标签
PostgreSQL , checkpoint , standby , CreateRestartPoint
背景
当数据库异常关闭时，数据库关闭时来不及或根本没有机会创建一个一致性的检查点，所以需要从上一个一致性检查点开始恢复。
实际上是数据库启动时检查控制文件中的数据库状态，如果状态不是shutdown的，那么说明数据库是异常关闭的（当然我们说，除了recovery状态），总之就需要从检查点开始恢复。
src/include/catalog/pg_control.h
/*
* System status indicator. Note this is stored in pg_control; if you change
* it, you must bump PG_CONTROL_VERSION
*/
typedef enum DBState
{
DB_STARTUP = 0,
DB_SHUTDOWNED,
DB_SHUTDOWNED_IN_RECOVERY,
DB_SHUTDOWNING,
DB_IN_CRASH_RECOVERY,
DB_IN_ARCHIVE_RECOVERY,
DB_IN_PRODUCTION
} DBState;
那么问题来了，对于standby，一直是出于recovery状态的，即使停库也可能是这个状态(实际上是shutdown_in_recovery)，所以STANDBY停库再起来，理论上来说又需要重上一个检查点开始恢复WAL，即使这个数据库已经恢复到了最新的WAL。
为了提高性能，缩短recovery时间，pg在standby上面也支持了checkpoint，叫做CreateRestartPoint。
CreateRestartPoint@src/backend/access/transam/xlog.c
/*
* Establish a restartpoint if possible.
*
* This is similar to CreateCheckPoint, but is used during WAL recovery
* to establish a point from which recovery can roll forward without
* replaying the entire recovery log.
*
* Returns true if a new restartpoint was established. We can only establish
* a restartpoint if we have replayed a safe checkpoint record since last
* restartpoint.
*/
bool
CreateRestartPoint(int flags)
{
当standby关闭，重启时，从restartpoint开始APPLY WAL，而不是上游节点（主节点）在WAL日志中写的检查点开始。
停库时，备库写检查点
/*
* This must be called ONCE during postmaster or standalone-backend shutdown
*/
void
ShutdownXLOG(int code, Datum arg)
{
/* Don't be chatty in standalone mode */
ereport(IsPostmasterEnvironment ? LOG : NOTICE,
(errmsg("shutting down")));
/*
* Signal walsenders to move to stopping state.
*/
WalSndInitStopping();
/*
* Wait for WAL senders to be in stopping state. This prevents commands
* from writing new WAL.
*/
WalSndWaitStopping();
if (RecoveryInProgress()) // 备库写检查点
CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
else
{
/*
* If archiving is enabled, rotate the last XLOG file so that all the
* remaining records are archived (postmaster wakes up the archiver
* process one more time at the end of shutdown). The checkpoint
* record will go to the next XLOG file and won't be archived (yet).
*/
if (XLogArchivingActive() && XLogArchiveCommandSet())
RequestXLogSwitch(false);
CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
}
ShutdownCLOG();
ShutdownCommitTs();
ShutdownSUBTRANS();
ShutdownMultiXact();
}
CreateRestartPoint
...
LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
ControlFile->checkPointCopy.redo < lastCheckPoint.redo)
{
ControlFile->checkPoint = lastCheckPointRecPtr;
ControlFile->checkPointCopy = lastCheckPoint;
ControlFile->time = (pg_time_t) time(NULL);
/*
* Ensure minRecoveryPoint is past the checkpoint record. Normally,
* this will have happened already while writing out dirty buffers,
* but not necessarily - e.g. because no buffers were dirtied. We do
* this because a non-exclusive base backup uses minRecoveryPoint to
* determine which WAL files must be included in the backup, and the
* file (or files) containing the checkpoint record must be included,
* at a minimum. Note that for an ordinary restart of recovery there's
* no value in having the minimum recovery point any earlier than this
* anyway, because redo will begin just after the checkpoint record.
*/
if (ControlFile->minRecoveryPoint < lastCheckPointEndPtr)
{
ControlFile->minRecoveryPoint = lastCheckPointEndPtr;
ControlFile->minRecoveryPointTLI = lastCheckPoint.ThisTimeLineID;
/* update local copy */
minRecoveryPoint = ControlFile->minRecoveryPoint;
minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
}
if (flags & CHECKPOINT_IS_SHUTDOWN)
ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY; // 备库正常关闭时，状态改写为DB_SHUTDOWNED_IN_RECOVERY
UpdateControlFile();
}
小结
PostgreSQL备库也可以写检查点，目的是避免每次重启备库都需要从上一个检查点（由主库产生，在WAL中回放出来的）APPLY后面所有的WAL。
从而forward 备库停库后重启时WAL的位点。
参考
CreateRestartPoint@src/backend/access/transam/xlog.c
