1 #LyX 1.6.7 created this file. For more info see http://www.lyx.org/
6 \use_default_options true
11 \font_typewriter default
12 \font_default_family default
19 \paperfontsize default
27 \paperorientation portrait
30 \paragraph_separation indent
32 \quotes_language english
35 \paperpagestyle default
36 \tracking_changes true
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
89 <lyxtabular version="3" rows="12" columns="3">
91 <column alignment="center" valignment="top" width="0">
92 <column alignment="center" valignment="top" width="0">
93 <column alignment="center" valignment="top" width="0">
95 <cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
98 \begin_layout Plain Layout
104 <cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
107 \begin_layout Plain Layout
113 <cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none">
116 \begin_layout Plain Layout
117 Lines of C Code Implementation
124 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
127 \begin_layout Plain Layout
133 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
136 \begin_layout Plain Layout
142 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
145 \begin_layout Plain Layout
153 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
156 \begin_layout Plain Layout
162 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
165 \begin_layout Plain Layout
171 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
174 \begin_layout Plain Layout
182 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
185 \begin_layout Plain Layout
191 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
194 \begin_layout Plain Layout
200 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
203 \begin_layout Plain Layout
211 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
214 \begin_layout Plain Layout
220 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
223 \begin_layout Plain Layout
229 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
232 \begin_layout Plain Layout
240 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
243 \begin_layout Plain Layout
249 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
252 \begin_layout Plain Layout
258 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
261 \begin_layout Plain Layout
269 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
272 \begin_layout Plain Layout
278 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
281 \begin_layout Plain Layout
287 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
290 \begin_layout Plain Layout
298 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
301 \begin_layout Plain Layout
307 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
310 \begin_layout Plain Layout
316 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
319 \begin_layout Plain Layout
327 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
330 \begin_layout Plain Layout
336 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
339 \begin_layout Plain Layout
345 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
348 \begin_layout Plain Layout
356 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
359 \begin_layout Plain Layout
365 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
368 \begin_layout Plain Layout
374 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
377 \begin_layout Plain Layout
385 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
388 \begin_layout Plain Layout
394 <cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
397 \begin_layout Plain Layout
403 <cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
406 \begin_layout Plain Layout
414 <cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
417 \begin_layout Plain Layout
423 <cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
426 \begin_layout Plain Layout
432 <cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none">
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
857 \change_deleted 0 1300360823
859 \change_inserted 0 1300360824
865 \begin_layout Subsection
866 TDB API Is Not POSIX Thread-safe
869 \begin_layout Standard
870 The TDB API uses an error code which can be queried after an operation to
871 determine what went wrong.
872 This programming model does not work with threads, unless specific additional
873 guarantees are given by the implementation.
874 In addition, even otherwise-independent threads cannot open the same TDB
876 \begin_inset CommandInset ref
878 reference "TDB-Files-Cannot"
885 \begin_layout Subsubsection
889 \begin_layout Standard
890 Reachitecting the API to include a tdb_errcode pointer would be a great
892 \change_inserted 0 1298979557
893 , but fortunately most functions return 0 on success and -1 on error: we
894 can change these to return 0 on success and a negative error code on error,
895 and the API remains similar to previous.
896 The tdb_fetch, tdb_firstkey and tdb_nextkey functions need to take a TDB_DATA
897 pointer and return an error code.
898 It is also simpler to have tdb_nextkey replace its key argument in place,
899 freeing up any old .dptr.
902 \begin_layout Standard
904 \change_deleted 0 1298979438
905 ; we are better to guarantee that the tdb_errcode is per-thread so the current
906 programming model can be maintained.
909 \begin_layout Standard
911 \change_deleted 0 1298979438
912 This requires dynamic per-thread allocations, which is awkward with POSIX
913 threads (pthread_key_create space is limited and we cannot simply allocate
914 a key for every TDB).
919 \begin_layout Standard
920 Internal locking is required to make sure that fcntl locks do not overlap
921 between threads, and also that the global list of tdbs is maintained.
924 \begin_layout Standard
925 The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe
926 version of the library, and otherwise no overhead will exist.
927 Alternatively, a hooking mechanism similar to that proposed for
928 \begin_inset CommandInset ref
930 reference "Proposed-Solution-locking-hook"
934 could be used to enable pthread locking at runtime.
937 \begin_layout Subsubsection
941 \begin_layout Standard
943 \change_inserted 0 1298979681
944 ; API has been changed but thread safety has not been implemented.
945 \change_deleted 0 1298979669
951 \begin_layout Subsection
952 *_nonblock Functions And *_mark Functions Expose Implementation
955 \begin_layout Standard
960 \begin_layout Plain Layout
961 Clustered TDB, see http://ctdb.samba.org
966 wishes to operate on TDB in a non-blocking manner.
967 This is currently done as follows:
970 \begin_layout Enumerate
971 Call the _nonblock variant of an API function (eg.
972 tdb_lockall_nonblock).
976 \begin_layout Enumerate
977 Fork a child process, and wait for it to call the normal variant (eg.
981 \begin_layout Enumerate
982 If the child succeeds, call the _mark variant to indicate we already have
987 \begin_layout Enumerate
988 Upon completion, tell the child to release the locks (eg.
992 \begin_layout Enumerate
993 Indicate to tdb that it should consider the locks removed (eg.
997 \begin_layout Standard
998 There are several issues with this approach.
999 Firstly, adding two new variants of each function clutters the API for
1000 an obscure use, and so not all functions have three variants.
1001 Secondly, it assumes that all paths of the functions ask for the same locks,
1002 otherwise the parent process will have to get a lock which the child doesn't
1003 have under some circumstances.
