1 #LyX 1.6.7 created this file. For more info see http://www.lyx.org/
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38 \author "Rusty Russell,,,"
45 TDB2: A Redesigning The Trivial DataBase
49 Rusty Russell, IBM Corporation
56 \begin_layout Abstract
57 The Trivial DataBase on-disk format is 32 bits; with usage cases heading
58 towards the 4G limit, that must change.
59 This required breakage provides an opportunity to revisit TDB's other design
60 decisions and reassess them.
67 \begin_layout Standard
68 The Trivial DataBase was originally written by Andrew Tridgell as a simple
69 key/data pair storage system with the same API as dbm, but allowing multiple
70 readers and writers while being small enough (< 1000 lines of C) to include
72 The simple design created in 1999 has proven surprisingly robust and performant
73 , used in Samba versions 3 and 4 as well as numerous other projects.
74 Its useful life was greatly increased by the (backwards-compatible!) addition
75 of transaction support in 2005.
78 \begin_layout Standard
79 The wider variety and greater demands of TDB-using code has lead to some
80 organic growth of the API, as well as some compromises on the implementation.
81 None of these, by themselves, are seen as show-stoppers, but the cumulative
82 effect is to a loss of elegance over the initial, simple TDB implementation.
83 Here is a table of the approximate number of lines of implementation code
84 and number of API functions at the end of each year:
87 \begin_layout Standard
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117 Lines of C Code Implementation
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435 \begin_layout Plain Layout
449 \begin_layout Standard
450 This review is an attempt to catalog and address all the known issues with
451 TDB and create solutions which address the problems without significantly
452 increasing complexity; all involved are far too aware of the dangers of
453 second system syndrome in rewriting a successful project like this.
456 \begin_layout Section
460 \begin_layout Subsection
461 tdb_open_ex Is Not Expandable
464 \begin_layout Standard
465 The tdb_open() call was expanded to tdb_open_ex(), which added an optional
466 hashing function and an optional logging function argument.
467 Additional arguments to open would require the introduction of a tdb_open_ex2
471 \begin_layout Subsubsection
473 \begin_inset CommandInset label
482 \begin_layout Standard
483 tdb_open() will take a linked-list of attributes:
486 \begin_layout LyX-Code
490 \begin_layout LyX-Code
491 TDB_ATTRIBUTE_LOG = 0,
494 \begin_layout LyX-Code
495 TDB_ATTRIBUTE_HASH = 1
498 \begin_layout LyX-Code
502 \begin_layout LyX-Code
503 struct tdb_attribute_base {
506 \begin_layout LyX-Code
507 enum tdb_attribute attr;
510 \begin_layout LyX-Code
511 union tdb_attribute *next;
514 \begin_layout LyX-Code
518 \begin_layout LyX-Code
519 struct tdb_attribute_log {
522 \begin_layout LyX-Code
523 struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_LOG */
526 \begin_layout LyX-Code
530 \begin_layout LyX-Code
534 \begin_layout LyX-Code
538 \begin_layout LyX-Code
539 struct tdb_attribute_hash {
542 \begin_layout LyX-Code
543 struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_HASH */
546 \begin_layout LyX-Code
547 tdb_hash_func hash_fn;
550 \begin_layout LyX-Code
554 \begin_layout LyX-Code
558 \begin_layout LyX-Code
559 union tdb_attribute {
562 \begin_layout LyX-Code
563 struct tdb_attribute_base base;
566 \begin_layout LyX-Code
567 struct tdb_attribute_log log;
570 \begin_layout LyX-Code
571 struct tdb_attribute_hash hash;
574 \begin_layout LyX-Code
578 \begin_layout Standard
579 This allows future attributes to be added, even if this expands the size
583 \begin_layout Subsubsection
587 \begin_layout Standard
591 \begin_layout Subsection
592 tdb_traverse Makes Impossible Guarantees
595 \begin_layout Standard
596 tdb_traverse (and tdb_firstkey/tdb_nextkey) predate transactions, and it
597 was thought that it was important to guarantee that all records which exist
598 at the start and end of the traversal would be included, and no record
599 would be included twice.
602 \begin_layout Standard
603 This adds complexity (see
604 \begin_inset CommandInset ref
606 reference "Reliable-Traversal-Adds"
610 ) and does not work anyway for records which are altered (in particular,
611 those which are expanded may be effectively deleted and re-added behind
615 \begin_layout Subsubsection
616 \begin_inset CommandInset label
618 name "traverse-Proposed-Solution"
625 \begin_layout Standard
626 Abandon the guarantee.
627 You will see every record if no changes occur during your traversal, otherwise
628 you will see some subset.
629 You can prevent changes by using a transaction or the locking API.
632 \begin_layout Subsubsection
636 \begin_layout Standard
638 Delete-during-traverse will still delete every record, too (assuming no
642 \begin_layout Subsection
643 Nesting of Transactions Is Fraught
646 \begin_layout Standard
647 TDB has alternated between allowing nested transactions and not allowing
649 Various paths in the Samba codebase assume that transactions will nest,
650 and in a sense they can: the operation is only committed to disk when the
651 outer transaction is committed.
652 There are two problems, however:
655 \begin_layout Enumerate
656 Canceling the inner transaction will cause the outer transaction commit
657 to fail, and will not undo any operations since the inner transaction began.
658 This problem is soluble with some additional internal code.
661 \begin_layout Enumerate
662 An inner transaction commit can be cancelled by the outer transaction.
663 This is desirable in the way which Samba's database initialization code
664 uses transactions, but could be a surprise to any users expecting a successful
665 transaction commit to expose changes to others.
668 \begin_layout Standard
669 The current solution is to specify the behavior at tdb_open(), with the
670 default currently that nested transactions are allowed.
671 This flag can also be changed at runtime.
674 \begin_layout Subsubsection
678 \begin_layout Standard
679 Given the usage patterns, it seems that the
680 \begin_inset Quotes eld
684 \begin_inset Quotes erd
687 behavior of disallowing nested transactions should become the default.
688 Additionally, it seems the outer transaction is the only code which knows
689 whether inner transactions should be allowed, so a flag to indicate this
690 could be added to tdb_transaction_start.
691 However, this behavior can be simulated with a wrapper which uses tdb_add_flags
692 () and tdb_remove_flags(), so the API should not be expanded for this relatively
696 \begin_layout Subsubsection
700 \begin_layout Standard
702 \change_deleted 0 1298979572
703 Incomplete; nesting flag is still defined as per tdb1.
704 \change_inserted 0 1298979584
705 Complete; the nesting flag has been removed.
710 \begin_layout Subsection
711 Incorrect Hash Function is Not Detected
714 \begin_layout Standard
715 tdb_open_ex() allows the calling code to specify a different hash function
716 to use, but does not check that all other processes accessing this tdb
717 are using the same hash function.
