PG中有个可以打印WAL日志信息的工具，pg_waldump，对于开发人员来说，掌握该工具是十分有必要的。
用法
具体用法这里不去详解，可见官网pg_waldump文档。
postgres@slpc:~$ pg_waldump --help pg_waldump decodes and displays PostgreSQL write-ahead logs for debugging. Usage: pg_waldump [OPTION]... [STARTSEG [ENDSEG]] Options: -b, --bkp-details output detailed information about backup blocks -e, --end=RECPTR stop reading at WAL location RECPTR -f, --follow keep retrying after reaching end of WAL -n, --limit=N number of records to display -p, --path=PATH directory in which to find log segment files or a directory with a ./pg_wal that contains such files (default: current directory, ./pg_wal, $PGDATA/pg_wal) -q, --quiet do not print any output, except for errors -r, --rmgr=RMGR only show records generated by resource manager RMGR; use --rmgr=list to list valid resource manager names -s, --start=RECPTR start reading at WAL location RECPTR -t, --timeline=TLI timeline from which to read log records (default: 1 or the value used in STARTSEG) -V, --version output version information, then exit -x, --xid=XID only show records with transaction ID XID -z, --stats[=record] show statistics instead of records (optionally, show per-record statistics) -?, --help show this help, then exit
具体的，我们举一个例子：
postgres=# create table t1(a int);
CREATE TABLE
postgres=# select pg_current_wal_lsn();
pg_current_wal_lsn
--------------------
0/1696C78
(1 row)
-- 插入一条数据
postgres=# insert into t1 values(1);
INSERT 0 1
postgres=# select pg_current_wal_insert_lsn();
pg_current_wal_insert_lsn
---------------------------
0/1696D18
(1 row)
postgres=# select txid_current();
txid_current
--------------
735
(1 row)
-- 查看WAL日志
postgres@slpc:~/pgsql$ pg_waldump -p pgdata/pg_wal/ -s 0/1696C78 -e 0/1696D18
-- 插入一条数据，堆表， 事务ID 735
rmgr: Heap len (rec/tot): 59/ 59, tx: 734, lsn: 0/01696C78, prev 0/01696C40, desc: INSERT+INIT off 1 flags 0x00, blkref #0: rel 1663/13010/16384 blk 0
rmgr: Transaction len (rec/tot): 34/ 34, tx: 734, lsn: 0/01696CB8, prev 0/01696C78, desc: COMMIT 2023-09-23 10:58:39.915785 CST
rmgr: Standby len (rec/tot): 54/ 54, tx: 0, lsn: 0/01696CE0, prev 0/01696CB8, desc: RUNNING_XACTS nextXid 735 latestCompletedXid 733 oldestRunningXid 734; 1 xacts: 734
-- 设置wal_debug = on
postgres=# set wal_debug = on;
SET
postgres=# select pg_current_wal_lsn();
pg_current_wal_lsn
--------------------
0/1696E28
(1 row)
postgres=# insert into t1 values(2);
LOG: INSERT @ 0/1696EC0: - Heap/INSERT: off 2 flags 0x00
LOG: INSERT @ 0/1696EE8: - Transaction/COMMIT: 2023-09-23 11:27:25.399199+08
LOG: xlog flush request 0/1696EE8; write 0/1696E28; flush 0/1696E28
INSERT 0 1
postgres=# select pg_current_wal_lsn();
pg_current_wal_lsn
--------------------
0/1696F20
(1 row)
postgres@slpc:~/pgsql$ pg_waldump -p pgdata/pg_wal/ -s 0/1696E28 -e 0/1696F20
rmgr: Heap len (rec/tot): 54/ 150, tx: 736, lsn: 0/01696E28, prev 0/01696DF0, desc: INSERT off 2 flags 0x00, blkref #0: rel 1663/13010/16384 blk 0 FPW
rmgr: Transaction len (rec/tot): 34/ 34, tx: 736, lsn: 0/01696EC0, prev 0/01696E28, desc: COMMIT 2023-09-23 11:27:25.399199 CST
rmgr: Standby len (rec/tot): 50/ 50, tx: 0, lsn: 0/01696EE8, prev 0/01696EC0, desc: RUNNING_XACTS nextXid 737 latestCompletedXid 736 oldestRunningXid 737
pg_waldump源码分析
pg_waldump的源码比较简单，在src/bin/pg_waldump中， 执行pg_waldump -p pgdata/pg_wal/ -s 0/1696E28 -e 0/1696F20进行调试。
main(int argc, char **argv)
--> XLogReaderAllocate(WalSegSz, waldir,XL_ROUTINE(.page_read = WALDumpReadPage,.segment_open = WALDumpOpenSegment, .segment_close = WALDumpCloseSegment), &private);
// 因为我们是读wal日志的一个范围，从起点到终点，所以，这里第1步要做的就是先找到起始的wal日志记录
--> XLogFindNextRecord(xlogreader_state, private.startptr);
for (;;)
{
// 开始读取后续WAL日志
/* try to read the next record */
record = XLogReadRecord(xlogreader_state, &errormsg);
