接前文 初步学习pg_control文件之九 看下面这个
XLogRecPtr checkPoint; /* last check point record ptr */
看看这个pointer究竟保留了什么
初始化的时候：
/*
* This func must be called ONCE on system install. It creates pg_control
* and the initial XLOG segment.
*/
void
BootStrapXLOG(void)
{ …
/*
* Set up information for the initial checkpoint record
*
* The initial checkpoint record is written to the beginning of the WAL
* segment with logid=0 logseg=1. The very first WAL segment, 0/0, is not
* used, so that we can use 0/0 to mean "before any valid WAL segment".
*/
checkPoint.redo.xlogid = ;
checkPoint.redo.xrecoff = XLogSegSize + SizeOfXLogLongPHD; ...
/* Now create pg_control */ memset(ControlFile, , sizeof(ControlFileData)); /* Initialize pg_control status fields */
ControlFile->system_identifier = sysidentifier;
ControlFile->state = DB_SHUTDOWNED;
ControlFile->time = checkPoint.time; ControlFile->checkPoint = checkPoint.redo;
ControlFile->checkPointCopy = checkPoint; …
WriteControlFile();
…
}
运行的时候：
/*
* ProcLastRecPtr points to the start of the last XLOG record inserted by the
* current backend. It is updated for all inserts. XactLastRecEnd points to
* end+1 of the last record, and is reset when we end a top-level transaction,
* or start a new one; so it can be used to tell if the current transaction has
* created any XLOG records.
*/
static XLogRecPtr ProcLastRecPtr = {, };
… /*
* Insert an XLOG record having the specified RMID and info bytes,
* with the body of the record being the data chunk(s) described by
* the rdata chain (see xlog.h for notes about rdata).
*
* Returns XLOG pointer to end of record (beginning of next record).
* This can be used as LSN for data pages affected by the logged action.
* (LSN is the XLOG point up to which the XLOG must be flushed to disk
* before the data page can be written out. This implements the basic
* WAL rule "write the log before the data".)
*
* NB: this routine feels free to scribble on the XLogRecData structs,
* though not on the data they reference. This is OK since the XLogRecData
* structs are always just temporaries in the calling code.
*/
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
{
…
/*
* In bootstrap mode, we don't actually log anything but XLOG resources;
* return a phony record pointer.
*/
if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
{
RecPtr.xlogid = ;
RecPtr.xrecoff = SizeOfXLogLongPHD; /* start of 1st chkpt record */
return RecPtr;
} …
/* Record begin of record in appropriate places */
ProcLastRecPtr = RecPtr;
Insert->PrevRecord = RecPtr;
…
}
关闭的时候：
/*
* Perform a checkpoint --- either during shutdown, or on-the-fly
*
* flags is a bitwise OR of the following:
* CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
* CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
* CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
* ignoring checkpoint_completion_target parameter.
* CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occured
* since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
* CHECKPOINT_END_OF_RECOVERY).
*
* Note: flags contains other bits, of interest here only for logging purposes.
* In particular note that this routine is synchronous and does not pay
* attention to CHECKPOINT_WAIT.
*/
void
CreateCheckPoint(int flags)
{
…
/*
* Update the control file.
*/
LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
if (shutdown)
ControlFile->state = DB_SHUTDOWNED;
ControlFile->prevCheckPoint = ControlFile->checkPoint;
ControlFile->checkPoint = ProcLastRecPtr;
ControlFile->checkPointCopy = checkPoint;
ControlFile->time = (pg_time_t) time(NULL);
/* crash recovery should always recover to the end of WAL */
MemSet(&ControlFile->minRecoveryPoint, , sizeof(XLogRecPtr));
UpdateControlFile();
LWLockRelease(ControlFileLock);
…
}
RecoveryMode或者Stand by 的时候：
/*
* 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)
{
XLogRecPtr lastCheckPointRecPtr;
CheckPoint lastCheckPoint;
uint32 _logId;
uint32 _logSeg;
TimestampTz xtime; /* use volatile pointer to prevent code rearrangement */
volatile XLogCtlData *xlogctl = XLogCtl; /*
* Acquire CheckpointLock to ensure only one restartpoint or checkpoint
* happens at a time.
*/
LWLockAcquire(CheckpointLock, LW_EXCLUSIVE); /* Get a local copy of the last safe checkpoint record. */
SpinLockAcquire(&xlogctl->info_lck);
lastCheckPointRecPtr = xlogctl->lastCheckPointRecPtr;
memcpy(&lastCheckPoint, &XLogCtl->lastCheckPoint, sizeof(CheckPoint));
SpinLockRelease(&xlogctl->info_lck); …
/*
* Update pg_control, using current time. Check that it still shows
* IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing;
* this is a quick hack to make sure nothing really bad happens if somehow
* we get here after the end-of-recovery checkpoint.
*/
LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
XLByteLT(ControlFile->checkPointCopy.redo, lastCheckPoint.redo))
{
ControlFile->prevCheckPoint = ControlFile->checkPoint;
ControlFile->checkPoint = lastCheckPointRecPtr;
ControlFile->checkPointCopy = lastCheckPoint;
ControlFile->time = (pg_time_t) time(NULL);
if (flags & CHECKPOINT_IS_SHUTDOWN)
ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
UpdateControlFile();
}
LWLockRelease(ControlFileLock); …
}
启动的时候：
/*
* This must be called ONCE during postmaster or standalone-backend startup
*/
void
StartupXLOG(void)
{
…
/*
* Read control file and check XLOG status looks valid.
*
* Note: in most control paths, *ControlFile is already valid and we need
* not do ReadControlFile() here, but might as well do it to be sure.
*/
ReadControlFile(); if (ControlFile->state < DB_SHUTDOWNED ||
ControlFile->state > DB_IN_PRODUCTION ||
!XRecOffIsValid(ControlFile->checkPoint.xrecoff))
ereport(FATAL,
(errmsg("control file contains invalid data"))); …
if (read_backup_label(&checkPointLoc, &backupEndRequired))
{
…
}
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, );
…
}
… /* REDO */
if (InRecovery)
{
…
ControlFile->prevCheckPoint = ControlFile->checkPoint;
ControlFile->checkPoint = checkPointLoc;
ControlFile->checkPointCopy = checkPoint;
…
} …
}