718 The result is that records are missing from tdb_fetch().
721 \begin_layout Subsubsection
725 \begin_layout Standard
726 The header should contain an example hash result (eg.
727 the hash of 0xdeadbeef), and tdb_open_ex() should check that the given
728 hash function produces the same answer, or fail the tdb_open call.
731 \begin_layout Subsubsection
735 \begin_layout Standard
739 \begin_layout Subsection
740 tdb_set_max_dead/TDB_VOLATILE Expose Implementation
743 \begin_layout Standard
744 In response to scalability issues with the free list (
745 \begin_inset CommandInset ref
747 reference "TDB-Freelist-Is"
751 ) two API workarounds have been incorporated in TDB: tdb_set_max_dead()
752 and the TDB_VOLATILE flag to tdb_open.
753 The latter actually calls the former with an argument of
754 \begin_inset Quotes eld
758 \begin_inset Quotes erd
764 \begin_layout Standard
765 This code allows deleted records to accumulate without putting them in the
767 On delete we iterate through each chain and free them in a batch if there
768 are more than max_dead entries.
769 These are never otherwise recycled except as a side-effect of a tdb_repack.
772 \begin_layout Subsubsection
776 \begin_layout Standard
777 With the scalability problems of the freelist solved, this API can be removed.
778 The TDB_VOLATILE flag may still be useful as a hint that store and delete
779 of records will be at least as common as fetch in order to allow some internal
780 tuning, but initially will become a no-op.
783 \begin_layout Subsubsection
787 \begin_layout Standard
789 \change_deleted 0 1300360559
791 TDB_VOLATILE still defined, but implementation should fail on unknown flags
793 \change_inserted 0 1300360588
795 Unknown flags cause tdb_open() to fail as well, so they can be detected
799 \begin_layout Subsection
800 \begin_inset CommandInset label
802 name "TDB-Files-Cannot"
806 TDB Files Cannot Be Opened Multiple Times In The Same Process
809 \begin_layout Standard
810 No process can open the same TDB twice; we check and disallow it.
811 This is an unfortunate side-effect of fcntl locks, which operate on a per-file
812 rather than per-file-descriptor basis, and do not nest.
813 Thus, closing any file descriptor on a file clears all the locks obtained
814 by this process, even if they were placed using a different file descriptor!
817 \begin_layout Standard
818 Note that even if this were solved, deadlock could occur if operations were
819 nested: this is a more manageable programming error in most cases.
822 \begin_layout Subsubsection
826 \begin_layout Standard
827 We could lobby POSIX to fix the perverse rules, or at least lobby Linux
828 to violate them so that the most common implementation does not have this
830 This would be a generally good idea for other fcntl lock users.
833 \begin_layout Standard
834 Samba uses a wrapper which hands out the same tdb_context to multiple callers
835 if this happens, and does simple reference counting.
836 We should do this inside the tdb library, which already emulates lock nesting
837 internally; it would need to recognize when deadlock occurs within a single
839 This would create a new failure mode for tdb operations (while we currently
840 handle locking failures, they are impossible in normal use and a process
841 encountering them can do little but give up).
844 \begin_layout Standard
845 I do not see benefit in an additional tdb_open flag to indicate whether
846 re-opening is allowed, as though there may be some benefit to adding a
847 call to detect when a tdb_context is shared, to allow other to create such
851 \begin_layout Subsubsection
855 \begin_layout Standard
859 \begin_layout Subsection
860 TDB API Is Not POSIX Thread-safe
863 \begin_layout Standard
864 The TDB API uses an error code which can be queried after an operation to
865 determine what went wrong.
866 This programming model does not work with threads, unless specific additional
867 guarantees are given by the implementation.
868 In addition, even otherwise-independent threads cannot open the same TDB
870 \begin_inset CommandInset ref
872 reference "TDB-Files-Cannot"
879 \begin_layout Subsubsection
883 \begin_layout Standard
884 Reachitecting the API to include a tdb_errcode pointer would be a great
886 \change_inserted 0 1298979557
887 , but fortunately most functions return 0 on success and -1 on error: we
888 can change these to return 0 on success and a negative error code on error,
889 and the API remains similar to previous.
890 The tdb_fetch, tdb_firstkey and tdb_nextkey functions need to take a TDB_DATA
891 pointer and return an error code.
892 It is also simpler to have tdb_nextkey replace its key argument in place,
893 freeing up any old .dptr.
896 \begin_layout Standard
898 \change_deleted 0 1298979438
899 ; we are better to guarantee that the tdb_errcode is per-thread so the current
900 programming model can be maintained.
903 \begin_layout Standard
905 \change_deleted 0 1298979438
906 This requires dynamic per-thread allocations, which is awkward with POSIX
907 threads (pthread_key_create space is limited and we cannot simply allocate
908 a key for every TDB).
913 \begin_layout Standard
914 Internal locking is required to make sure that fcntl locks do not overlap
915 between threads, and also that the global list of tdbs is maintained.
918 \begin_layout Standard
919 The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe
920 version of the library, and otherwise no overhead will exist.
921 Alternatively, a hooking mechanism similar to that proposed for
922 \begin_inset CommandInset ref
924 reference "Proposed-Solution-locking-hook"
928 could be used to enable pthread locking at runtime.
931 \begin_layout Subsubsection
935 \begin_layout Standard
937 \change_inserted 0 1298979681
938 ; API has been changed but thread safety has not been implemented.
939 \change_deleted 0 1298979669
945 \begin_layout Subsection
946 *_nonblock Functions And *_mark Functions Expose Implementation
949 \begin_layout Standard
954 \begin_layout Plain Layout
955 Clustered TDB, see http://ctdb.samba.org
960 wishes to operate on TDB in a non-blocking manner.
961 This is currently done as follows:
964 \begin_layout Enumerate
965 Call the _nonblock variant of an API function (eg.
966 tdb_lockall_nonblock).
970 \begin_layout Enumerate
971 Fork a child process, and wait for it to call the normal variant (eg.
975 \begin_layout Enumerate
976 If the child succeeds, call the _mark variant to indicate we already have
981 \begin_layout Enumerate
982 Upon completion, tell the child to release the locks (eg.
986 \begin_layout Enumerate
987 Indicate to tdb that it should consider the locks removed (eg.
991 \begin_layout Standard
992 There are several issues with this approach.
993 Firstly, adding two new variants of each function clutters the API for
994 an obscure use, and so not all functions have three variants.
995 Secondly, it assumes that all paths of the functions ask for the same locks,
996 otherwise the parent process will have to get a lock which the child doesn't
997 have under some circumstances.