1004 I don't believe this is currently the case, but it constrains the implementatio
1009 \begin_layout Subsubsection
1010 \begin_inset CommandInset label
1012 name "Proposed-Solution-locking-hook"
1019 \begin_layout Standard
1020 Implement a hook for locking methods, so that the caller can control the
1021 calls to create and remove fcntl locks.
1022 In this scenario, ctdbd would operate as follows:
1025 \begin_layout Enumerate
1026 Call the normal API function, eg tdb_lockall().
1029 \begin_layout Enumerate
1030 When the lock callback comes in, check if the child has the lock.
1031 Initially, this is always false.
1033 Otherwise, try to obtain it in non-blocking mode.
1034 If that fails, return EWOULDBLOCK.
1037 \begin_layout Enumerate
1038 Release locks in the unlock callback as normal.
1041 \begin_layout Enumerate
1042 If tdb_lockall() fails, see if we recorded a lock failure; if so, call the
1043 child to repeat the operation.
1046 \begin_layout Enumerate
1047 The child records what locks it obtains, and returns that information to
1051 \begin_layout Enumerate
1052 When the child has succeeded, goto 1.
1055 \begin_layout Standard
1056 This is flexible enough to handle any potential locking scenario, even when
1057 lock requirements change.
1058 It can be optimized so that the parent does not release locks, just tells
1059 the child which locks it doesn't need to obtain.
1062 \begin_layout Standard
1063 It also keeps the complexity out of the API, and in ctdbd where it is needed.
1066 \begin_layout Subsubsection
1070 \begin_layout Standard
1074 \begin_layout Subsection
1075 tdb_chainlock Functions Expose Implementation
1078 \begin_layout Standard
1079 tdb_chainlock locks some number of records, including the record indicated
1081 This gave atomicity guarantees; no-one can start a transaction, alter,
1082 read or delete that key while the lock is held.
1085 \begin_layout Standard
1086 It also makes the same guarantee for any other key in the chain, which is
1087 an internal implementation detail and potentially a cause for deadlock.
1090 \begin_layout Subsubsection
1094 \begin_layout Standard
1096 It would be nice to have an explicit single entry lock which effected no
1098 Unfortunately, this won't work for an entry which doesn't exist.
1099 Thus while chainlock may be implemented more efficiently for the existing
1100 case, it will still have overlap issues with the non-existing case.
1101 So it is best to keep the current (lack of) guarantee about which records
1102 will be effected to avoid constraining our implementation.
1105 \begin_layout Subsection
1106 Signal Handling is Not Race-Free
1109 \begin_layout Standard
1110 The tdb_setalarm_sigptr() call allows the caller's signal handler to indicate
1111 that the tdb locking code should return with a failure, rather than trying
1112 again when a signal is received (and errno == EAGAIN).
1113 This is usually used to implement timeouts.
1116 \begin_layout Standard
1117 Unfortunately, this does not work in the case where the signal is received
1118 before the tdb code enters the fcntl() call to place the lock: the code
1119 will sleep within the fcntl() code, unaware that the signal wants it to
1121 In the case of long timeouts, this does not happen in practice.
1124 \begin_layout Subsubsection
1128 \begin_layout Standard
1129 The locking hooks proposed in
1130 \begin_inset CommandInset ref
1132 reference "Proposed-Solution-locking-hook"
1136 would allow the user to decide on whether to fail the lock acquisition
1138 This allows the caller to choose their own compromise: they could narrow
1139 the race by checking immediately before the fcntl call.
1143 \begin_layout Plain Layout
1144 It may be possible to make this race-free in some implementations by having
1145 the signal handler alter the struct flock to make it invalid.
1146 This will cause the fcntl() lock call to fail with EINVAL if the signal
1147 occurs before the kernel is entered, otherwise EAGAIN.
1155 \begin_layout Subsubsection
1159 \begin_layout Standard
1163 \begin_layout Subsection
1164 The API Uses Gratuitous Typedefs, Capitals
1167 \begin_layout Standard
1168 typedefs are useful for providing source compatibility when types can differ
1169 across implementations, or arguably in the case of function pointer definitions
1170 which are hard for humans to parse.
1171 Otherwise it is simply obfuscation and pollutes the namespace.
1174 \begin_layout Standard
1175 Capitalization is usually reserved for compile-time constants and macros.
1178 \begin_layout Description
1179 TDB_CONTEXT There is no reason to use this over 'struct tdb_context'; the
1180 definition isn't visible to the API user anyway.
1183 \begin_layout Description
1184 TDB_DATA There is no reason to use this over struct TDB_DATA; the struct
1185 needs to be understood by the API user.
1188 \begin_layout Description
1190 \begin_inset space ~
1193 TDB_DATA This would normally be called 'struct tdb_data'.
1196 \begin_layout Description
1198 \begin_inset space ~
1201 TDB_ERROR Similarly, this would normally be enum tdb_error.
1204 \begin_layout Subsubsection
1208 \begin_layout Standard
1210 Introducing lower case variants would please pedants like myself, but if
1211 it were done the existing ones should be kept.
1212 There is little point forcing a purely cosmetic change upon tdb users.
1215 \begin_layout Subsection
1216 \begin_inset CommandInset label
1218 name "tdb_log_func-Doesnt-Take"
1222 tdb_log_func Doesn't Take The Private Pointer
1225 \begin_layout Standard
1226 For API compatibility reasons, the logging function needs to call tdb_get_loggin
1227 g_private() to retrieve the pointer registered by the tdb_open_ex for logging.
1230 \begin_layout Subsubsection
1234 \begin_layout Standard
1235 It should simply take an extra argument, since we are prepared to break
1239 \begin_layout Subsubsection
1243 \begin_layout Standard
1247 \begin_layout Subsection
1248 Various Callback Functions Are Not Typesafe
1251 \begin_layout Standard
1252 The callback functions in tdb_set_logging_function (after
1253 \begin_inset CommandInset ref
1255 reference "tdb_log_func-Doesnt-Take"
1259 is resolved), tdb_parse_record, tdb_traverse, tdb_traverse_read and tdb_check
1260 all take void * and must internally convert it to the argument type they
1264 \begin_layout Standard
1265 If this type changes, the compiler will not produce warnings on the callers,
1266 since it only sees void *.
