背景
当我们使用了多个ZFS卷或者文件系统时，如果一个实例的多个部分，如表空间，放在了不同的zfs上，再使用基于ZFS快照的备份时，可能出现多个文件系统不一致的情况。
例如控制文件是新的，但是数据是旧的。
保物理备份的一致性检查
基于文件的物理备份，为了保证备份的一致性，在备份开始时，需要做一个检查点，同时打开FULL PAGE WRTIE，同时还会生成backup_label文件记录备份开始时的WAL文件，检查点位置等信息。
backup_label文件内容示例
START WAL LOCATION: 42/EB000060 (file 0000000100000042000000EB)
CHECKPOINT LOCATION: 42/EB000098
BACKUP METHOD: pg_start_backup
BACKUP FROM: master
START TIME: 2016-08-23 19:43:32 CST
LABEL: a 同时backup_label也指出了当前备份是从主节点还是从 standby备份的
使用read_backup_label解析backup_label文件
优先从backup_label中得到备份开始的WAL位置，检查点的WAL位置，进行恢复。
而不是控制文件的信息，因为控制文件可能在备份过程中，经过检查点后，发生了变化。
/*
* 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* problem that pg_control might have been archived one or more checkpoints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. If this backup_label
* was created during recovery, *backupFromStandby is set to TRUE.
*/
static bool
read_backup_label(XLogRecPtr *checkPointLoc, bool *backupEndRequired,
bool *backupFromStandby)
{
char startxlogfilename[MAXFNAMELEN];
TimeLineID tli;
FILE *lfp;
char ch;
char backuptype[20];
char backupfrom[20];
uint32 hi,
lo;
*backupEndRequired = false;
*backupFromStandby = 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",
&hi, &lo, &tli, startxlogfilename, &ch) != 5 || ch != '\n')
ereport(FATAL,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
RedoStartLSN = ((uint64) hi) << 32 | lo;
if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
&hi, &lo, &ch) != 3 || ch != '\n')
ereport(FATAL,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
*checkPointLoc = ((uint64) hi) << 32 | lo;
/*
* BACKUP METHOD and BACKUP FROM lines are new in 9.2. We can't restore
* from an older backup anyway, but since the information on it is not
* strictly required, don't error out if it's missing for some reason.
*/
if (fscanf(lfp, "BACKUP METHOD: %19s\n", backuptype) == 1)
{
if (strcmp(backuptype, "streamed") == 0)
*backupEndRequired = true;
}
if (fscanf(lfp, "BACKUP FROM: %19s\n", backupfrom) == 1)
{
if (strcmp(backupfrom, "standby") == 0)
*backupFromStandby = true;
}
if (ferror(lfp) || FreeFile(lfp))
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not read file \"%s\": %m",
BACKUP_LABEL_FILE)));
return true;
} 两种物理备份模式
物理备份分exclusive和non-exclusive两种模式。
* There are two kind of backups: exclusive and non-exclusive. An exclusive
* backup is started with pg_start_backup(), and there can be only one active
* at a time. The backup and tablespace map files of an exclusive backup are
* written to $PGDATA/backup_label and $PGDATA/tablespace_map, and they are
* removed by pg_stop_backup().
*
* A non-exclusive backup is used for the streaming base backups (see
* src/backend/replication/basebackup.c). The difference to exclusive backups
* is that the backup label and tablespace map files are not written to disk.
* Instead, their would-be contents are returned in *labelfile and *tblspcmapfile,
* and the caller is responsible for including them in the backup archive as
* 'backup_label' and 'tablespace_map'. There can be many non-exclusive backups
* active at the same time, and they don't conflict with an exclusive backup
* either.
*
* tblspcmapfile is required mainly for tar format in windows as native windows
* utilities are not able to create symlinks while extracting files from tar.
* However for consistency, the same is used for all platforms.
*
* needtblspcmapfile is true for the cases (exclusive backup and for
* non-exclusive backup only when tar format is used for taking backup)
* when backup needs to generate tablespace_map file, it is used to
* embed escape character before newline character in tablespace path.