998 I don't believe this is currently the case, but it constrains the implementatio
1003 \begin_layout Subsubsection
1004 \begin_inset CommandInset label
1006 name "Proposed-Solution-locking-hook"
1013 \begin_layout Standard
1014 Implement a hook for locking methods, so that the caller can control the
1015 calls to create and remove fcntl locks.
1016 In this scenario, ctdbd would operate as follows:
1019 \begin_layout Enumerate
1020 Call the normal API function, eg tdb_lockall().
1023 \begin_layout Enumerate
1024 When the lock callback comes in, check if the child has the lock.
1025 Initially, this is always false.
1027 Otherwise, try to obtain it in non-blocking mode.
1028 If that fails, return EWOULDBLOCK.
1031 \begin_layout Enumerate
1032 Release locks in the unlock callback as normal.
1035 \begin_layout Enumerate
1036 If tdb_lockall() fails, see if we recorded a lock failure; if so, call the
1037 child to repeat the operation.
1040 \begin_layout Enumerate
1041 The child records what locks it obtains, and returns that information to
1045 \begin_layout Enumerate
1046 When the child has succeeded, goto 1.
1049 \begin_layout Standard
1050 This is flexible enough to handle any potential locking scenario, even when
1051 lock requirements change.
1052 It can be optimized so that the parent does not release locks, just tells
1053 the child which locks it doesn't need to obtain.
1056 \begin_layout Standard
1057 It also keeps the complexity out of the API, and in ctdbd where it is needed.
1060 \begin_layout Subsubsection
1064 \begin_layout Standard
1068 \begin_layout Subsection
1069 tdb_chainlock Functions Expose Implementation
1072 \begin_layout Standard
1073 tdb_chainlock locks some number of records, including the record indicated
1075 This gave atomicity guarantees; no-one can start a transaction, alter,
1076 read or delete that key while the lock is held.
1079 \begin_layout Standard
1080 It also makes the same guarantee for any other key in the chain, which is
1081 an internal implementation detail and potentially a cause for deadlock.
1084 \begin_layout Subsubsection
1088 \begin_layout Standard
1090 It would be nice to have an explicit single entry lock which effected no
1092 Unfortunately, this won't work for an entry which doesn't exist.
1093 Thus while chainlock may be implemented more efficiently for the existing
1094 case, it will still have overlap issues with the non-existing case.
1095 So it is best to keep the current (lack of) guarantee about which records
1096 will be effected to avoid constraining our implementation.
1099 \begin_layout Subsection
1100 Signal Handling is Not Race-Free
1103 \begin_layout Standard
1104 The tdb_setalarm_sigptr() call allows the caller's signal handler to indicate
1105 that the tdb locking code should return with a failure, rather than trying
1106 again when a signal is received (and errno == EAGAIN).
1107 This is usually used to implement timeouts.
1110 \begin_layout Standard
1111 Unfortunately, this does not work in the case where the signal is received
1112 before the tdb code enters the fcntl() call to place the lock: the code
1113 will sleep within the fcntl() code, unaware that the signal wants it to
1115 In the case of long timeouts, this does not happen in practice.
1118 \begin_layout Subsubsection
1122 \begin_layout Standard
1123 The locking hooks proposed in
1124 \begin_inset CommandInset ref
1126 reference "Proposed-Solution-locking-hook"
1130 would allow the user to decide on whether to fail the lock acquisition
1132 This allows the caller to choose their own compromise: they could narrow
1133 the race by checking immediately before the fcntl call.
1137 \begin_layout Plain Layout
1138 It may be possible to make this race-free in some implementations by having
1139 the signal handler alter the struct flock to make it invalid.
1140 This will cause the fcntl() lock call to fail with EINVAL if the signal
1141 occurs before the kernel is entered, otherwise EAGAIN.
1149 \begin_layout Subsubsection
1153 \begin_layout Standard
1157 \begin_layout Subsection
1158 The API Uses Gratuitous Typedefs, Capitals
1161 \begin_layout Standard
1162 typedefs are useful for providing source compatibility when types can differ
1163 across implementations, or arguably in the case of function pointer definitions
1164 which are hard for humans to parse.
1165 Otherwise it is simply obfuscation and pollutes the namespace.
1168 \begin_layout Standard
1169 Capitalization is usually reserved for compile-time constants and macros.
1172 \begin_layout Description
1173 TDB_CONTEXT There is no reason to use this over 'struct tdb_context'; the
1174 definition isn't visible to the API user anyway.
1177 \begin_layout Description
1178 TDB_DATA There is no reason to use this over struct TDB_DATA; the struct
1179 needs to be understood by the API user.
1182 \begin_layout Description
1184 \begin_inset space ~
1187 TDB_DATA This would normally be called 'struct tdb_data'.
1190 \begin_layout Description
1192 \begin_inset space ~
1195 TDB_ERROR Similarly, this would normally be enum tdb_error.
1198 \begin_layout Subsubsection
1202 \begin_layout Standard
1204 Introducing lower case variants would please pedants like myself, but if
1205 it were done the existing ones should be kept.
1206 There is little point forcing a purely cosmetic change upon tdb users.
1209 \begin_layout Subsection
1210 \begin_inset CommandInset label
1212 name "tdb_log_func-Doesnt-Take"
1216 tdb_log_func Doesn't Take The Private Pointer
1219 \begin_layout Standard
1220 For API compatibility reasons, the logging function needs to call tdb_get_loggin
1221 g_private() to retrieve the pointer registered by the tdb_open_ex for logging.
1224 \begin_layout Subsubsection
1228 \begin_layout Standard
1229 It should simply take an extra argument, since we are prepared to break
1233 \begin_layout Subsubsection
1237 \begin_layout Standard
1241 \begin_layout Subsection
1242 Various Callback Functions Are Not Typesafe
1245 \begin_layout Standard
1246 The callback functions in tdb_set_logging_function (after
1247 \begin_inset CommandInset ref
1249 reference "tdb_log_func-Doesnt-Take"
1253 is resolved), tdb_parse_record, tdb_traverse, tdb_traverse_read and tdb_check
1254 all take void * and must internally convert it to the argument type they
1258 \begin_layout Standard
1259 If this type changes, the compiler will not produce warnings on the callers,
1260 since it only sees void *.
1263 \begin_layout Subsubsection
1267 \begin_layout Standard
1268 With careful use of macros, we can create callback functions which give
1269 a warning when used on gcc and the types of the callback and its private
1271 Unsupported compilers will not give a warning, which is no worse than now.
1272 In addition, the callbacks become clearer, as they need not use void *
1273 for their parameter.