1269 \begin_layout Subsubsection
1273 \begin_layout Standard
1274 With careful use of macros, we can create callback functions which give
1275 a warning when used on gcc and the types of the callback and its private
1277 Unsupported compilers will not give a warning, which is no worse than now.
1278 In addition, the callbacks become clearer, as they need not use void *
1279 for their parameter.
1282 \begin_layout Standard
1283 See CCAN's typesafe_cb module at http://ccan.ozlabs.org/info/typesafe_cb.html
1286 \begin_layout Subsubsection
1290 \begin_layout Standard
1292 \change_deleted 0 1300360712
1294 \change_inserted 0 1300360716
1300 \begin_layout Subsection
1301 TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, tdb_reopen_all Problematic
1304 \begin_layout Standard
1305 The TDB_CLEAR_IF_FIRST flag to tdb_open indicates that the TDB file should
1306 be cleared if the caller discovers it is the only process with the TDB
1308 However, if any caller does not specify TDB_CLEAR_IF_FIRST it will not
1309 be detected, so will have the TDB erased underneath them (usually resulting
1313 \begin_layout Standard
1314 There is a similar issue on fork(); if the parent exits (or otherwise closes
1315 the tdb) before the child calls tdb_reopen_all() to establish the lock
1316 used to indicate the TDB is opened by someone, a TDB_CLEAR_IF_FIRST opener
1317 at that moment will believe it alone has opened the TDB and will erase
1321 \begin_layout Subsubsection
1325 \begin_layout Standard
1326 Remove TDB_CLEAR_IF_FIRST.
1327 Other workarounds are possible, but see
1328 \begin_inset CommandInset ref
1330 reference "TDB_CLEAR_IF_FIRST-Imposes-Performance"
1337 \begin_layout Subsubsection
1341 \begin_layout Standard
1343 \change_deleted 0 1298979699
1344 Incomplete, TDB_CLEAR_IF_FIRST still defined, but not implemented.
1345 \change_inserted 0 1298979700
1351 \begin_layout Subsection
1352 Extending The Header Is Difficult
1355 \begin_layout Standard
1356 We have reserved (zeroed) words in the TDB header, which can be used for
1358 If the future features are compulsory, the version number must be updated
1359 to prevent old code from accessing the database.
1360 But if the future feature is optional, we have no way of telling if older
1361 code is accessing the database or not.
1364 \begin_layout Subsubsection
1368 \begin_layout Standard
1369 The header should contain a
1370 \begin_inset Quotes eld
1374 \begin_inset Quotes erd
1378 This is divided into two 32-bit parts:
1381 \begin_layout Enumerate
1382 The lower part reflects the format variant understood by code accessing
1386 \begin_layout Enumerate
1387 The upper part reflects the format variant you must understand to write
1388 to the database (otherwise you can only open for reading).
1391 \begin_layout Standard
1392 The latter field can only be written at creation time, the former should
1393 be written under the OPEN_LOCK when opening the database for writing, if
1394 the variant of the code is lower than the current lowest variant.
1397 \begin_layout Standard
1398 This should allow backwards-compatible features to be added, and detection
1399 if older code (which doesn't understand the feature) writes to the database.
1402 \begin_layout Subsubsection
1406 \begin_layout Standard
1408 \change_deleted 0 1300360753
1410 \change_inserted 0 1300360754
1416 \begin_layout Subsection
1417 Record Headers Are Not Expandible
1420 \begin_layout Standard
1421 If we later want to add (say) checksums on keys and data, it would require
1422 another format change, which we'd like to avoid.
1425 \begin_layout Subsubsection
1429 \begin_layout Standard
1430 We often have extra padding at the tail of a record.
1431 If we ensure that the first byte (if any) of this padding is zero, we will
1432 have a way for future changes to detect code which doesn't understand a
1433 new format: the new code would write (say) a 1 at the tail, and thus if
1434 there is no tail or the first byte is 0, we would know the extension is
1435 not present on that record.
1438 \begin_layout Subsubsection
1442 \begin_layout Standard
1444 \change_deleted 0 1300360766
1446 \change_inserted 0 1300360767
1452 \begin_layout Subsection
1453 TDB Does Not Use Talloc
1456 \begin_layout Standard
1457 Many users of TDB (particularly Samba) use the talloc allocator, and thus
1458 have to wrap TDB in a talloc context to use it conveniently.
1461 \begin_layout Subsubsection
1465 \begin_layout Standard
1466 The allocation within TDB is not complicated enough to justify the use of
1467 talloc, and I am reluctant to force another (excellent) library on TDB
1469 Nonetheless a compromise is possible.
1471 \begin_inset CommandInset ref
1473 reference "attributes"
1477 ) can be added later to tdb_open() to provide an alternate allocation mechanism,
1478 specifically for talloc but usable by any other allocator (which would
1480 \begin_inset Quotes eld
1484 \begin_inset Quotes erd
1490 \begin_layout Standard
1491 This would form a talloc heirarchy as expected, but the caller would still
1492 have to attach a destructor to the tdb context returned from tdb_open to
1494 All TDB_DATA fields would be children of the tdb_context, and the caller
1495 would still have to manage them (using talloc_free() or talloc_steal()).