*
* Returns the minimum WAL position that must be present to restore from this
* backup, and the corresponding timeline ID in *starttli_p.
*
* Every successfully started non-exclusive backup must be stopped by calling
* do_pg_stop_backup() or do_pg_abort_backup().
*
* It is the responsibility of the caller of this function to verify the
* permissions of the calling user! exclusive模式是使用pg_statrt_backup()函数调用产生的，这个函数只能在主节点调用
在备节点调用报错如下
XLogRecPtr
do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
char **labelfile, DIR *tblspcdir, List **tablespaces,
char **tblspcmapfile, bool infotbssize,
bool needtblspcmapfile)
{
bool exclusive = (labelfile == NULL);
...
/*
* Currently only non-exclusive backup can be taken during recovery.
*/
if (backup_started_in_recovery && exclusive)
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("recovery is in progress"),
errhint("WAL control functions cannot be executed during recovery."))); 在standby备份如何保证输出backup_label
在standby备份，不能通过直接调用pg_start_backup()函数来生成backup_label文件，但是PG提供了pg_basebackup命令，是以non-exclusive的模式进行备份的。
labelfile非空，所以不会触发以上告警。
startptr = do_pg_start_backup(opt->label, opt->fastcheckpoint, &starttli,
&labelfile, tblspcdir, &tablespaces,
&tblspc_map_file,
opt->progress, opt->sendtblspcmapfile); 这个labelfile是在最开始的时候备份的，最后备份控制文件。
if (ti->path == NULL)
{
struct stat statbuf;
/* In the main tar, include the backup_label first... */
sendFileWithContent(BACKUP_LABEL_FILE, labelfile);
/*
* Send tablespace_map file if required and then the bulk of
* the files.
*/
if (tblspc_map_file && opt->sendtblspcmapfile)
{
sendFileWithContent(TABLESPACE_MAP, tblspc_map_file);
sendDir(".", 1, false, tablespaces, false);
}
else
sendDir(".", 1, false, tablespaces, true);
/* ... and pg_control after everything else. */
if (lstat(XLOG_CONTROL_FILE, &statbuf) != 0)
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not stat control file \"%s\": %m",
XLOG_CONTROL_FILE)));
sendFile(XLOG_CONTROL_FILE, XLOG_CONTROL_FILE, &statbuf, false);
}
else
sendTablespace(ti->path, false); 通过以上方法，我们备份时的检查点信息得以保存在backup_label中，因此pg_basebackup的备份集，在恢复时可以拿到正确的WAL位置进行恢复，而不需要用到控制文件的检查点位置。
因为控制文件可能不是最初的位置。
zfs快照, 如何保证多个zfs文件系统的一致性
其实从pg_basebackup的备份步骤我们就能得知，备份集需要一个最老的检查点位置。
那我们不使用pg_basebackup，如何能保证在整个备份过程中控制文件是最老的呢？
因此最靠谱的方法是，优先给控制文件所在的ZFS打快照，然后再给其他ZFS文件系统打快照，这样就很好的解决了需要最老的控制文件的这个问题。
例子
回到我写的《PostgreSQL 最佳实践 - 块级增量备份(ZFS篇)方案与实战》
https://yq.aliyun.com/articles/59363
以上案例中一个standby集群用的是一个ZFS(所有表空间, 以及$PGDATA), 当数据库集群使用多个ZFS时, 因为快照只能基于单个volume或filesystem产生, 所以当我们使用了多个zfs filesystem是要得到数据库一致的备份, 怎么办呢?
例如 :
1. $PGDATA使用文件系统 zfs/pg_root
2. tbs1使用文件系统 zfs/tbs1
3. tbs2使用文件系统 zfs/tbs2
4. pg_xlog使用文件系统 zfs/pg_xlog
要使用zfs snapshot来备份这个PostgreSQL, 应该怎么做?
前面已经从代码中分析了，我们需要最老的控制文件即可。
首选创建控制文件所在zfs的快照, 再创建其他文件系统的快照.
pg_xlog的快照可以不创建, 而通过归档来恢复.