1276 \begin_layout Standard
1277 See CCAN's typesafe_cb module at http://ccan.ozlabs.org/info/typesafe_cb.html
1280 \begin_layout Subsubsection
1284 \begin_layout Standard
1288 \begin_layout Subsection
1289 TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, tdb_reopen_all Problematic
1292 \begin_layout Standard
1293 The TDB_CLEAR_IF_FIRST flag to tdb_open indicates that the TDB file should
1294 be cleared if the caller discovers it is the only process with the TDB
1296 However, if any caller does not specify TDB_CLEAR_IF_FIRST it will not
1297 be detected, so will have the TDB erased underneath them (usually resulting
1301 \begin_layout Standard
1302 There is a similar issue on fork(); if the parent exits (or otherwise closes
1303 the tdb) before the child calls tdb_reopen_all() to establish the lock
1304 used to indicate the TDB is opened by someone, a TDB_CLEAR_IF_FIRST opener
1305 at that moment will believe it alone has opened the TDB and will erase
1309 \begin_layout Subsubsection
1313 \begin_layout Standard
1314 Remove TDB_CLEAR_IF_FIRST.
1315 Other workarounds are possible, but see
1316 \begin_inset CommandInset ref
1318 reference "TDB_CLEAR_IF_FIRST-Imposes-Performance"
1325 \begin_layout Subsubsection
1329 \begin_layout Standard
1331 \change_deleted 0 1298979699
1332 Incomplete, TDB_CLEAR_IF_FIRST still defined, but not implemented.
1333 \change_inserted 0 1298979700
1339 \begin_layout Subsection
1340 Extending The Header Is Difficult
1343 \begin_layout Standard
1344 We have reserved (zeroed) words in the TDB header, which can be used for
1346 If the future features are compulsory, the version number must be updated
1347 to prevent old code from accessing the database.
1348 But if the future feature is optional, we have no way of telling if older
1349 code is accessing the database or not.
1352 \begin_layout Subsubsection
1356 \begin_layout Standard
1357 The header should contain a
1358 \begin_inset Quotes eld
1362 \begin_inset Quotes erd
1366 This is divided into two 32-bit parts:
1369 \begin_layout Enumerate
1370 The lower part reflects the format variant understood by code accessing
1374 \begin_layout Enumerate
1375 The upper part reflects the format variant you must understand to write
1376 to the database (otherwise you can only open for reading).
1379 \begin_layout Standard
1380 The latter field can only be written at creation time, the former should
1381 be written under the OPEN_LOCK when opening the database for writing, if
1382 the variant of the code is lower than the current lowest variant.
1385 \begin_layout Standard
1386 This should allow backwards-compatible features to be added, and detection
1387 if older code (which doesn't understand the feature) writes to the database.
1390 \begin_layout Subsubsection
1394 \begin_layout Standard
1398 \begin_layout Subsection
1399 Record Headers Are Not Expandible
1402 \begin_layout Standard
1403 If we later want to add (say) checksums on keys and data, it would require
1404 another format change, which we'd like to avoid.
1407 \begin_layout Subsubsection
1411 \begin_layout Standard
1412 We often have extra padding at the tail of a record.
1413 If we ensure that the first byte (if any) of this padding is zero, we will
1414 have a way for future changes to detect code which doesn't understand a
1415 new format: the new code would write (say) a 1 at the tail, and thus if
1416 there is no tail or the first byte is 0, we would know the extension is
1417 not present on that record.
1420 \begin_layout Subsubsection
1424 \begin_layout Standard
1428 \begin_layout Subsection
1429 TDB Does Not Use Talloc
1432 \begin_layout Standard
1433 Many users of TDB (particularly Samba) use the talloc allocator, and thus
1434 have to wrap TDB in a talloc context to use it conveniently.
1437 \begin_layout Subsubsection
1441 \begin_layout Standard
1442 The allocation within TDB is not complicated enough to justify the use of
1443 talloc, and I am reluctant to force another (excellent) library on TDB
1445 Nonetheless a compromise is possible.
1447 \begin_inset CommandInset ref
1449 reference "attributes"
1453 ) can be added later to tdb_open() to provide an alternate allocation mechanism,
1454 specifically for talloc but usable by any other allocator (which would
1456 \begin_inset Quotes eld
1460 \begin_inset Quotes erd
1466 \begin_layout Standard
1467 This would form a talloc heirarchy as expected, but the caller would still
1468 have to attach a destructor to the tdb context returned from tdb_open to
1470 All TDB_DATA fields would be children of the tdb_context, and the caller
1471 would still have to manage them (using talloc_free() or talloc_steal()).
1474 \begin_layout Subsubsection
1478 \begin_layout Standard
1482 \begin_layout Section
1483 Performance And Scalability Issues
1486 \begin_layout Subsection
1487 \begin_inset CommandInset label
1489 name "TDB_CLEAR_IF_FIRST-Imposes-Performance"
1493 TDB_CLEAR_IF_FIRST Imposes Performance Penalty
1496 \begin_layout Standard
1497 When TDB_CLEAR_IF_FIRST is specified, a 1-byte read lock is placed at offset
1500 While these locks never conflict in normal tdb usage, they do add substantial
1501 overhead for most fcntl lock implementations when the kernel scans to detect
1502 if a lock conflict exists.
1503 This is often a single linked list, making the time to acquire and release
1504 a fcntl lock O(N) where N is the number of processes with the TDB open,
1505 not the number actually doing work.
1508 \begin_layout Standard
1509 In a Samba server it is common to have huge numbers of clients sitting idle,
1510 and thus they have weaned themselves off the TDB_CLEAR_IF_FIRST flag.
1514 \begin_layout Plain Layout
1515 There is a flag to tdb_reopen_all() which is used for this optimization:
1516 if the parent process will outlive the child, the child does not need the
1518 This is a workaround for this very performance issue.
1526 \begin_layout Subsubsection
1530 \begin_layout Standard
1532 It was a neat idea, but even trivial servers tend to know when they are
1533 initializing for the first time and can simply unlink the old tdb at that
1537 \begin_layout Subsubsection
1541 \begin_layout Standard
1543 \change_deleted 0 1298979837
1544 Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing.
1545 \change_inserted 0 1298979837
1551 \begin_layout Subsection
1552 TDB Files Have a 4G Limit
1555 \begin_layout Standard
1556 This seems to be becoming an issue (so much for
1557 \begin_inset Quotes eld
1561 \begin_inset Quotes erd
1564 !), particularly for ldb.
1567 \begin_layout Subsubsection
1571 \begin_layout Standard
1572 A new, incompatible TDB format which uses 64 bit offsets internally rather
1574 For simplicity of endian conversion (which TDB does on the fly if required),
1575 all values will be 64 bit on disk.