1498 \begin_layout Subsubsection
1502 \begin_layout Standard
1506 \begin_layout Section
1507 Performance And Scalability Issues
1510 \begin_layout Subsection
1511 \begin_inset CommandInset label
1513 name "TDB_CLEAR_IF_FIRST-Imposes-Performance"
1517 TDB_CLEAR_IF_FIRST Imposes Performance Penalty
1520 \begin_layout Standard
1521 When TDB_CLEAR_IF_FIRST is specified, a 1-byte read lock is placed at offset
1524 While these locks never conflict in normal tdb usage, they do add substantial
1525 overhead for most fcntl lock implementations when the kernel scans to detect
1526 if a lock conflict exists.
1527 This is often a single linked list, making the time to acquire and release
1528 a fcntl lock O(N) where N is the number of processes with the TDB open,
1529 not the number actually doing work.
1532 \begin_layout Standard
1533 In a Samba server it is common to have huge numbers of clients sitting idle,
1534 and thus they have weaned themselves off the TDB_CLEAR_IF_FIRST flag.
1538 \begin_layout Plain Layout
1539 There is a flag to tdb_reopen_all() which is used for this optimization:
1540 if the parent process will outlive the child, the child does not need the
1542 This is a workaround for this very performance issue.
1550 \begin_layout Subsubsection
1554 \begin_layout Standard
1556 It was a neat idea, but even trivial servers tend to know when they are
1557 initializing for the first time and can simply unlink the old tdb at that
1561 \begin_layout Subsubsection
1565 \begin_layout Standard
1567 \change_deleted 0 1298979837
1568 Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing.
1569 \change_inserted 0 1298979837
1575 \begin_layout Subsection
1576 TDB Files Have a 4G Limit
1579 \begin_layout Standard
1580 This seems to be becoming an issue (so much for
1581 \begin_inset Quotes eld
1585 \begin_inset Quotes erd
1588 !), particularly for ldb.
1591 \begin_layout Subsubsection
1595 \begin_layout Standard
1596 A new, incompatible TDB format which uses 64 bit offsets internally rather
1598 For simplicity of endian conversion (which TDB does on the fly if required),
1599 all values will be 64 bit on disk.
1600 In practice, some upper bits may be used for other purposes, but at least
1601 56 bits will be available for file offsets.
1604 \begin_layout Standard
1605 tdb_open() will automatically detect the old version, and even create them
1606 if TDB_VERSION6 is specified to tdb_open.
1609 \begin_layout Standard
1610 32 bit processes will still be able to access TDBs larger than 4G (assuming
1611 that their off_t allows them to seek to 64 bits), they will gracefully
1612 fall back as they fail to mmap.
1613 This can happen already with large TDBs.
1616 \begin_layout Standard
1617 Old versions of tdb will fail to open the new TDB files (since 28 August
1618 2009, commit 398d0c29290: prior to that any unrecognized file format would
1619 be erased and initialized as a fresh tdb!)
1622 \begin_layout Subsubsection
1626 \begin_layout Standard
1630 \begin_layout Subsection
1631 TDB Records Have a 4G Limit
1634 \begin_layout Standard
1635 This has not been a reported problem, and the API uses size_t which can
1636 be 64 bit on 64 bit platforms.
1637 However, other limits may have made such an issue moot.
1640 \begin_layout Subsubsection
1644 \begin_layout Standard
1645 Record sizes will be 64 bit, with an error returned on 32 bit platforms
1646 which try to access such records (the current implementation would return
1647 TDB_ERR_OOM in a similar case).
1648 It seems unlikely that 32 bit keys will be a limitation, so the implementation
1649 may not support this (see
1650 \begin_inset CommandInset ref
1652 reference "sub:Records-Incur-A"
1659 \begin_layout Subsubsection
1663 \begin_layout Standard
1667 \begin_layout Subsection
1668 Hash Size Is Determined At TDB Creation Time
1671 \begin_layout Standard
1672 TDB contains a number of hash chains in the header; the number is specified
1673 at creation time, and defaults to 131.
1674 This is such a bottleneck on large databases (as each hash chain gets quite
1675 long), that LDB uses 10,000 for this hash.
1676 In general it is impossible to know what the 'right' answer is at database
1680 \begin_layout Subsubsection
1681 \begin_inset CommandInset label
1683 name "sub:Hash-Size-Solution"
1690 \begin_layout Standard
1691 After comprehensive performance testing on various scalable hash variants
1695 \begin_layout Plain Layout
1696 http://rusty.ozlabs.org/?p=89 and http://rusty.ozlabs.org/?p=94 This was annoying
1697 because I was previously convinced that an expanding tree of hashes would
1698 be very close to optimal.
1703 , it became clear that it is hard to beat a straight linear hash table which
1704 doubles in size when it reaches saturation.
1705 Unfortunately, altering the hash table introduces serious locking complications
1706 : the entire hash table needs to be locked to enlarge the hash table, and
1707 others might be holding locks.
1708 Particularly insidious are insertions done under tdb_chainlock.
1711 \begin_layout Standard
1712 Thus an expanding layered hash will be used: an array of hash groups, with
1713 each hash group exploding into pointers to lower hash groups once it fills,
1714 turning into a hash tree.
1715 This has implications for locking: we must lock the entire group in case
1716 we need to expand it, yet we don't know how deep the tree is at that point.
1719 \begin_layout Standard
1720 Note that bits from the hash table entries should be stolen to hold more
1721 hash bits to reduce the penalty of collisions.
1722 We can use the otherwise-unused lower 3 bits.
1723 If we limit the size of the database to 64 exabytes, we can use the top
1724 8 bits of the hash entry as well.
1725 These 11 bits would reduce false positives down to 1 in 2000 which is more
1726 than we need: we can use one of the bits to indicate that the extra hash
1728 This means we can choose not to re-hash all entries when we expand a hash
1729 group; simply use the next bits we need and mark them invalid.
