6_PostgreSQL WAL文件名解析

内容概述

WAL 是"Write Ahead Logging"简称，和Oracle数据库中的Redo Log功能一样，用于记录数据库中的事务日志，本小节解析WAL文件名？

WAL存储位置

WAL文件存储在 $PGDATA/pg_wal目录下，默认大小16M,文件名由24位16进制数字组成。
[postgres@enmo pgdata]$ ls -ltr $PGDATA/pg_wal/
total 1048576
drwx------ 2 postgres dba        6 Oct 25 11:08 archive_status
-rw------- 1 postgres dba 16777216 Oct 25 11:21 00000001000000020000007A
-rw------- 1 postgres dba 16777216 Oct 25 11:21 00000001000000020000007B
-rw------- 1 postgres dba 16777216 Oct 25 11:21 00000001000000020000007C
-rw------- 1 postgres dba 16777216 Oct 25 11:21 00000001000000020000007D
-rw------- 1 postgres dba 16777216 Oct 25 11:21 00000001000000020000007E
-rw------- 1 postgres dba 16777216 Oct 25 11:21 00000001000000020000007F
-rw------- 1 postgres dba 16777216 Oct 25 11:21 000000010000000200000080
-rw------- 1 postgres dba 16777216 Oct 25 11:21 000000010000000200000081
-rw------- 1 postgres dba 16777216 Oct 25 11:21 000000010000000200000082
-rw------- 1 postgres dba 16777216 Oct 25 11:26 000000010000000200000083
[postgres@enmo pgdata]$ 

获取当前WAL日志

postgres=# select pg_current_wal_lsn();
 pg_current_wal_lsn 
--------------------
 2/830045B0
(1 row)

postgres=# select pg_walfile_name('2/830045B0');
     pg_walfile_name      
--------------------------
 000000010000000200000083
(1 row)

postgres=# SELECT pg_walfile_name(pg_current_wal_lsn());
     pg_walfile_name      
--------------------------
 000000010000000200000083
(1 row)

postgres=# 

如何通过LSN获取WAL文件名

timelineID: 时间线，前32bit长的16进制数，类似于Oracle中的incarnation。
LogId: 中间32bit长的16进制数，实际为LSN的高32bit。LogSeg进位时加1。
LogSeg: 后32bit长的16进制数，实际为LSN的低32bit再除以WAL大小的结果。取值为000000-0000FF

pg_walfile_name源码执行过程

pg_walfile_name函数

 * Compute an xlog file name given a WAL location,
 * such as is returned by pg_backup_stop() or pg_switch_wal().
 */
Datum
pg_walfile_name(PG_FUNCTION_ARGS)
{
	XLogSegNo	xlogsegno;
    //根据输入LSN计算locationpoint: typedef uint64 XLogRecPtr;
	XLogRecPtr	locationpoint = PG_GETARG_LSN(0);  
	char		xlogfilename[MAXFNAMELEN];

	if (RecoveryInProgress())
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("recovery is in progress"),
				 errhint("%s cannot be executed during recovery.",
						 "pg_walfile_name()")));

//根据locationpoint和wal_segment_size求xlogsegno值
	XLByteToPrevSeg(locationpoint, xlogsegno, wal_segment_size); 
    
    //根据xlogsegno/wal_segment_size/GetWALInsertionTimeLine()，获取xlogfilename
	XLogFileName(xlogfilename, GetWALInsertionTimeLine(), xlogsegno,
				 wal_segment_size);

	PG_RETURN_TEXT_P(cstring_to_text(xlogfilename));
}

GetWALInsertionTimeLine函数

/*
 * GetWALInsertionTimeLine -- Returns the current timeline of a system that
 * is not in recovery.
 */
TimeLineID
GetWALInsertionTimeLine(void)
{
	Assert(XLogCtl->SharedRecoveryState == RECOVERY_STATE_DONE);

	/* Since the value can't be changing, no lock is required. */
	return XLogCtl->InsertTimeLineID;
}

XLogFileName函数

/*
 * Generate a WAL segment file name.  Do not use this macro in a helper
 * function allocating the result generated.
 */
#define XLogFileName(fname, tli, logSegNo, wal_segsz_bytes)	\
	snprintf(fname, MAXFNAMELEN, "%08X%08X%08X", tli,		\
			 (uint32) ((logSegNo) / XLogSegmentsPerXLogId(wal_segsz_bytes)), \
			 (uint32) ((logSegNo) % XLogSegmentsPerXLogId(wal_segsz_bytes)))