1576 In practice, some upper bits may be used for other purposes, but at least
1577 56 bits will be available for file offsets.
1580 \begin_layout Standard
1581 tdb_open() will automatically detect the old version, and even create them
1582 if TDB_VERSION6 is specified to tdb_open.
1585 \begin_layout Standard
1586 32 bit processes will still be able to access TDBs larger than 4G (assuming
1587 that their off_t allows them to seek to 64 bits), they will gracefully
1588 fall back as they fail to mmap.
1589 This can happen already with large TDBs.
1592 \begin_layout Standard
1593 Old versions of tdb will fail to open the new TDB files (since 28 August
1594 2009, commit 398d0c29290: prior to that any unrecognized file format would
1595 be erased and initialized as a fresh tdb!)
1598 \begin_layout Subsubsection
1602 \begin_layout Standard
1606 \begin_layout Subsection
1607 TDB Records Have a 4G Limit
1610 \begin_layout Standard
1611 This has not been a reported problem, and the API uses size_t which can
1612 be 64 bit on 64 bit platforms.
1613 However, other limits may have made such an issue moot.
1616 \begin_layout Subsubsection
1620 \begin_layout Standard
1621 Record sizes will be 64 bit, with an error returned on 32 bit platforms
1622 which try to access such records (the current implementation would return
1623 TDB_ERR_OOM in a similar case).
1624 It seems unlikely that 32 bit keys will be a limitation, so the implementation
1625 may not support this (see
1626 \begin_inset CommandInset ref
1628 reference "sub:Records-Incur-A"
1635 \begin_layout Subsubsection
1639 \begin_layout Standard
1643 \begin_layout Subsection
1644 Hash Size Is Determined At TDB Creation Time
1647 \begin_layout Standard
1648 TDB contains a number of hash chains in the header; the number is specified
1649 at creation time, and defaults to 131.
1650 This is such a bottleneck on large databases (as each hash chain gets quite
1651 long), that LDB uses 10,000 for this hash.
1652 In general it is impossible to know what the 'right' answer is at database
1656 \begin_layout Subsubsection
1657 \begin_inset CommandInset label
1659 name "sub:Hash-Size-Solution"
1666 \begin_layout Standard
1667 After comprehensive performance testing on various scalable hash variants
1671 \begin_layout Plain Layout
1672 http://rusty.ozlabs.org/?p=89 and http://rusty.ozlabs.org/?p=94 This was annoying
1673 because I was previously convinced that an expanding tree of hashes would
1674 be very close to optimal.
1679 , it became clear that it is hard to beat a straight linear hash table which
1680 doubles in size when it reaches saturation.
1681 Unfortunately, altering the hash table introduces serious locking complications
1682 : the entire hash table needs to be locked to enlarge the hash table, and
1683 others might be holding locks.
1684 Particularly insidious are insertions done under tdb_chainlock.
1687 \begin_layout Standard
1688 Thus an expanding layered hash will be used: an array of hash groups, with
1689 each hash group exploding into pointers to lower hash groups once it fills,
1690 turning into a hash tree.
1691 This has implications for locking: we must lock the entire group in case
1692 we need to expand it, yet we don't know how deep the tree is at that point.
1695 \begin_layout Standard
1696 Note that bits from the hash table entries should be stolen to hold more
1697 hash bits to reduce the penalty of collisions.
1698 We can use the otherwise-unused lower 3 bits.
1699 If we limit the size of the database to 64 exabytes, we can use the top
1700 8 bits of the hash entry as well.
1701 These 11 bits would reduce false positives down to 1 in 2000 which is more
1702 than we need: we can use one of the bits to indicate that the extra hash
1704 This means we can choose not to re-hash all entries when we expand a hash
1705 group; simply use the next bits we need and mark them invalid.
1708 \begin_layout Subsubsection
1712 \begin_layout Standard
1716 \begin_layout Subsection
1717 \begin_inset CommandInset label
1719 name "TDB-Freelist-Is"
1723 TDB Freelist Is Highly Contended
1726 \begin_layout Standard
1727 TDB uses a single linked list for the free list.
1728 Allocation occurs as follows, using heuristics which have evolved over
1732 \begin_layout Enumerate
1733 Get the free list lock for this whole operation.
1736 \begin_layout Enumerate
1737 Multiply length by 1.25, so we always over-allocate by 25%.
1740 \begin_layout Enumerate
1741 Set the slack multiplier to 1.
1744 \begin_layout Enumerate
1745 Examine the current freelist entry: if it is > length but < the current
1746 best case, remember it as the best case.
1749 \begin_layout Enumerate
1750 Multiply the slack multiplier by 1.05.
1753 \begin_layout Enumerate
1754 If our best fit so far is less than length * slack multiplier, return it.
1755 The slack will be turned into a new free record if it's large enough.
1758 \begin_layout Enumerate
1759 Otherwise, go onto the next freelist entry.
1762 \begin_layout Standard
1763 Deleting a record occurs as follows:
1766 \begin_layout Enumerate
1767 Lock the hash chain for this whole operation.
1770 \begin_layout Enumerate
1771 Walk the chain to find the record, keeping the prev pointer offset.
1774 \begin_layout Enumerate
1775 If max_dead is non-zero:
1779 \begin_layout Enumerate
1780 Walk the hash chain again and count the dead records.
1783 \begin_layout Enumerate
1784 If it's more than max_dead, bulk free all the dead ones (similar to steps
1785 4 and below, but the lock is only obtained once).
1788 \begin_layout Enumerate
1789 Simply mark this record as dead and return.
1794 \begin_layout Enumerate
1795 Get the free list lock for the remainder of this operation.
1798 \begin_layout Enumerate
1799 \begin_inset CommandInset label
1801 name "right-merging"
1805 Examine the following block to see if it is free; if so, enlarge the current
1806 block and remove that block from the free list.
1807 This was disabled, as removal from the free list was O(entries-in-free-list).
1810 \begin_layout Enumerate
1811 Examine the preceeding block to see if it is free: for this reason, each
1812 block has a 32-bit tailer which indicates its length.
1813 If it is free, expand it to cover our new block and return.
1816 \begin_layout Enumerate
1817 Otherwise, prepend ourselves to the free list.
1820 \begin_layout Standard
1821 Disabling right-merging (step
1822 \begin_inset CommandInset ref
1824 reference "right-merging"
1828 ) causes fragmentation; the other heuristics proved insufficient to address
1829 this, so the final answer to this was that when we expand the TDB file
1830 inside a transaction commit, we repack the entire tdb.
1833 \begin_layout Standard
1834 The single list lock limits our allocation rate; due to the other issues
1835 this is not currently seen as a bottleneck.