1732 \begin_layout Subsubsection
1736 \begin_layout Standard
1740 \begin_layout Subsection
1741 \begin_inset CommandInset label
1743 name "TDB-Freelist-Is"
1747 TDB Freelist Is Highly Contended
1750 \begin_layout Standard
1751 TDB uses a single linked list for the free list.
1752 Allocation occurs as follows, using heuristics which have evolved over
1756 \begin_layout Enumerate
1757 Get the free list lock for this whole operation.
1760 \begin_layout Enumerate
1761 Multiply length by 1.25, so we always over-allocate by 25%.
1764 \begin_layout Enumerate
1765 Set the slack multiplier to 1.
1768 \begin_layout Enumerate
1769 Examine the current freelist entry: if it is > length but < the current
1770 best case, remember it as the best case.
1773 \begin_layout Enumerate
1774 Multiply the slack multiplier by 1.05.
1777 \begin_layout Enumerate
1778 If our best fit so far is less than length * slack multiplier, return it.
1779 The slack will be turned into a new free record if it's large enough.
1782 \begin_layout Enumerate
1783 Otherwise, go onto the next freelist entry.
1786 \begin_layout Standard
1787 Deleting a record occurs as follows:
1790 \begin_layout Enumerate
1791 Lock the hash chain for this whole operation.
1794 \begin_layout Enumerate
1795 Walk the chain to find the record, keeping the prev pointer offset.
1798 \begin_layout Enumerate
1799 If max_dead is non-zero:
1803 \begin_layout Enumerate
1804 Walk the hash chain again and count the dead records.
1807 \begin_layout Enumerate
1808 If it's more than max_dead, bulk free all the dead ones (similar to steps
1809 4 and below, but the lock is only obtained once).
1812 \begin_layout Enumerate
1813 Simply mark this record as dead and return.
1818 \begin_layout Enumerate
1819 Get the free list lock for the remainder of this operation.
1822 \begin_layout Enumerate
1823 \begin_inset CommandInset label
1825 name "right-merging"
1829 Examine the following block to see if it is free; if so, enlarge the current
1830 block and remove that block from the free list.
1831 This was disabled, as removal from the free list was O(entries-in-free-list).
1834 \begin_layout Enumerate
1835 Examine the preceeding block to see if it is free: for this reason, each
1836 block has a 32-bit tailer which indicates its length.
1837 If it is free, expand it to cover our new block and return.
1840 \begin_layout Enumerate
1841 Otherwise, prepend ourselves to the free list.
1844 \begin_layout Standard
1845 Disabling right-merging (step
1846 \begin_inset CommandInset ref
1848 reference "right-merging"
1852 ) causes fragmentation; the other heuristics proved insufficient to address
1853 this, so the final answer to this was that when we expand the TDB file
1854 inside a transaction commit, we repack the entire tdb.
1857 \begin_layout Standard
1858 The single list lock limits our allocation rate; due to the other issues
1859 this is not currently seen as a bottleneck.
1862 \begin_layout Subsubsection
1866 \begin_layout Standard
1867 The first step is to remove all the current heuristics, as they obviously
1868 interact, then examine them once the lock contention is addressed.
1871 \begin_layout Standard
1872 The free list must be split to reduce contention.
1873 Assuming perfect free merging, we can at most have 1 free list entry for
1875 This implies that the number of free lists is related to the size of the
1876 hash table, but as it is rare to walk a large number of free list entries
1877 we can use far fewer, say 1/32 of the number of hash buckets.
1880 \begin_layout Standard
1881 It seems tempting to try to reuse the hash implementation which we use for
1882 records here, but we have two ways of searching for free entries: for allocatio
1883 n we search by size (and possibly zone) which produces too many clashes
1884 for our hash table to handle well, and for coalescing we search by address.
1885 Thus an array of doubly-linked free lists seems preferable.
1888 \begin_layout Standard
1889 There are various benefits in using per-size free lists (see
1890 \begin_inset CommandInset ref
1892 reference "sub:TDB-Becomes-Fragmented"
1896 ) but it's not clear this would reduce contention in the common case where
1897 all processes are allocating/freeing the same size.
1898 Thus we almost certainly need to divide in other ways: the most obvious
1899 is to divide the file into zones, and using a free list (or table of free
1901 This approximates address ordering.
1904 \begin_layout Standard
1905 Unfortunately it is difficult to know what heuristics should be used to
1906 determine zone sizes, and our transaction code relies on being able to
1908 \begin_inset Quotes eld
1912 \begin_inset Quotes erd
1915 by simply appending to the file (difficult if it would need to create a
1917 Thus we use a linked-list of free tables; currently we only ever create
1918 one, but if there is more than one we choose one at random to use.