那么就比较清晰了, 以上快照的步骤如下 :
STIME=`date +%F%T`
1. zfs snapshot zp1/pg_root@$STIME
2. zfs snapshot zp1/tbs1@$STIME
3. zfs snapshot zp1/tbs2@$STIME 验证
验证一下这种方法的有效性
还记得我以前写过一篇使用老的控制文件来恢复一个异常的standby吗?
某数据库的流复制standby因为主库产生的XLOG过多, 延迟后触发了recovery.conf的restore命令, 但是restore里面用了sudo 进行copy并未保持源文件的owner, 属性等特征, 使用了root owner, 导致COPY完的xlog不能被postgresql 正常读取.
同事在处理这个事情, 因为没有了解实际情况, 上去就使用了pg_resetxlog修改standby的nextXID(resetxlog在这个场景属于扯淡的操作, 千万不要这么干), 接着大家知道的, standby无法正常完成standby的工作了.
这个数据库有10几个T, 要重新做standby的话, 拷贝的数据量太大了, 即使使用rsync重做, 工作量也比较大(表空间过多, 目录过多).
有没有省事的方法呢?
首先来分析一下事件,
1\. xlog不能被postgresql 正常读取(这个很好修复, 修改一下restore command就可以了, 或者不要用sudo 来拷贝). 2014-11-30 10:05:54.505 CST,,,3695,,5435f48b.e6f,6,,2014-10-09 10:35:55 CST,1/0,0,LOG,00000,"restored log file ""0000000300002A9D0000004D"" from archive",,,,,,,,"RestoreArchivedFile, xlogarchive.c:254",""
2014-11-30 10:05:54.534 CST,,,3695,,5435f48b.e6f,7,,2014-10-09 10:35:55 CST,1/0,0,PANIC,42501,"could not open file ""pg_xlog/0000000300002A9D0000004D"": Permission denied",,,,,,,,"XLogFileRead, xlog.c:2696","" ```
2\. pg_resetxlog对控制文件造成了持久性伤害, 没有办法修复. pg_resetxlog -x 90010254 -f $PGDATA 为了快速恢复, 可以找到更早的控制文件, 刚好这套数据库的备份系统使用的是ZFS快照做的, 每天会创建一个快照, 所以可以取早些时间的快照, 拿到控制文件, 替换掉standby被人为"破坏"的控制文件, 启动standby后, 将从控制文件开始恢复需要的xlog. (只要这些xlog归档还在就可以了).
操作过程 :
1\. shutdown standby
2\. mount zfs snapshot old then standby crashed time.
3\. copy $PGDATA/global/pg_control from snapshot to standby.
4\. startup standby.
5\. umount snapshot.
注意, 拿到的控制文件必须是standby crash之前的控制文件, 并且控制文件至今的所有xlog归档必须都在. postgresql standby将从控制文件需要的xlog开始恢复数据块. 所以这么做是完全可以的.
现在standby数据库已经完全恢复了 882 postgres 20 0 2337m 15m 14m S 0.0 0.1 0:00.10 /opt/pgsql9.3.2/bin/postgres
883 postgres 20 0 157m 1072 476 S 0.0 0.0 0:00.14 postgres: logger process
884 postgres 20 0 2338m 2.0g 2.0g S 0.0 8.6 5:16.35 postgres: startup process recovering 0000000300002AA50000003F
889 postgres 20 0 2338m 2.0g 2.0g S 0.0 8.6 0:35.69 postgres: checkpointer process
890 postgres 20 0 2338m 2.0g 2.0g S 0.0 8.6 0:34.98 postgres: writer process
905 postgres 20 0 159m 1212 456 S 0.0 0.0 0:02.62 postgres: stats collector process
1922 postgres 20 0 105m 1632 1180 S 0.0 0.0 0:00.01 -bash
1959 postgres 20 0 137m 1764 1292 S 0.0 0.0 0:00.00 psql
1960 postgres 20 0 2340m 6780 4684 S 0.0 0.0 0:00.01 postgres: postgres postgres [local] idle
4309 postgres 20 0 2346m 7080 1748 S 0.0 0.0 0:00.67 postgres: wal receiver process streaming 2AA5/3F0429C0