1838 \begin_layout Subsubsection
1842 \begin_layout Standard
1843 The first step is to remove all the current heuristics, as they obviously
1844 interact, then examine them once the lock contention is addressed.
1847 \begin_layout Standard
1848 The free list must be split to reduce contention.
1849 Assuming perfect free merging, we can at most have 1 free list entry for
1851 This implies that the number of free lists is related to the size of the
1852 hash table, but as it is rare to walk a large number of free list entries
1853 we can use far fewer, say 1/32 of the number of hash buckets.
1856 \begin_layout Standard
1857 It seems tempting to try to reuse the hash implementation which we use for
1858 records here, but we have two ways of searching for free entries: for allocatio
1859 n we search by size (and possibly zone) which produces too many clashes
1860 for our hash table to handle well, and for coalescing we search by address.
1861 Thus an array of doubly-linked free lists seems preferable.
1864 \begin_layout Standard
1865 There are various benefits in using per-size free lists (see
1866 \begin_inset CommandInset ref
1868 reference "sub:TDB-Becomes-Fragmented"
1872 ) but it's not clear this would reduce contention in the common case where
1873 all processes are allocating/freeing the same size.
1874 Thus we almost certainly need to divide in other ways: the most obvious
1875 is to divide the file into zones, and using a free list (or table of free
1877 This approximates address ordering.
1880 \begin_layout Standard
1881 Unfortunately it is difficult to know what heuristics should be used to
1882 determine zone sizes, and our transaction code relies on being able to
1884 \begin_inset Quotes eld
1888 \begin_inset Quotes erd
1891 by simply appending to the file (difficult if it would need to create a
1893 Thus we use a linked-list of free tables; currently we only ever create
1894 one, but if there is more than one we choose one at random to use.
1895 In future we may use heuristics to add new free tables on contention.
1896 We only expand the file when all free tables are exhausted.
1899 \begin_layout Standard
1900 The basic algorithm is as follows.
1904 \begin_layout Enumerate
1905 Identify the correct free list.
1908 \begin_layout Enumerate
1909 Lock the corresponding list.
1912 \begin_layout Enumerate
1913 Re-check the list (we didn't have a lock, sizes could have changed): relock
1917 \begin_layout Enumerate
1918 Place the freed entry in the list.
1921 \begin_layout Standard
1922 Allocation is a little more complicated, as we perform delayed coalescing
1926 \begin_layout Enumerate
1927 Pick a free table; usually the previous one.
1930 \begin_layout Enumerate
1931 Lock the corresponding list.
1934 \begin_layout Enumerate
1935 If the top entry is -large enough, remove it from the list and return it.
1938 \begin_layout Enumerate
1939 Otherwise, coalesce entries in the list.If there was no entry large enough,
1940 unlock the list and try the next largest list
1943 \begin_layout Enumerate
1944 If no list has an entry which meets our needs, try the next free table.
1947 \begin_layout Enumerate
1948 If no zone satisfies, expand the file.
1951 \begin_layout Standard
1952 This optimizes rapid insert/delete of free list entries by not coalescing
1954 First-fit address ordering ordering seems to be fairly good for keeping
1955 fragmentation low (see
1956 \begin_inset CommandInset ref
1958 reference "sub:TDB-Becomes-Fragmented"
1963 Note that address ordering does not need a tailer to coalesce, though if
1964 we needed one we could have one cheaply: see
1965 \begin_inset CommandInset ref
1967 reference "sub:Records-Incur-A"
1975 \begin_layout Standard
1976 Each free entry has the free table number in the header: less than 255.
1977 It also contains a doubly-linked list for easy deletion.
1980 \begin_layout Subsection
1981 \begin_inset CommandInset label
1983 name "sub:TDB-Becomes-Fragmented"
1987 TDB Becomes Fragmented
1990 \begin_layout Standard
1991 Much of this is a result of allocation strategy
1995 \begin_layout Plain Layout
1996 The Memory Fragmentation Problem: Solved? Johnstone & Wilson 1995 ftp://ftp.cs.ute
1997 xas.edu/pub/garbage/malloc/ismm98.ps
2002 and deliberate hobbling of coalescing; internal fragmentation (aka overallocati
2003 on) is deliberately set at 25%, and external fragmentation is only cured
2004 by the decision to repack the entire db when a transaction commit needs
2005 to enlarge the file.
2008 \begin_layout Subsubsection
2012 \begin_layout Standard
2013 The 25% overhead on allocation works in practice for ldb because indexes
2014 tend to expand by one record at a time.
2015 This internal fragmentation can be resolved by having an
2016 \begin_inset Quotes eld
2020 \begin_inset Quotes erd
2023 bit in the header to note entries that have previously expanded, and allocating
2024 more space for them.
2027 \begin_layout Standard
2028 There are is a spectrum of possible solutions for external fragmentation:
2029 one is to use a fragmentation-avoiding allocation strategy such as best-fit
2030 address-order allocator.
2031 The other end of the spectrum would be to use a bump allocator (very fast
2032 and simple) and simply repack the file when we reach the end.
2035 \begin_layout Standard
2036 There are three problems with efficient fragmentation-avoiding allocators:
2037 they are non-trivial, they tend to use a single free list for each size,
2038 and there's no evidence that tdb allocation patterns will match those recorded
2039 for general allocators (though it seems likely).
2042 \begin_layout Standard
2043 Thus we don't spend too much effort on external fragmentation; we will be
2044 no worse than the current code if we need to repack on occasion.
2045 More effort is spent on reducing freelist contention, and reducing overhead.
2048 \begin_layout Subsection
2049 \begin_inset CommandInset label
2051 name "sub:Records-Incur-A"
2055 Records Incur A 28-Byte Overhead
2058 \begin_layout Standard
2059 Each TDB record has a header as follows:
2062 \begin_layout LyX-Code
2066 \begin_layout LyX-Code
2067 tdb_off_t next; /* offset of the next record in the list */
2070 \begin_layout LyX-Code
2071 tdb_len_t rec_len; /* total byte length of record */
2074 \begin_layout LyX-Code
2075 tdb_len_t key_len; /* byte length of key */
2078 \begin_layout LyX-Code
2079 tdb_len_t data_len; /* byte length of data */
2082 \begin_layout LyX-Code
2083 uint32_t full_hash; /* the full 32 bit hash of the key */
2086 \begin_layout LyX-Code
2087 uint32_t magic; /* try to catch errors */
2090 \begin_layout LyX-Code
2091 /* the following union is implied:
2094 \begin_layout LyX-Code
2098 \begin_layout LyX-Code
2099 char record[rec_len];
2102 \begin_layout LyX-Code
2106 \begin_layout LyX-Code
2110 \begin_layout LyX-Code
2111 char data[data_len];
2114 \begin_layout LyX-Code
2118 \begin_layout LyX-Code
2119 uint32_t totalsize; (tailer)
2122 \begin_layout LyX-Code
2126 \begin_layout LyX-Code
2130 \begin_layout LyX-Code
2134 \begin_layout Standard
2135 Naively, this would double to a 56-byte overhead on a 64 bit implementation.
2138 \begin_layout Subsubsection
2142 \begin_layout Standard
2143 We can use various techniques to reduce this for an allocated block:
2146 \begin_layout Enumerate
2147 The 'next' pointer is not required, as we are using a flat hash table.
2150 \begin_layout Enumerate
2151 'rec_len' can instead be expressed as an addition to key_len and data_len
2152 (it accounts for wasted or overallocated length in the record).