1919 In future we may use heuristics to add new free tables on contention.
1920 We only expand the file when all free tables are exhausted.
1923 \begin_layout Standard
1924 The basic algorithm is as follows.
1928 \begin_layout Enumerate
1929 Identify the correct free list.
1932 \begin_layout Enumerate
1933 Lock the corresponding list.
1936 \begin_layout Enumerate
1937 Re-check the list (we didn't have a lock, sizes could have changed): relock
1941 \begin_layout Enumerate
1942 Place the freed entry in the list.
1945 \begin_layout Standard
1946 Allocation is a little more complicated, as we perform delayed coalescing
1950 \begin_layout Enumerate
1951 Pick a free table; usually the previous one.
1954 \begin_layout Enumerate
1955 Lock the corresponding list.
1958 \begin_layout Enumerate
1959 If the top entry is -large enough, remove it from the list and return it.
1962 \begin_layout Enumerate
1963 Otherwise, coalesce entries in the list.If there was no entry large enough,
1964 unlock the list and try the next largest list
1967 \begin_layout Enumerate
1968 If no list has an entry which meets our needs, try the next free table.
1971 \begin_layout Enumerate
1972 If no zone satisfies, expand the file.
1975 \begin_layout Standard
1976 This optimizes rapid insert/delete of free list entries by not coalescing
1978 First-fit address ordering ordering seems to be fairly good for keeping
1979 fragmentation low (see
1980 \begin_inset CommandInset ref
1982 reference "sub:TDB-Becomes-Fragmented"
1987 Note that address ordering does not need a tailer to coalesce, though if
1988 we needed one we could have one cheaply: see
1989 \begin_inset CommandInset ref
1991 reference "sub:Records-Incur-A"
1999 \begin_layout Standard
2000 Each free entry has the free table number in the header: less than 255.
2001 It also contains a doubly-linked list for easy deletion.
2004 \begin_layout Subsection
2005 \begin_inset CommandInset label
2007 name "sub:TDB-Becomes-Fragmented"
2011 TDB Becomes Fragmented
2014 \begin_layout Standard
2015 Much of this is a result of allocation strategy
2019 \begin_layout Plain Layout
2020 The Memory Fragmentation Problem: Solved? Johnstone & Wilson 1995 ftp://ftp.cs.ute
2021 xas.edu/pub/garbage/malloc/ismm98.ps
2026 and deliberate hobbling of coalescing; internal fragmentation (aka overallocati
2027 on) is deliberately set at 25%, and external fragmentation is only cured
2028 by the decision to repack the entire db when a transaction commit needs
2029 to enlarge the file.
2032 \begin_layout Subsubsection
2036 \begin_layout Standard
2037 The 25% overhead on allocation works in practice for ldb because indexes
2038 tend to expand by one record at a time.
2039 This internal fragmentation can be resolved by having an
2040 \begin_inset Quotes eld
2044 \begin_inset Quotes erd
2047 bit in the header to note entries that have previously expanded, and allocating
2048 more space for them.
2051 \begin_layout Standard
2052 There are is a spectrum of possible solutions for external fragmentation:
2053 one is to use a fragmentation-avoiding allocation strategy such as best-fit
2054 address-order allocator.
2055 The other end of the spectrum would be to use a bump allocator (very fast
2056 and simple) and simply repack the file when we reach the end.
2059 \begin_layout Standard
2060 There are three problems with efficient fragmentation-avoiding allocators:
2061 they are non-trivial, they tend to use a single free list for each size,
2062 and there's no evidence that tdb allocation patterns will match those recorded
2063 for general allocators (though it seems likely).
2066 \begin_layout Standard
2067 Thus we don't spend too much effort on external fragmentation; we will be
2068 no worse than the current code if we need to repack on occasion.
2069 More effort is spent on reducing freelist contention, and reducing overhead.
2072 \begin_layout Subsection
2073 \begin_inset CommandInset label
2075 name "sub:Records-Incur-A"
2079 Records Incur A 28-Byte Overhead
2082 \begin_layout Standard
2083 Each TDB record has a header as follows:
2086 \begin_layout LyX-Code
2090 \begin_layout LyX-Code
2091 tdb_off_t next; /* offset of the next record in the list */
2094 \begin_layout LyX-Code
2095 tdb_len_t rec_len; /* total byte length of record */
2098 \begin_layout LyX-Code
2099 tdb_len_t key_len; /* byte length of key */
2102 \begin_layout LyX-Code
2103 tdb_len_t data_len; /* byte length of data */
2106 \begin_layout LyX-Code
2107 uint32_t full_hash; /* the full 32 bit hash of the key */
2110 \begin_layout LyX-Code
2111 uint32_t magic; /* try to catch errors */
2114 \begin_layout LyX-Code
2115 /* the following union is implied:
2118 \begin_layout LyX-Code
2122 \begin_layout LyX-Code
2123 char record[rec_len];
2126 \begin_layout LyX-Code
2130 \begin_layout LyX-Code
2134 \begin_layout LyX-Code
2135 char data[data_len];
2138 \begin_layout LyX-Code
2142 \begin_layout LyX-Code
2143 uint32_t totalsize; (tailer)
2146 \begin_layout LyX-Code
2150 \begin_layout LyX-Code
2154 \begin_layout LyX-Code
2158 \begin_layout Standard
2159 Naively, this would double to a 56-byte overhead on a 64 bit implementation.
2162 \begin_layout Subsubsection
2166 \begin_layout Standard
2167 We can use various techniques to reduce this for an allocated block:
2170 \begin_layout Enumerate
2171 The 'next' pointer is not required, as we are using a flat hash table.
2174 \begin_layout Enumerate
2175 'rec_len' can instead be expressed as an addition to key_len and data_len
2176 (it accounts for wasted or overallocated length in the record).
2177 Since the record length is always a multiple of 8, we can conveniently
2178 fit it in 32 bits (representing up to 35 bits).
2181 \begin_layout Enumerate
2182 'key_len' and 'data_len' can be reduced.
2183 I'm unwilling to restrict 'data_len' to 32 bits, but instead we can combine
2184 the two into one 64-bit field and using a 5 bit value which indicates at
2185 what bit to divide the two.
2186 Keys are unlikely to scale as fast as data, so I'm assuming a maximum key
2190 \begin_layout Enumerate
2191 'full_hash' is used to avoid a memcmp on the
2192 \begin_inset Quotes eld
2196 \begin_inset Quotes erd
2199 case, but this is diminishing returns after a handful of bits (at 10 bits,
2200 it reduces 99.9% of false memcmp).
2201 As an aside, as the lower bits are already incorporated in the hash table
2202 resolution, the upper bits should be used here.
2203 Note that it's not clear that these bits will be a win, given the extra
2204 bits in the hash table itself (see
2205 \begin_inset CommandInset ref
2207 reference "sub:Hash-Size-Solution"
2214 \begin_layout Enumerate
2215 'magic' does not need to be enlarged: it currently reflects one of 5 values
2216 (used, free, dead, recovery, and unused_recovery).
