Shulou(Shulou.com)06/01 Report--

< replica, but for now we * don't.) * 检查是否希望执行异步提交. * 如synchronous_commit=off,可以允许异步执行XLOG刷新,或者如果当前事务没有执行 * WAL-logged操作或者不能分配XID. * 如果事务只写入临时和/或unlogged的表，那么即使它有一个xid，它也不会写入任何WAL。 * 如果事务执行HOT pruning,那么可以在没有XID的情况下写入WAL. * 在crash的情况下,此类事务引起的问题将无关紧要;临时表可以随时废弃,unlogged表将被阶段, * 而HOT pruning在稍后将被再次执行. * (鉴于上述情况，您可能认为在本例中根本没有必要发出XLOG记录，但我们目前并不尝试这样做。 * 至少在热备份模式下，它肯定会导致问题，因为在这种模式下，KnownAssignedXids机器需要跟踪每个XID分配。 * 可能只在wal_level < replica时跳过它是可以的，但是现在我们不这样做。) * * However, if we're doing cleanup of any non-temp rels or committing any * command that wanted to force sync commit, then we must flush XLOG * immediately. (We must not allow asynchronous commit if there are any * non-temp tables to be deleted, because we might delete the files before * the COMMIT record is flushed to disk. We do allow asynchronous commit * if all to-be-deleted tables are temporary though, since they are lost * anyway if we crash.) * 但是，如果我们正在清理任何非临时的临时记录或提交想要强制同步提交的命令，那么我们必须立即刷新XLOG。 * (如存在非临时表的删除操作,则不允许异步提交,因为我们可能在COMMIT 记录刷到磁盘前已删除了文件. * 但如果将被删除的是临时表,我们确实可以允许异步提交,因为临时表在crash也会丢弃) */ if ((wrote_xlog && markXidCommitted && synchronous_commit >

SYNCHRONOUS_COMMIT_OFF) | | forceSyncCommit | | nrels > 0) {XLogFlush (XactLastRecEnd); / * * Now we may update the CLOG, if we wrote a COMMIT record above * now let's update CLOG if we have written COMMIT WAL Record on it. * / if (markXidCommitted) TransactionIdCommitTree (xid, nchildren, children);} else {/ / Asynchronous submission / * * Asynchronous commit case: * Asynchronous submission: * * This enables possible committed transaction loss in the case of a * postmaster crash because WAL buffers are left unwritten. Ideally we * could issue the WAL write without the fsync, but some * wal_sync_methods do not allow separate write/fsync. * this may result in the loss of committed transactions in the event of a postmaster crash, * because WAL buffer is not persisted. * ideally, we can issue WAL write without fsync, * but some wal_sync_methods do not allow separate write/fsync. * Report the latest async commit LSN, so that the WAL writer knows to * flush this commit. * report the final asynchronous submission LSN and notify the WAL writer to refresh the commit * / XLogSetAsyncXactLSN (XactLastRecEnd); / * We must not immediately update the CLOG, since we didn't flush the * XLOG. Instead, we store the LSN up to which the XLOG must be * flushed before the CLOG may be updated. * We can't update CLOG right away because we haven't refreshed XLOG yet. * on the contrary, we store LSN until the XLOG must be refreshed before the CLOG may have been updated. * / if (markXidCommitted) TransactionIdAsyncCommitTree (xid, nchildren, children, XactLastRecEnd);} / * If we entered a commit critical section, leave it now, and let * checkpoints proceed. * if you have entered the critical area of commit and have finished your work, you can leave and let checkpoints perform the relevant operations. * / if (markXidCommitted) {MyPgXact- > delayChkpt = false; END_CRIT_SECTION ();} / * Compute latestXid while we have the child XIDs handy * / / if you have a child XIDs, calculate the final latestXid latestXid = TransactionIdLatest (xid, nchildren, children); / * Wait for synchronous replication, if required. Similar to the decision * above about using committing asynchronously we only want to wait if * this backend assigned an xid and wrote WAL. No need to wait if an xid * was assigned due to temporary/unlogged tables or due to HOT pruning. * wait for synchronous replication if necessary. * similar to the above decision to use asynchronous commit, we only want to wait if the process has allocated and written WAL. * temporary / unlog table or HOT pruning, there is no need to wait for whether the transaction ID is allocated. * Note that at this stage we have marked clog, but still show as running * in the procarray and continue to hold locks. * Note that in this scenario, we must mark clog, but it still appears as running in procarray and holds the lock all the time. * / if (wrote_xlog & & markXidCommitted) SyncRepWaitForLSN (XactLastRecEnd, true); / * remember end of last commit record * / / record the location of the last commit record XactLastCommitEnd = XactLastRecEnd; / * Reset XactLastRecEnd until the next transaction writes something * / / reset XactLastRecEnd until the next transaction writes data. XactLastRecEnd = 0 cleanup: / * Clean up local data * / / clear local data if (rels) pfree (rels); / / return XID return latestXid;} 3. Track and analyze

Insert data, execute commit

10:57:56 (xdb@ [local]: 5432) testdb=# begin;BEGIN10:57:59 (xdb@ [local]: 5432) testdb=#* insert into t_session1 values (1); INSERT 0 110 local 58 01 (xdb@ [local]: 5432) testdb=#* commit