2153 Since the record length is always a multiple of 8, we can conveniently
2154 fit it in 32 bits (representing up to 35 bits).
2157 \begin_layout Enumerate
2158 'key_len' and 'data_len' can be reduced.
2159 I'm unwilling to restrict 'data_len' to 32 bits, but instead we can combine
2160 the two into one 64-bit field and using a 5 bit value which indicates at
2161 what bit to divide the two.
2162 Keys are unlikely to scale as fast as data, so I'm assuming a maximum key
2166 \begin_layout Enumerate
2167 'full_hash' is used to avoid a memcmp on the
2168 \begin_inset Quotes eld
2172 \begin_inset Quotes erd
2175 case, but this is diminishing returns after a handful of bits (at 10 bits,
2176 it reduces 99.9% of false memcmp).
2177 As an aside, as the lower bits are already incorporated in the hash table
2178 resolution, the upper bits should be used here.
2179 Note that it's not clear that these bits will be a win, given the extra
2180 bits in the hash table itself (see
2181 \begin_inset CommandInset ref
2183 reference "sub:Hash-Size-Solution"
2190 \begin_layout Enumerate
2191 'magic' does not need to be enlarged: it currently reflects one of 5 values
2192 (used, free, dead, recovery, and unused_recovery).
2193 It is useful for quick sanity checking however, and should not be eliminated.
2196 \begin_layout Enumerate
2197 'tailer' is only used to coalesce free blocks (so a block to the right can
2198 find the header to check if this block is free).
2199 This can be replaced by a single 'free' bit in the header of the following
2200 block (and the tailer only exists in free blocks).
2204 \begin_layout Plain Layout
2205 This technique from Thomas Standish.
2206 Data Structure Techniques.
2207 Addison-Wesley, Reading, Massachusetts, 1980.
2212 The current proposed coalescing algorithm doesn't need this, however.
2215 \begin_layout Standard
2216 This produces a 16 byte used header like this:
2219 \begin_layout LyX-Code
2220 struct tdb_used_record {
2223 \begin_layout LyX-Code
2224 uint32_t used_magic : 16,
2227 \begin_layout LyX-Code
2231 \begin_layout LyX-Code
2235 \begin_layout LyX-Code
2239 \begin_layout LyX-Code
2240 uint32_t extra_octets;
2243 \begin_layout LyX-Code
2244 uint64_t key_and_data_len;
2247 \begin_layout LyX-Code
2251 \begin_layout Standard
2252 And a free record like this:
2255 \begin_layout LyX-Code
2256 struct tdb_free_record {
2259 \begin_layout LyX-Code
2260 uint64_t free_magic: 8,
2263 \begin_layout LyX-Code
2267 \begin_layout LyX-Code
2271 \begin_layout LyX-Code
2272 uint64_t free_table: 8,
2275 \begin_layout LyX-Code
2279 \begin_layout LyX-Code
2283 \begin_layout LyX-Code
2287 \begin_layout Standard
2289 \change_deleted 0 1291206079
2292 Note that by limiting valid offsets to 56 bits, we can pack everything we
2293 need into 3 64-byte words, meaning our minimum record size is 8 bytes.
2296 \begin_layout Subsubsection
2300 \begin_layout Standard
2304 \begin_layout Subsection
2305 Transaction Commit Requires 4 fdatasync
2308 \begin_layout Standard
2309 The current transaction algorithm is:
2312 \begin_layout Enumerate
2313 write_recovery_data();
2316 \begin_layout Enumerate
2320 \begin_layout Enumerate
2321 write_recovery_header();
2324 \begin_layout Enumerate
2328 \begin_layout Enumerate
2329 overwrite_with_new_data();
2332 \begin_layout Enumerate
2336 \begin_layout Enumerate
2337 remove_recovery_header();
2340 \begin_layout Enumerate
2344 \begin_layout Standard
2345 On current ext3, each sync flushes all data to disk, so the next 3 syncs
2346 are relatively expensive.
2347 But this could become a performance bottleneck on other filesystems such
2351 \begin_layout Subsubsection
2355 \begin_layout Standard
2356 Neil Brown points out that this is overzealous, and only one sync is needed:
2359 \begin_layout Enumerate
2360 Bundle the recovery data, a transaction counter and a strong checksum of
2364 \begin_layout Enumerate
2365 Strong checksum that whole bundle.
2368 \begin_layout Enumerate
2369 Store the bundle in the database.
2372 \begin_layout Enumerate
2373 Overwrite the oldest of the two recovery pointers in the header (identified
2374 using the transaction counter) with the offset of this bundle.
2377 \begin_layout Enumerate
2381 \begin_layout Enumerate
2382 Write the new data to the file.
2385 \begin_layout Standard
2386 Checking for recovery means identifying the latest bundle with a valid checksum
2387 and using the new data checksum to ensure that it has been applied.
2388 This is more expensive than the current check, but need only be done at
2390 For running databases, a separate header field can be used to indicate
2391 a transaction in progress; we need only check for recovery if this is set.
2394 \begin_layout Subsubsection
2398 \begin_layout Standard
2402 \begin_layout Subsection
2403 \begin_inset CommandInset label
2405 name "sub:TDB-Does-Not"
2409 TDB Does Not Have Snapshot Support
2412 \begin_layout Subsubsection
2413 Proposed SolutionNone.
2414 At some point you say
2415 \begin_inset Quotes eld
2419 \begin_inset Quotes erd
2423 \begin_inset CommandInset ref
2425 reference "replay-attribute"
2432 \begin_layout Standard
2433 But as a thought experiment, if we implemented transactions to only overwrite
2434 free entries (this is tricky: there must not be a header in each entry
2435 which indicates whether it is free, but use of presence in metadata elsewhere),
2436 and a pointer to the hash table, we could create an entirely new commit
2437 without destroying existing data.
2438 Then it would be easy to implement snapshots in a similar way.
2441 \begin_layout Standard
2442 This would not allow arbitrary changes to the database, such as tdb_repack
2443 does, and would require more space (since we have to preserve the current
2444 and future entries at once).
