table size,
- since that is such an expensive operation already.
- In the case of increasing the file size, there is an optimization we can
- use: if we use M in the formula above as the file size rounded up to the
- next power of 2, we only need reshuffle free lists when the file size crosses
- a power of 2 boundary,
-\emph on
-and
-\emph default
-reshuffling the free lists is trivial: we simply merge every consecutive
- pair of free lists.
-d2164 107
-d2276 2
-d2280 59
-d2346 2
-d2363 2
-d2366 2
-d2371 2
-d2382 2
-d2389 57
-d2458 13
-d2474 32
-a2505 2
-We could implement snapshots using a similar method to the above, only using
- multiple different hash tables/free tables.
-@
-
-
-1.1
-log
-@Initial revision
-@
-text
-@d1 1
-a1 1
-#LyX 1.6.4 created this file. For more info see http://www.lyx.org/
-d36 3
-a38 3
-\tracking_changes false
-\output_changes false
-\author ""
-d662 5
-a666 1
- behavior of disallowing transactions should become the default.
-d1215 21
-d1527 2
-d1533 3
-a1535 1
- The algorithm for freeing is simple:
-d1642 26
-@
projects / ccan / blobdiff