PostgreSQL的backuplabel file 初步学习

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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 * problem that pg_control might have been archived one or more checkpoints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[];                        

    *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) !=  || ch != '\n')
        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) !=  || ch != '\n')
        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) == )
    {
        if (strcmp(backuptype, "streamed") == )
            *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, );                                    

        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, );
        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, );
            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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磨砺技术珠矶，践行数据之道，追求卓越价值