2217 It is useful for quick sanity checking however, and should not be eliminated.
2220 \begin_layout Enumerate
2221 'tailer' is only used to coalesce free blocks (so a block to the right can
2222 find the header to check if this block is free).
2223 This can be replaced by a single 'free' bit in the header of the following
2224 block (and the tailer only exists in free blocks).
2228 \begin_layout Plain Layout
2229 This technique from Thomas Standish.
2230 Data Structure Techniques.
2231 Addison-Wesley, Reading, Massachusetts, 1980.
2236 The current proposed coalescing algorithm doesn't need this, however.
2239 \begin_layout Standard
2240 This produces a 16 byte used header like this:
2243 \begin_layout LyX-Code
2244 struct tdb_used_record {
2247 \begin_layout LyX-Code
2248 uint32_t used_magic : 16,
2251 \begin_layout LyX-Code
2255 \begin_layout LyX-Code
2259 \begin_layout LyX-Code
2263 \begin_layout LyX-Code
2264 uint32_t extra_octets;
2267 \begin_layout LyX-Code
2268 uint64_t key_and_data_len;
2271 \begin_layout LyX-Code
2275 \begin_layout Standard
2276 And a free record like this:
2279 \begin_layout LyX-Code
2280 struct tdb_free_record {
2283 \begin_layout LyX-Code
2284 uint64_t free_magic: 8,
2287 \begin_layout LyX-Code
2291 \begin_layout LyX-Code
2295 \begin_layout LyX-Code
2296 uint64_t free_table: 8,
2299 \begin_layout LyX-Code
2303 \begin_layout LyX-Code
2307 \begin_layout LyX-Code
2311 \begin_layout Standard
2313 \change_deleted 0 1291206079
2316 Note that by limiting valid offsets to 56 bits, we can pack everything we
2317 need into 3 64-byte words, meaning our minimum record size is 8 bytes.
2320 \begin_layout Subsubsection
2324 \begin_layout Standard
2328 \begin_layout Subsection
2329 Transaction Commit Requires 4 fdatasync
2332 \begin_layout Standard
2333 The current transaction algorithm is:
2336 \begin_layout Enumerate
2337 write_recovery_data();
2340 \begin_layout Enumerate
2344 \begin_layout Enumerate
2345 write_recovery_header();
2348 \begin_layout Enumerate
2352 \begin_layout Enumerate
2353 overwrite_with_new_data();
2356 \begin_layout Enumerate
2360 \begin_layout Enumerate
2361 remove_recovery_header();
2364 \begin_layout Enumerate
2368 \begin_layout Standard
2369 On current ext3, each sync flushes all data to disk, so the next 3 syncs
2370 are relatively expensive.
2371 But this could become a performance bottleneck on other filesystems such
2375 \begin_layout Subsubsection
2379 \begin_layout Standard
2380 Neil Brown points out that this is overzealous, and only one sync is needed:
2383 \begin_layout Enumerate
2384 Bundle the recovery data, a transaction counter and a strong checksum of
2388 \begin_layout Enumerate
2389 Strong checksum that whole bundle.
2392 \begin_layout Enumerate
2393 Store the bundle in the database.
2396 \begin_layout Enumerate
2397 Overwrite the oldest of the two recovery pointers in the header (identified
2398 using the transaction counter) with the offset of this bundle.
2401 \begin_layout Enumerate
2405 \begin_layout Enumerate
2406 Write the new data to the file.
2409 \begin_layout Standard
2410 Checking for recovery means identifying the latest bundle with a valid checksum
2411 and using the new data checksum to ensure that it has been applied.
2412 This is more expensive than the current check, but need only be done at
2414 For running databases, a separate header field can be used to indicate
2415 a transaction in progress; we need only check for recovery if this is set.
2418 \begin_layout Subsubsection
2422 \begin_layout Standard
2426 \begin_layout Subsection
2427 \begin_inset CommandInset label
2429 name "sub:TDB-Does-Not"
2433 TDB Does Not Have Snapshot Support
2436 \begin_layout Subsubsection
2437 Proposed SolutionNone.
2438 At some point you say
2439 \begin_inset Quotes eld
2443 \begin_inset Quotes erd
2447 \begin_inset CommandInset ref
2449 reference "replay-attribute"
2456 \begin_layout Standard
2457 But as a thought experiment, if we implemented transactions to only overwrite
2458 free entries (this is tricky: there must not be a header in each entry
2459 which indicates whether it is free, but use of presence in metadata elsewhere),
2460 and a pointer to the hash table, we could create an entirely new commit
2461 without destroying existing data.
2462 Then it would be easy to implement snapshots in a similar way.
2465 \begin_layout Standard
2466 This would not allow arbitrary changes to the database, such as tdb_repack
2467 does, and would require more space (since we have to preserve the current
2468 and future entries at once).
2469 If we used hash trees rather than one big hash table, we might only have
2470 to rewrite some sections of the hash, too.
2473 \begin_layout Standard
2474 We could then implement snapshots using a similar method, using multiple
2475 different hash tables/free tables.
2478 \begin_layout Subsubsection
2482 \begin_layout Standard
2486 \begin_layout Subsection
2487 Transactions Cannot Operate in Parallel
2490 \begin_layout Standard
2491 This would be useless for ldb, as it hits the index records with just about
2493 It would add significant complexity in resolving clashes, and cause the
2494 all transaction callers to write their code to loop in the case where the
2495 transactions spuriously failed.
2498 \begin_layout Subsubsection
2502 \begin_layout Standard
2504 \begin_inset CommandInset ref
2506 reference "replay-attribute"
2511 We could solve a small part of the problem by providing read-only transactions.
2512 These would allow one write transaction to begin, but it could not commit
2513 until all r/o transactions are done.
