head 1.6; access; symbols; locks; strict; comment @# @; 1.6 date 2010.08.02.00.21.43; author rusty; state Exp; branches; next 1.5; 1.5 date 2010.08.02.00.21.16; author rusty; state Exp; branches; next 1.4; 1.4 date 2010.05.10.13.09.11; author rusty; state Exp; branches; next 1.3; 1.3 date 2010.05.10.11.58.37; author rusty; state Exp; branches; next 1.2; 1.2 date 2010.05.10.05.35.13; author rusty; state Exp; branches; next 1.1; 1.1 date 2010.05.04.02.29.16; author rusty; state Exp; branches; next ; desc @First draft @ 1.6 log @Commit changes @ text @#LyX 1.6.5 created this file. For more info see http://www.lyx.org/ \lyxformat 345 \begin_document \begin_header \textclass article \use_default_options true \language english \inputencoding auto \font_roman default \font_sans default \font_typewriter default \font_default_family default \font_sc false \font_osf false \font_sf_scale 100 \font_tt_scale 100 \graphics default \paperfontsize default \use_hyperref false \papersize default \use_geometry false \use_amsmath 1 \use_esint 1 \cite_engine basic \use_bibtopic false \paperorientation portrait \secnumdepth 3 \tocdepth 3 \paragraph_separation indent \defskip medskip \quotes_language english \papercolumns 1 \papersides 1 \paperpagestyle default \tracking_changes true \output_changes true \author "" \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 26-July-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 \begin_inset Text \begin_layout Plain Layout Year End \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout API Functions \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout Lines of C Code Implementation \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 1999 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 13 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 1195 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2000 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 24 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 1725 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2001 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 32 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2228 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2002 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 35 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2481 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2003 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 35 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2552 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2004 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 40 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2584 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2005 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 38 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2647 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2006 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 52 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 3754 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2007 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 66 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 4398 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2008 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 71 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 4768 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 2009 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 73 \end_layout \end_inset \begin_inset Text \begin_layout Plain Layout 5715 \end_layout \end_inset \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 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. There are three details which become important: \end_layout \begin_layout Enumerate On encountering a full bucket, we use the next bucket. \end_layout \begin_layout Enumerate Extra hash bits are stored with the offset, to reduce comparisons. \end_layout \begin_layout Enumerate A marker entry is used on deleting an entry. \end_layout \begin_layout Standard 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 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 One possible optimization is to only re-check the hash size on an insert or a lookup miss. \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 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. \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. \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; \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 LyX-Code \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 @ 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 \change_inserted 0 1272940179 nested \change_unchanged transactions should become the default. a1210 2 \change_inserted 0 1272944650 a1214 2 \change_inserted 0 1272944763 a1218 2 \change_unchanged a1223 2 \change_unchanged a1301 2 \change_inserted 0 1273478114 a1310 2 \change_unchanged d1515 1 a1515 11 The free list \change_deleted 0 1273469807 should \change_inserted 0 1273469810 must \change_unchanged be split \change_deleted 0 1273469815 into multiple lists \change_unchanged to reduce contention. a1520 2 \change_inserted 0 1273470006 a1523 2 \change_inserted 0 1273492055 a1539 2 \change_inserted 0 1273483888 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). \change_inserted 0 1273484187 d1556 1 a1556 7 \change_deleted 0 1273484194 The algorithm for f \change_inserted 0 1273484194 F \change_unchanged reeing is simple: d1560 1 a1560 7 Identify the correct \change_deleted 0 1273482856 free list \change_inserted 0 1273482857 zone \change_unchanged . d1564 1 a1564 7 Lock the \change_inserted 0 1273482895 corresponding \change_unchanged list \change_inserted 0 1273482863 . a1567 2 \change_inserted 0 1273482909 d1573 1 a1573 13 \change_deleted 0 1273482885 , and p \change_inserted 0 1273482888 P \change_unchanged lace the freed entry \change_deleted 0 1273492415 at the head \change_inserted 0 1273492415 in the list for that zone \change_unchanged . d1577 2 a1578 7 Allocation is a little more complicated, as we \change_deleted 0 1273483240 merge entries as we walk the list: \change_inserted 0 1273484250 perform delayed coalescing at this point: \change_unchanged d1582 1 a1582 19 Pick a \change_deleted 0 1273482955 free list; \change_inserted 0 1273482957 zone \change_unchanged either the \change_deleted 0 1273482962 list \change_inserted 0 1273482962 zone \change_unchanged we last freed \change_deleted 0 1273482966 o \change_inserted 0 1273482966 i \change_unchanged nto, or based on a d1594 1 a1594 9 Lock th \change_inserted 0 1273482980 e corresponding \change_deleted 0 1273482973 at \change_unchanged list. \change_inserted 0 1273482982 a1597 2 \change_inserted 0 1273483084 a1598 53 \change_unchanged \end_layout \begin_layout Enumerate If the top entry is \change_deleted 0 1273492155 well-sized, \change_inserted 0 1273492159 -large enough, \change_unchanged remove it from the list and return it. \end_layout \begin_layout Enumerate Otherwise, \change_inserted 0 1273492206 coalesce entries in the list. \change_deleted 0 1273492200 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 \change_deleted 0 1273485554 Goto step 3. \end_layout \end_deeper \begin_layout Enumerate \change_inserted 0 1273485311 If there was no entry large enough, unlock the list and try the next zone. d1602 1 a1602 5 \change_deleted 0 1273483646 Repeat step 3 with each entry in the list. \change_unchanged d1606 2 a1607 5 \change_deleted 0 1273483668 Unlock the list and repeat step 2 with the next list. \change_unchanged d1611 1 a1611 7 If no \change_deleted 0 1273483671 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. \change_inserted 0 1273483750 \end_layout \begin_layout Standard \change_inserted 0 1273492450 a1644 2 \change_inserted 0 1273470441 a1654 2 \change_inserted 0 1273476556 a1659 2 \change_inserted 0 1273470423 a1661 2 \change_unchanged a1672 2 \change_inserted 0 1273476847 a1676 2 \change_inserted 0 1273476886 a1691 2 \change_inserted 0 1273477233 a1699 2 \change_inserted 0 1273477534 a1706 2 \change_inserted 0 1273482700 a1712 2 \change_inserted 0 1273478079 a1722 2 \change_inserted 0 1273477839 a1726 2 \change_inserted 0 1273477925 a1730 2 \change_inserted 0 1273477925 a1734 2 \change_inserted 0 1273477925 a1738 2 \change_inserted 0 1273477925 a1742 2 \change_inserted 0 1273477925 a1746 2 \change_inserted 0 1273477925 a1750 2 \change_inserted 0 1273477925 a1754 2 \change_inserted 0 1273477925 a1758 2 \change_inserted 0 1273477925 a1762 2 \change_inserted 0 1273477925 a1766 2 \change_inserted 0 1273477925 a1770 2 \change_inserted 0 1273477925 a1774 2 \change_inserted 0 1273477925 a1778 2 \change_inserted 0 1273477925 a1782 2 \change_inserted 0 1273477925 a1786 2 \change_inserted 0 1273477925 a1790 2 \change_inserted 0 1273477925 a1794 2 \change_inserted 0 1273477925 a1798 2 \change_inserted 0 1273492522 a1802 2 \change_inserted 0 1273492530 a1806 2 \change_inserted 0 1273492546 a1810 2 \change_inserted 0 1273478239 a1814 2 \change_inserted 0 1273479960 a1821 2 \change_inserted 0 1273480265 a1830 2 \change_inserted 0 1273480354 a1845 2 \change_inserted 0 1273478968 a1851 2 \change_inserted 0 1273492604 a1859 2 \change_inserted 0 1273479572 a1862 2 \change_unchanged a1870 2 \change_inserted 0 1273480282 a1874 2 \change_inserted 0 1273478931 a1878 2 \change_inserted 0 1273481549 a1882 2 \change_inserted 0 1273481557 a1886 2 \change_inserted 0 1273480307 a1890 2 \change_inserted 0 1273480335 a1894 2 \change_inserted 0 1273479897 a1898 2 \change_inserted 0 1273479653 a1902 2 \change_inserted 0 1273480371 a1906 2 \change_inserted 0 1273480464 a1910 2 \change_inserted 0 1273480399 a1914 2 \change_inserted 0 1273480425 a1918 2 \change_inserted 0 1273480453 a1922 2 \change_inserted 0 1273480455 a1926 2 \change_inserted 0 1273480450 a1930 2 \change_inserted 0 1273480452 a1935 2 \change_inserted 0 1273478830 a1942 5 \change_deleted 0 1273481604 In theory, we could get away with 2: one after we write the new data, and one to somehow atomically change over to it. \change_inserted 0 1273481632 a1946 2 \change_inserted 0 1273481724 a1950 2 \change_inserted 0 1273481713 a1954 2 \change_inserted 0 1273481717 a1958 2 \change_inserted 0 1273481730 a1962 2 \change_inserted 0 1273481736 a1966 2 \change_inserted 0 1273481744 a1970 2 \change_inserted 0 1273481748 a1974 2 \change_inserted 0 1273482185 a1978 2 \change_inserted 0 1273482259 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 \change_inserted 0 1273482102 a1993 2 \change_inserted 0 1273482061 a1998 2 \change_inserted 0 1273482063 a2002 2 \change_inserted 0 1273482072 a2006 2 \change_inserted 0 1273482139 a2011 2 \change_inserted 0 1273482364 a2015 2 \change_inserted 0 1273482163 a2019 2 \change_inserted 0 1273482493 a2037 2 \change_inserted 0 1273482536 a2046 2 \change_unchanged a2049 2 \change_inserted 0 1273482641 a2058 2 \change_inserted 0 1273481827 d2067 2 a2068 11 We could \change_inserted 0 1273481829 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 @