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看代码：
/* File path names (all relative to $PGDATA) */
#define BACKUP_LABEL_FILE backup_label
#define BACKUP_LABEL_OLD backup_label.old
在特定条件下，会有一个文件，名为 backup_label
在StartupXLOG执行时，通过 read_backup_label 函数来进行读取：
/*
* read_backup_label: check to see if a backup_label file is present
* If we see a backup_label during recovery, we assume that we are recovering
* from a backup dump file, and we therefore roll forward from the checkpoint
* identified by the label file, NOT what pg_control says. This avoids the
* problem that pg_control might have been archived one or more checkpoints
* later than the start of the dump, and so if we rely on it as the start
* point, we will fail to restore a consistent database state.
*
* Returns TRUE if a backup_label was found (and fills the checkpoint
* location and its REDO location into *checkPointLoc and RedoStartLSN,
* respectively); returns FALSE if not. If this backup_label came from a
* streamed backup, *backupEndRequired is set to TRUE.
*/
static bool
read_backup_label(XLogRecPtr *checkPointLoc, bool *backupEndRequired)
{
char startxlogfilename[MAXFNAMELEN];
TimeLineID tli;
FILE *lfp;
char ch;
char backuptype[20];
*backupEndRequired = false;
/*
* See if label file is present
*/
lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
if (!lfp)
{
if (errno != ENOENT)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not read file "%s": %m",
BACKUP_LABEL_FILE)));
return false; /* it's not there, all is fine */
}
/*
* Read and parse the START WAL LOCATION and CHECKPOINT lines (this code
* is pretty crude, but we are not expecting any variability in the file
* format).
*/
if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %08X%16s)%c",
&RedoStartLSN.xlogid, &RedoStartLSN.xrecoff, &tli,
startxlogfilename, &ch) != 5 || ch != '
')
ereport(FATAL,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("invalid data in file "%s"", BACKUP_LABEL_FILE)));
if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
&checkPointLoc->xlogid, &checkPointLoc->xrecoff,
&ch) != 3 || ch != '
')
ereport(FATAL,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("invalid data in file "%s"", BACKUP_LABEL_FILE)));
/*
* BACKUP METHOD line didn't exist in 9.1beta3 and earlier, so don't
* error out if it doesn't exist.
*/
if (fscanf(lfp, "BACKUP METHOD: %19s", backuptype) == 1)
{
if (strcmp(backuptype, "streamed") == 0)
*backupEndRequired = true;
}
if (ferror(lfp) || FreeFile(lfp))
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not read file "%s": %m",
BACKUP_LABEL_FILE)));
return true;
}
看StarupXLOG函数：
/*
* This must be called ONCE during postmaster or standalone-backend startup
*/
void
StartupXLOG(void)
{
…
if (read_backup_label(&checkPointLoc, &backupEndRequired))
{
/*
* When a backup_label file is present, we want to roll forward from
* the checkpoint it identifies, rather than using pg_control.
*/
record = ReadCheckpointRecord(checkPointLoc, 0);
if (record != NULL)
{
memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
ereport(DEBUG1,
(errmsg("checkpoint record is at %X/%X",
checkPointLoc.xlogid, checkPointLoc.xrecoff)));
InRecovery = true; /* force recovery even if SHUTDOWNED */
/*
* Make sure that REDO location exists. This may not be the case
* if there was a crash during an online backup, which left a
* backup_label around that references a WAL segment that's
* already been archived.
*/
if (XLByteLT(checkPoint.redo, checkPointLoc))
{
if (!ReadRecord(&(checkPoint.redo), LOG, false))
ereport(FATAL,
(errmsg("could not find redo location referenced by checkpoint record"),
errhint("If you are not restoring from a backup,
try removing the file "%s/backup_label".", DataDir)));
}
}
else
{
ereport(FATAL,
(errmsg("could not locate required checkpoint record"),
errhint("If you are not restoring from a backup,
try removing the file "%s/backup_label".", DataDir)));
wasShutdown = false; /* keep compiler quiet */
}
/* set flag to delete it later */
haveBackupLabel = true;
}
else
{
/*
* Get the last valid checkpoint record. If the latest one according
* to pg_control is broken, try the next-to-last one.
*/
checkPointLoc = ControlFile->checkPoint;
RedoStartLSN = ControlFile->checkPointCopy.redo;
record = ReadCheckpointRecord(checkPointLoc, 1);
if (record != NULL)
{
ereport(DEBUG1,
(errmsg("checkpoint record is at %X/%X",
checkPointLoc.xlogid, checkPointLoc.xrecoff)));
}
else if (StandbyMode)
{
/*
* The last valid checkpoint record required for a streaming
* recovery exists in neither standby nor the primary.
*/
ereport(PANIC,
(errmsg("could not locate a valid checkpoint record")));
}
else
{
checkPointLoc = ControlFile->prevCheckPoint;
record = ReadCheckpointRecord(checkPointLoc, 2);
if (record != NULL)
{
ereport(LOG,
(errmsg("using previous checkpoint record at %X/%X",
checkPointLoc.xlogid, checkPointLoc.xrecoff)));
InRecovery = true; /* force recovery even if SHUTDOWNED */
}
else
ereport(PANIC,
(errmsg("could not locate a valid checkpoint record")));
}
memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
}
…
}
通过运行发现，上一次正常关闭或者崩溃，都不会产生 backup_label 文件。
关于这个文件，大家都是这么说的：
http://jsoc.stanford.edu/production/postgres_backup_restore.html.bk!
http://raghavt.blogspot.com/2011/05/postgresql-90-backup-recovery.html
http://www.network-theory.co.uk/docs/postgresql/vol3/MakingaBaseBackup.html
就是：pg_start_backup，会生成 backup_label文件。pg_stop_backup，会删除backup_label文件。
而如果StartupXLOG函数运行时，发现了backup_label文件，那么意味着它处正在从online backup中恢复的过程中。
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