PostgreSQL 源码解读（115）- 后台进程#3（checkpointer进程#2）
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本节简单介绍了PostgreSQL的后台进程:checkpointer,主要分析CreateCheckPoint函数的实现逻辑。
一、数据结构
CheckPoint
CheckPoint XLOG record结构体.
typedef struct CheckPoint
{
//在开始创建CheckPoint时下一个可用的RecPtr(比如REDO的开始点)
XLogRecPtr redo;
//当前的时间线
TimeLineID ThisTimeLineID;
//上一个时间线(如该记录正在开启一条新的时间线,否则等于当前时间线)
TimeLineID PrevTimeLineID;
//是否full-page-write
bool fullPageWrites;
//nextXid的高阶位
uint32 nextXidEpoch;
//下一个free的XID
TransactionId nextXid;
//下一个free的OID
Oid nextOid;
//下一个fredd的MultiXactId
MultiXactId nextMulti;
//下一个空闲的MultiXact偏移
MultiXactOffset nextMultiOffset;
//集群范围内的最小datfrozenxid
TransactionId oldestXid;
//最小datfrozenxid所在的database
Oid oldestXidDB;
//集群范围内的最小datminmxid
MultiXactId oldestMulti;
//最小datminmxid所在的database
Oid oldestMultiDB;
//checkpoint的时间戳
pg_time_t time;
//带有有效提交时间戳的最老Xid
TransactionId oldestCommitTsXid;
//带有有效提交时间戳的最新Xid
TransactionId newestCommitTsXid;
TransactionId oldestActiveXid;
} CheckPoint;
#define XLOG_CHECKPOINT_SHUTDOWN 0x00
#define XLOG_CHECKPOINT_ONLINE 0x10
#define XLOG_NOOP 0x20
#define XLOG_NEXTOID 0x30
#define XLOG_SWITCH 0x40
#define XLOG_BACKUP_END 0x50
#define XLOG_PARAMETER_CHANGE 0x60
#define XLOG_RESTORE_POINT 0x70
#define XLOG_FPW_CHANGE 0x80
#define XLOG_END_OF_RECOVERY 0x90
#define XLOG_FPI_FOR_HINT 0xA0
#define XLOG_FPI 0xB0
CheckpointerShmem
checkpointer进程和其他后台进程之间通讯的共享内存结构.
typedef struct
{
RelFileNode rnode;//表空间/数据库/Relation信息
ForkNumber forknum;//fork编号
BlockNumber segno;
} CheckpointerRequest;
typedef struct
{
//checkpoint进程的pid(为0则进程未启动)
pid_t checkpointer_pid;
//用于保护所有的ckpt_*域
slock_t ckpt_lck;
//在checkpoint启动时计数
int ckpt_started;
//在checkpoint完成时计数
int ckpt_done;
//在checkpoint失败时计数
int ckpt_failed;
//检查点标记,在xlog.h中定义
int ckpt_flags;
//计数后台进程缓存写的次数
uint32 num_backend_writes;
//计数后台进程fsync调用次数
uint32 num_backend_fsync;
//当前的请求编号
int num_requests;
//最大的请求编号
int max_requests;
//请求数组
CheckpointerRequest requests[FLEXIBLE_ARRAY_MEMBER];
} CheckpointerShmemStruct;
//静态变量(CheckpointerShmemStruct结构体指针)
static CheckpointerShmemStruct *CheckpointerShmem;
VirtualTransactionId
最顶层的事务通过VirtualTransactionIDs定义.
typedef struct
{
BackendId backendId;
LocalTransactionId localTransactionId;
} VirtualTransactionId;
二、源码解读
CreateCheckPoint函数,执行checkpoint,不管是在shutdown过程还是在运行中.
void
CreateCheckPoint(int flags)
{
bool shutdown;//是否处于shutdown?