2514 This would require a new RO_TRANSACTION_LOCK, which would be upgraded on
2518 \begin_layout Subsubsection
2522 \begin_layout Standard
2526 \begin_layout Subsection
2527 Default Hash Function Is Suboptimal
2530 \begin_layout Standard
2531 The Knuth-inspired multiplicative hash used by tdb is fairly slow (especially
2532 if we expand it to 64 bits), and works best when the hash bucket size is
2533 a prime number (which also means a slow modulus).
2534 In addition, it is highly predictable which could potentially lead to a
2535 Denial of Service attack in some TDB uses.
2538 \begin_layout Subsubsection
2542 \begin_layout Standard
2543 The Jenkins lookup3 hash
2547 \begin_layout Plain Layout
2548 http://burtleburtle.net/bob/c/lookup3.c
2553 is a fast and superbly-mixing hash.
2554 It's used by the Linux kernel and almost everything else.
2555 This has the particular properties that it takes an initial seed, and produces
2556 two 32 bit hash numbers, which we can combine into a 64-bit hash.
2559 \begin_layout Standard
2560 The seed should be created at tdb-creation time from some random source,
2561 and placed in the header.
2562 This is far from foolproof, but adds a little bit of protection against
2566 \begin_layout Subsubsection
2570 \begin_layout Standard
2574 \begin_layout Subsection
2575 \begin_inset CommandInset label
2577 name "Reliable-Traversal-Adds"
2581 Reliable Traversal Adds Complexity
2584 \begin_layout Standard
2585 We lock a record during traversal iteration, and try to grab that lock in
2587 If that grab on delete fails, we simply mark it deleted and continue onwards;
2588 traversal checks for this condition and does the delete when it moves off
2592 \begin_layout Standard
2593 If traversal terminates, the dead record may be left indefinitely.
2596 \begin_layout Subsubsection
2600 \begin_layout Standard
2601 Remove reliability guarantees; see
2602 \begin_inset CommandInset ref
2604 reference "traverse-Proposed-Solution"
2611 \begin_layout Subsubsection
2615 \begin_layout Standard
2619 \begin_layout Subsection
2620 Fcntl Locking Adds Overhead
2623 \begin_layout Standard
2624 Placing a fcntl lock means a system call, as does removing one.
2625 This is actually one reason why transactions can be faster (everything
2626 is locked once at transaction start).
2627 In the uncontended case, this overhead can theoretically be eliminated.
2630 \begin_layout Subsubsection
2634 \begin_layout Standard
2638 \begin_layout Standard
2639 We tried this before with spinlock support, in the early days of TDB, and
2640 it didn't make much difference except in manufactured benchmarks.
2643 \begin_layout Standard
2644 We could use spinlocks (with futex kernel support under Linux), but it means
2645 that we lose automatic cleanup when a process dies with a lock.
2646 There is a method of auto-cleanup under Linux, but it's not supported by
2647 other operating systems.
2648 We could reintroduce a clear-if-first-style lock and sweep for dead futexes
2649 on open, but that wouldn't help the normal case of one concurrent opener
2651 Increasingly elaborate repair schemes could be considered, but they require
2652 an ABI change (everyone must use them) anyway, so there's no need to do
2653 this at the same time as everything else.
2656 \begin_layout Subsection
2657 Some Transactions Don't Require Durability
2660 \begin_layout Standard
2661 Volker points out that gencache uses a CLEAR_IF_FIRST tdb for normal (fast)
2662 usage, and occasionally empties the results into a transactional TDB.
2663 This kind of usage prioritizes performance over durability: as long as
2664 we are consistent, data can be lost.
2667 \begin_layout Standard
2668 This would be more neatly implemented inside tdb: a
2669 \begin_inset Quotes eld
2673 \begin_inset Quotes erd
2676 transaction commit (ie.
2677 syncless) which meant that data may be reverted on a crash.
2680 \begin_layout Subsubsection
2684 \begin_layout Standard
2688 \begin_layout Standard
2689 Unfortunately any transaction scheme which overwrites old data requires
2690 a sync before that overwrite to avoid the possibility of corruption.
2693 \begin_layout Standard
2694 It seems possible to use a scheme similar to that described in
2695 \begin_inset CommandInset ref
2697 reference "sub:TDB-Does-Not"
2701 ,where transactions are committed without overwriting existing data, and
2702 an array of top-level pointers were available in the header.
2703 If the transaction is
2704 \begin_inset Quotes eld
2708 \begin_inset Quotes erd
2711 then we would not need a sync at all: existing processes would pick up
2712 the new hash table and free list and work with that.
2715 \begin_layout Standard
2716 At some later point, a sync would allow recovery of the old data into the
2717 free lists (perhaps when the array of top-level pointers filled).
2718 On crash, tdb_open() would examine the array of top levels, and apply the
2719 transactions until it encountered an invalid checksum.
2722 \begin_layout Subsection
2723 Tracing Is Fragile, Replay Is External
2726 \begin_layout Standard
2727 The current TDB has compile-time-enabled tracing code, but it often breaks
2728 as it is not enabled by default.
2729 In a similar way, the ctdb code has an external wrapper which does replay
2730 tracing so it can coordinate cluster-wide transactions.
2733 \begin_layout Subsubsection
2735 \begin_inset CommandInset label
2737 name "replay-attribute"
2744 \begin_layout Standard
2745 Tridge points out that an attribute can be later added to tdb_open (see
2747 \begin_inset CommandInset ref
2749 reference "attributes"
2753 ) to provide replay/trace hooks, which could become the basis for this and
2754 future parallel transactions and snapshot support.
2757 \begin_layout Subsubsection
2761 \begin_layout Standard