[ccan] / ccan / tdb2 / doc / design.lyx

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\author "Rusty Russell,,," 
\author "" 
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\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

\change_deleted 0 1283307542
26-July
\change_inserted 0 1283307544
1-September
\change_unchanged
-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
\end_layout

\end_inset
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\begin_layout Plain Layout
API Functions
\end_layout

\end_inset
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\begin_inset Text

\begin_layout Plain Layout
Lines of C Code Implementation
\end_layout

\end_inset
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<row>
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\begin_inset Text

\begin_layout Plain Layout
1999
\end_layout

\end_inset
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\begin_inset Text

\begin_layout Plain Layout
13
\end_layout

\end_inset
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\begin_layout Plain Layout
1195
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2000
\end_layout

\end_inset
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\begin_inset Text

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24
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1725
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2001
\end_layout

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\begin_inset Text

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32
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2228
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2002
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35
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2003
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35
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2004
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40
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2005
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38
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2006
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52
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2007
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66
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2008
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71
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2009
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5715
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</cell>
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</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
\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 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 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 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 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 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 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; 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
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).
\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.
\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 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 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 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 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 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 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 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 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

\change_inserted 0 1283336713
\begin_inset CommandInset label
LatexCommand label
name "sub:Hash-Size-Solution"

\end_inset


\change_unchanged
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.
 
\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
 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

\change_inserted 0 1283336187
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

\change_inserted 0 1283336586
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.
\change_unchanged

\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
\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.
\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.
\change_inserted 0 1283336910

\end_layout

\begin_layout Standard

\change_inserted 0 1283337052
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.
\change_unchanged

\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 set of free
 lists) for each.
 This approximates address ordering.
\end_layout

\begin_layout Standard
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.
\end_layout

\begin_layout Standard
The basic algorithm is as follows.
 Freeing is simple:
\end_layout

\begin_layout Enumerate
Identify the correct zone.
\end_layout

\begin_layout Enumerate
Lock the corresponding list.
\end_layout

\begin_layout Enumerate
Re-check the zone (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 for that zone.
\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 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.
\end_layout

\begin_layout Enumerate
Lock the corresponding list.
\end_layout

\begin_layout Enumerate
Re-check the zone: relock if necessary.
\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 zone.
\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
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_inserted 0 1283309850

\end_layout

\begin_layout Standard

\change_inserted 0 1283337216
\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 open

\begin_layout Plain Layout

\change_inserted 0 1283310945
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.
\change_unchanged

\end_layout

\end_inset


\change_unchanged

\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.

\change_inserted 0 1283336739
 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

).
\change_unchanged

\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 magic : 16,
\end_layout

\begin_layout LyX-Code
                 prev_is_free: 1,
\end_layout

\begin_layout LyX-Code
                 key_data_divide: 5,
\end_layout

\begin_layout LyX-Code
                 top_hash: 10;
\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
        uint32_t free_magic;
\end_layout

\begin_layout LyX-Code
        uint64_t total_length;
\change_inserted 0 1283337133

\end_layout

\begin_layout LyX-Code

\change_inserted 0 1283337139
        uint64_t prev, next;
\change_unchanged

\end_layout

\begin_layout LyX-Code
        ...
\end_layout

\begin_layout LyX-Code
        uint64_t tailer;
\end_layout

\begin_layout LyX-Code
};
\end_layout

\begin_layout Standard

\change_inserted 0 1283337235
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_unchanged

\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 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 Solution
\end_layout

\begin_layout Standard
None.
 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
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 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
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 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 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 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

\end_body
\end_document