CheckPoint checkPoint;//checkpoint
XLogRecPtr recptr;//XLOG Record位置
XLogSegNo _logSegNo;//LSN(uint64)
XLogCtlInsert *Insert = &XLogCtl->Insert;//控制器
uint32 freespace;//空闲空间
XLogRecPtr PriorRedoPtr;//上一个Redo point
XLogRecPtr curInsert;//当前插入的位置
XLogRecPtr last_important_lsn;//上一个重要的LSN
VirtualTransactionId *vxids;//虚拟事务ID
int nvxids;
if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
shutdown = true;
else
shutdown = false;
//验证
if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
elog(ERROR, "can't create a checkpoint during recovery");
InitXLogInsert();
LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);
MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
CheckpointStats.ckpt_start_t = GetCurrentTimestamp();
START_CRIT_SECTION();
if (shutdown)
{
//shutdown = T
//更新control file(pg_control文件)
LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
ControlFile->state = DB_SHUTDOWNING;
ControlFile->time = (pg_time_t) time(NULL);
UpdateControlFile();
LWLockRelease(ControlFileLock);
}
smgrpreckpt();
//填充Checkpoint XLOG Record
MemSet(&checkPoint, 0, sizeof(checkPoint));
checkPoint.time = (pg_time_t) time(NULL);//时间
if (!shutdown && XLogStandbyInfoActive())
checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
else
checkPoint.oldestActiveXid = InvalidTransactionId;
last_important_lsn = GetLastImportantRecPtr();
WALInsertLockAcquireExclusive();
curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);
if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
CHECKPOINT_FORCE)) == 0)
{
if (last_important_lsn == ControlFile->checkPoint)
{
WALInsertLockRelease();
LWLockRelease(CheckpointLock);
END_CRIT_SECTION();
ereport(DEBUG1,
(errmsg("checkpoint skipped because system is idle")));
return;
}
}
if (flags & CHECKPOINT_END_OF_RECOVERY)
LocalSetXLogInsertAllowed();
checkPoint.ThisTimeLineID = ThisTimeLineID;
if (flags & CHECKPOINT_END_OF_RECOVERY)
checkPoint.PrevTimeLineID = XLogCtl->PrevTimeLineID;
else
checkPoint.PrevTimeLineID = ThisTimeLineID;
checkPoint.fullPageWrites = Insert->fullPageWrites;
freespace = INSERT_FREESPACE(curInsert);//获取空闲空间
if (freespace == 0)
{
//没有空闲空间了
if (XLogSegmentOffset(curInsert, wal_segment_size) == 0)
curInsert += SizeOfXLogLongPHD;//新的WAL segment file,偏移为LONG header
else
curInsert += SizeOfXLogShortPHD;//原WAL segment file,偏移为常规的header
}
checkPoint.redo = curInsert;
RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;
WALInsertLockRelease();
//同时,更新RedoRecPtr的info_lck-protected拷贝锁.
SpinLockAcquire(&XLogCtl->info_lck);
XLogCtl->RedoRecPtr = checkPoint.redo;
SpinLockRelease(&XLogCtl->info_lck);
if (log_checkpoints)
LogCheckpointStart(flags, false);
TRACE_POSTGRESQL_CHECKPOINT_START(flags);
LWLockAcquire(XidGenLock, LW_SHARED);
checkPoint.nextXid = ShmemVariableCache->nextXid;
checkPoint.oldestXid = ShmemVariableCache->oldestXid;
checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
LWLockRelease(XidGenLock);
LWLockAcquire(CommitTsLock, LW_SHARED);
checkPoint.oldestCommitTsXid = ShmemVariableCache->oldestCommitTsXid;
checkPoint.newestCommitTsXid = ShmemVariableCache->newestCommitTsXid;
LWLockRelease(CommitTsLock);
//如果我们从上一个checkpoint开始wrapped around，则增加XID epoch
checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
checkPoint.nextXidEpoch++;
LWLockAcquire(OidGenLock, LW_SHARED);
checkPoint.nextOid = ShmemVariableCache->nextOid;
if (!shutdown)
checkPoint.nextOid += ShmemVariableCache->oidCount;
LWLockRelease(OidGenLock);
MultiXactGetCheckptMulti(shutdown,
&checkPoint.nextMulti,
&checkPoint.nextMultiOffset,
&checkPoint.oldestMulti,
&checkPoint.oldestMultiDB);
END_CRIT_SECTION();
vxids = GetVirtualXIDsDelayingChkpt(&nvxids);//获取虚拟事务XID
if (nvxids > 0)
{
do
{
//等待10ms
pg_usleep(10000L);
} while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids));
}
pfree(vxids);
//把共享内存中的数据刷到磁盘上,并执行fsync
CheckPointGuts(checkPoint.redo, flags);
if (!shutdown && XLogStandbyInfoActive())
LogStandbySnapshot();
START_CRIT_SECTION();//进入critical section.