/* perform any per-record work */
if (!config.quiet)
{
if (config.stats == true)
XLogDumpCountRecord(&config, &stats, xlogreader_state);
else
XLogDumpDisplayRecord(&config, xlogreader_state);
}
/* check whether we printed enough */
config.already_displayed_records++;
if (config.stop_after_records > 0 &&
config.already_displayed_records >= config.stop_after_records)
break;
}
--> XLogReaderFree(xlogreader_state);
更详细的代码可以查看：
int main(int argc, char **argv)
{
uint32 xlogid;
uint32 xrecoff;
XLogReaderState *xlogreader_state;
XLogDumpPrivate private;
XLogDumpConfig config;
XLogDumpStats stats;
XLogRecord *record;
XLogRecPtr first_record;
private.timeline = 1;
private.startptr = InvalidXLogRecPtr; // 起点
private.endptr = InvalidXLogRecPtr; // 终点
private.endptr_reached = false;
while ((option = getopt_long(argc, argv, "be:fn:p:qr:s:t:x:z", long_options, &optindex)) != -1)
{
switch (option)
{
case 'e':
if (sscanf(optarg, "%X/%X", &xlogid, &xrecoff) != 2)
{
pg_log_error("could not parse end WAL location \"%s\"",
optarg);
goto bad_argument;
}
private.endptr = (uint64) xlogid << 32 | xrecoff;
break;
case 's':
if (sscanf(optarg, "%X/%X", &xlogid, &xrecoff) != 2)
{
pg_log_error("could not parse start WAL location \"%s\"",
optarg);
goto bad_argument;
}
else
private.startptr = (uint64) xlogid << 32 | xrecoff;
break;
// ...
}
}
// ...
/* done with argument parsing, do the actual work */
/* we have everything we need, start reading */
// 初始化XLogReader，分配读取所需的内存空间
xlogreader_state = XLogReaderAllocate(WalSegSz, waldir, XL_ROUTINE(.page_read = WALDumpReadPage,
.segment_open = WALDumpOpenSegment,
.segment_close = WALDumpCloseSegment), &private);
/* first find a valid recptr to start from */
// 找到第一个lsn >= startptr的有效日志记录
first_record = XLogFindNextRecord(xlogreader_state, private.startptr);
if (first_record == InvalidXLogRecPtr)
fatal_error("could not find a valid record after %X/%X",
LSN_FORMAT_ARGS(private.startptr));
/*
* Display a message that we're skipping data if `from` wasn't a pointer
* to the start of a record and also wasn't a pointer to the beginning of
* a segment (e.g. we were used in file mode).
*/
if (first_record != private.startptr &&
XLogSegmentOffset(private.startptr, WalSegSz) != 0)
printf(ngettext("first record is after %X/%X, at %X/%X, skipping over %u byte\n",
"first record is after %X/%X, at %X/%X, skipping over %u bytes\n",
(first_record - private.startptr)),
LSN_FORMAT_ARGS(private.startptr),
LSN_FORMAT_ARGS(first_record),
(uint32) (first_record - private.startptr));
for (;;)
{
/* try to read the next record */
record = XLogReadRecord(xlogreader_state, &errormsg);
if (!record)
{
if (!config.follow || private.endptr_reached)
break;
else
{
pg_usleep(1000000L); /* 1 second */
continue;
}
}
/* apply all specified filters */
if (config.filter_by_rmgr != -1 &&
config.filter_by_rmgr != record->xl_rmid)
continue;
if (config.filter_by_xid_enabled &&
config.filter_by_xid != record->xl_xid)
continue;
/* perform any per-record work */
if (!config.quiet)
{
if (config.stats == true)
XLogDumpCountRecord(&config, &stats, xlogreader_state);
else
XLogDumpDisplayRecord(&config, xlogreader_state);
}
/* check whether we printed enough */
config.already_displayed_records++;
if (config.stop_after_records > 0 &&
config.already_displayed_records >= config.stop_after_records)
break;
}
if (config.stats == true && !config.quiet)
XLogDumpDisplayStats(&config, &stats);
if (errormsg)
fatal_error("error in WAL record at %X/%X: %s",
LSN_FORMAT_ARGS(xlogreader_state->ReadRecPtr),
errormsg);
XLogReaderFree(xlogreader_state);
return EXIT_SUCCESS;
bad_argument:
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
return EXIT_FAILURE;
}
参考文档：
pg_waldump
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