Start gdb and set breakpoint

(gdb) b RecordTransactionCommitBreakpoint 2 at 0x547528: file xact.c, line 1141. (gdb) cContinuing.Breakpoint 2, RecordTransactionCommit () at xact.c:11411141 TransactionId xid = GetTopTransactionIdIfAny (); (gdb)

View the call stack

(gdb) bt#0 RecordTransactionCommit () at xact.c:1141#1 0x00000000005483f2 in CommitTransaction () at xact.c:2070#2 0x0000000000549078 in CommitTransactionCommand () at xact.c:2831#3 0x00000000008c8ea9 in finish_xact_command () at postgres.c:2523#4 0x00000000008c6b5d in exec_simple_query (query_string=0x2c97ec8 "commit ") at postgres.c:1170#5 0x00000000008cae70 in PostgresMain (argc=1, argv=0x2cc3dc8, dbname=0x2cc3c30" testdb ", username=0x2c94ba8" xdb ") at postgres.c:4182#6 0x000000000082642b in BackendRun (port=0x2cb9c00) at postmaster.c:4361#7 0x0000000000825b8f in BackendStartup (port=0x2cb9c00) at postmaster.c:4033#8 0x0000000000821f1c in ServerLoop () at postmaster.c:1706#9 0x00000000008217b4 in PostmasterMain (argc=1, argv=0x2c92b60) at postmaster.c:1379#10 0x00000000007488ef in main (argc=1, argv=0x2c92b60) at main.c:228 (gdb)

Get transaction ID

(gdb) p xid$3 = 2411 (gdb)

Set other variables, markXidCommitted-> True

(gdb) n1143 TransactionId latestXid = InvalidTransactionId; (gdb) 1148 int nmsgs = 0; (gdb) 1149 SharedInvalidationMessage * invalMessages = NULL; (gdb) 1150 bool RelcacheInitFileInval = false; (gdb) 1154 nrels = smgrGetPendingDeletes (true, & rels); (gdb) 1155 nchildren = xactGetCommittedChildren (& children); (gdb) 1156 if (XLogStandbyInfoActive ()) (gdb) 1159 wrote_xlog = (XactLastRecEnd! = 0) (gdb) 1165 if (! markXidCommitted) (gdb) p latestXid$4 = 0 (gdb) p markXidCommitted$5 = true (gdb) p nrels$6 = 0 (gdb) p nchildren$7 = 0 (gdb) p wrote_xlog$8 = true (gdb)

MarkXidCommitted is T, enter the corresponding processing logic.

Begin to enter the key part of the submission and insert the commit XLOG record.

(gdb) n1214 replorigin = (replorigin_session_origin! = InvalidRepOriginId & & (gdb) 1221 BufmgrCommit (); (gdb) p replorigin$9 = false (gdb)

Enter the commit section and set the current transaction timestamp

(gdb) n1240 START_CRIT_SECTION (); (gdb) 1241 MyPgXact- > delayChkpt = true; (gdb) 1243 SetCurrentTransactionStopTimestamp (); (gdb) p * MyPgXact$10 = {xid = 2411, xmin = 0, vacuumFlags = 0'\ 000mm, overflowed = false, delayChkpt = true, nxids = 0'\ 000'} (gdb)

Insert XLOG

(gdb) n1245 XactLogCommitRecord (xactStopTimestamp, (gdb) 1252 if (replorigin) (gdb))

Set up commit transaction data

(gdb) 1267 if (! replorigin | | replorigin_session_origin_timestamp = = 0) (gdb) 1268 replorigin_session_origin_timestamp = xactStopTimest (gdb) 1270 TransactionTreeSetCommitTsData (xid, nchildren, children, (gdb) 1300 if ((wrote_xlog & markXidCommitted & & (gdb))

Synchronously refresh XLOG

(gdb) 1301 synchronous_commit > SYNCHRONOUS_COMMIT_OFF) | | (gdb) 1300 if ((wrote_xlog & markXidCommitted & & (gdb) 1304 XLogFlush (XactLastRecEnd); (gdb) 1309 if (markXidCommitted) (gdb))

Update CLOG if we have written COMMIT WAL Record.

(gdb) 1310 TransactionIdCommitTree (xid, nchildren, children); (gdb) 1309 if (markXidCommitted) (gdb)

Exit the critical area of submission

(gdb) 1340 if (markXidCommitted) (gdb) 1342 MyPgXact- > delayChkpt = false; (gdb) 1343 END_CRIT_SECTION (); (gdb)

Calculate the final latestXid

(gdb) 1347 latestXid = TransactionIdLatest (xid, nchildren, children); (gdb) n1358 if (wrote_xlog & & markXidCommitted) (gdb) p latestXid$11 = 2411 (gdb)

Record the location of the last commit record

(gdb) n1359 SyncRepWaitForLSN (XactLastRecEnd, true); (gdb) 1362 XactLastCommitEnd = XactLastRecEnd; (gdb) 1365 XactLastRecEnd = 0; (gdb) 1368 if (rels) (gdb) 1371 return latestXid; (gdb) p XactLastCommitEnd$12 = 5522364896 (gdb)

Return, complete the call

(gdb) n1372} (gdb) CommitTransaction () at xact.c:20872087 TRACE_POSTGRESQL_TRANSACTION_COMMIT (MyProc- > lxid); (gdb)

DONE!

IV. Reference materials

How Postgres Makes Transactions Atomic

PG Source Code

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