2445 If we used hash trees rather than one big hash table, we might only have
2446 to rewrite some sections of the hash, too.
2449 \begin_layout Standard
2450 We could then implement snapshots using a similar method, using multiple
2451 different hash tables/free tables.
2454 \begin_layout Subsubsection
2458 \begin_layout Standard
2462 \begin_layout Subsection
2463 Transactions Cannot Operate in Parallel
2466 \begin_layout Standard
2467 This would be useless for ldb, as it hits the index records with just about
2469 It would add significant complexity in resolving clashes, and cause the
2470 all transaction callers to write their code to loop in the case where the
2471 transactions spuriously failed.
2474 \begin_layout Subsubsection
2478 \begin_layout Standard
2480 \begin_inset CommandInset ref
2482 reference "replay-attribute"
2487 We could solve a small part of the problem by providing read-only transactions.
2488 These would allow one write transaction to begin, but it could not commit
2489 until all r/o transactions are done.
2490 This would require a new RO_TRANSACTION_LOCK, which would be upgraded on
2494 \begin_layout Subsubsection
2498 \begin_layout Standard
2502 \begin_layout Subsection
2503 Default Hash Function Is Suboptimal
2506 \begin_layout Standard
2507 The Knuth-inspired multiplicative hash used by tdb is fairly slow (especially
2508 if we expand it to 64 bits), and works best when the hash bucket size is
2509 a prime number (which also means a slow modulus).
2510 In addition, it is highly predictable which could potentially lead to a
2511 Denial of Service attack in some TDB uses.
2514 \begin_layout Subsubsection
2518 \begin_layout Standard
2519 The Jenkins lookup3 hash
2523 \begin_layout Plain Layout
2524 http://burtleburtle.net/bob/c/lookup3.c
2529 is a fast and superbly-mixing hash.
2530 It's used by the Linux kernel and almost everything else.
2531 This has the particular properties that it takes an initial seed, and produces
2532 two 32 bit hash numbers, which we can combine into a 64-bit hash.
2535 \begin_layout Standard
2536 The seed should be created at tdb-creation time from some random source,
2537 and placed in the header.
2538 This is far from foolproof, but adds a little bit of protection against
2542 \begin_layout Subsubsection
2546 \begin_layout Standard
2550 \begin_layout Subsection
2551 \begin_inset CommandInset label
2553 name "Reliable-Traversal-Adds"
2557 Reliable Traversal Adds Complexity
2560 \begin_layout Standard
2561 We lock a record during traversal iteration, and try to grab that lock in
2563 If that grab on delete fails, we simply mark it deleted and continue onwards;
2564 traversal checks for this condition and does the delete when it moves off
2568 \begin_layout Standard
2569 If traversal terminates, the dead record may be left indefinitely.
2572 \begin_layout Subsubsection
2576 \begin_layout Standard
2577 Remove reliability guarantees; see
2578 \begin_inset CommandInset ref
2580 reference "traverse-Proposed-Solution"
2587 \begin_layout Subsubsection
2591 \begin_layout Standard
2595 \begin_layout Subsection
2596 Fcntl Locking Adds Overhead
2599 \begin_layout Standard
2600 Placing a fcntl lock means a system call, as does removing one.
2601 This is actually one reason why transactions can be faster (everything
2602 is locked once at transaction start).
2603 In the uncontended case, this overhead can theoretically be eliminated.
2606 \begin_layout Subsubsection
2610 \begin_layout Standard
2614 \begin_layout Standard
2615 We tried this before with spinlock support, in the early days of TDB, and
2616 it didn't make much difference except in manufactured benchmarks.
2619 \begin_layout Standard
2620 We could use spinlocks (with futex kernel support under Linux), but it means
2621 that we lose automatic cleanup when a process dies with a lock.
2622 There is a method of auto-cleanup under Linux, but it's not supported by
2623 other operating systems.
2624 We could reintroduce a clear-if-first-style lock and sweep for dead futexes
2625 on open, but that wouldn't help the normal case of one concurrent opener
2627 Increasingly elaborate repair schemes could be considered, but they require
2628 an ABI change (everyone must use them) anyway, so there's no need to do
2629 this at the same time as everything else.
2632 \begin_layout Subsection
2633 Some Transactions Don't Require Durability
2636 \begin_layout Standard
2637 Volker points out that gencache uses a CLEAR_IF_FIRST tdb for normal (fast)
2638 usage, and occasionally empties the results into a transactional TDB.
2639 This kind of usage prioritizes performance over durability: as long as
2640 we are consistent, data can be lost.
2643 \begin_layout Standard
2644 This would be more neatly implemented inside tdb: a
2645 \begin_inset Quotes eld
2649 \begin_inset Quotes erd
2652 transaction commit (ie.
2653 syncless) which meant that data may be reverted on a crash.
2656 \begin_layout Subsubsection
2660 \begin_layout Standard
2664 \begin_layout Standard
2665 Unfortunately any transaction scheme which overwrites old data requires
2666 a sync before that overwrite to avoid the possibility of corruption.
2669 \begin_layout Standard
2670 It seems possible to use a scheme similar to that described in
2671 \begin_inset CommandInset ref
2673 reference "sub:TDB-Does-Not"
2677 ,where transactions are committed without overwriting existing data, and
2678 an array of top-level pointers were available in the header.
2679 If the transaction is
2680 \begin_inset Quotes eld
2684 \begin_inset Quotes erd
2687 then we would not need a sync at all: existing processes would pick up
2688 the new hash table and free list and work with that.
2691 \begin_layout Standard
2692 At some later point, a sync would allow recovery of the old data into the
2693 free lists (perhaps when the array of top-level pointers filled).
2694 On crash, tdb_open() would examine the array of top levels, and apply the
2695 transactions until it encountered an invalid checksum.
2698 \begin_layout Subsection
2699 Tracing Is Fragile, Replay Is External
2702 \begin_layout Standard
2703 The current TDB has compile-time-enabled tracing code, but it often breaks
2704 as it is not enabled by default.
2705 In a similar way, the ctdb code has an external wrapper which does replay
2706 tracing so it can coordinate cluster-wide transactions.
2709 \begin_layout Subsubsection
2711 \begin_inset CommandInset label
2713 name "replay-attribute"
2720 \begin_layout Standard
2721 Tridge points out that an attribute can be later added to tdb_open (see
2723 \begin_inset CommandInset ref
2725 reference "attributes"
2729 ) to provide replay/trace hooks, which could become the basis for this and
2730 future parallel transactions and snapshot support.
2733 \begin_layout Subsubsection
2737 \begin_layout Standard