XLogBeginInsert();//开始插入
XLogRegisterData((char *) (&checkPoint), sizeof(checkPoint));//注册数据
recptr = XLogInsert(RM_XLOG_ID,
shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
XLOG_CHECKPOINT_ONLINE);//执行插入
XLogFlush(recptr);//刷盘
if (shutdown)
{
//关闭过程中
if (flags & CHECKPOINT_END_OF_RECOVERY)
LocalXLogInsertAllowed = -1;
else
LocalXLogInsertAllowed = 0;
}
if (shutdown && checkPoint.redo != ProcLastRecPtr)
ereport(PANIC,
(errmsg("concurrent write-ahead log activity while database system is shutting down")));
PriorRedoPtr = ControlFile->checkPointCopy.redo;
LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
if (shutdown)
ControlFile->state = DB_SHUTDOWNED;
ControlFile->checkPoint = ProcLastRecPtr;
ControlFile->checkPointCopy = checkPoint;
ControlFile->time = (pg_time_t) time(NULL);
//crash recovery通常来说应恢复至WAL的末尾
ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
ControlFile->minRecoveryPointTLI = 0;
SpinLockAcquire(&XLogCtl->ulsn_lck);
ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
SpinLockRelease(&XLogCtl->ulsn_lck);
UpdateControlFile();
LWLockRelease(ControlFileLock);
//更新checkpoint XID/epoch的共享内存拷贝
SpinLockAcquire(&XLogCtl->info_lck);
XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
XLogCtl->ckptXid = checkPoint.nextXid;
SpinLockRelease(&XLogCtl->info_lck);
END_CRIT_SECTION();
smgrpostckpt();
if (PriorRedoPtr != InvalidXLogRecPtr)
UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);
XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);
KeepLogSeg(recptr, &_logSegNo);
_logSegNo--;
RemoveOldXlogFiles(_logSegNo, RedoRecPtr, recptr);
if (!shutdown)
PreallocXlogFiles(recptr);
if (!RecoveryInProgress())
TruncateSUBTRANS(GetOldestXmin(NULL, PROCARRAY_FLAGS_DEFAULT));
//实际的工作已完成,除了记录日志已经更新统计信息.
LogCheckpointEnd(false);
TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
NBuffers,
CheckpointStats.ckpt_segs_added,
CheckpointStats.ckpt_segs_removed,
CheckpointStats.ckpt_segs_recycled);
//释放锁
LWLockRelease(CheckpointLock);
}
static void
CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
{
CheckPointCLOG();
CheckPointCommitTs();
CheckPointSUBTRANS();
CheckPointMultiXact();
CheckPointPredicate();
CheckPointRelationMap();
CheckPointReplicationSlots();
CheckPointSnapBuild();
CheckPointLogicalRewriteHeap();
CheckPointBuffers(flags);
CheckPointReplicationOrigin();
CheckPointTwoPhase(checkPointRedo);
}
三、跟踪分析
更新数据,执行checkpoint.
testdb=# update t_wal_ckpt set c2 = 'C2_'||substr(c2,4,40);
UPDATE 1
testdb=# checkpoint;
启动gdb,设置信号控制,设置断点,进入CreateCheckPoint
(gdb) handle SIGINT print nostop pass
SIGINT is used by the debugger.
Are you sure you want to change it? (y or n) y
Signal Stop Print Pass to program Description
SIGINT No Yes Yes Interrupt
(gdb)
(gdb) b CreateCheckPoint
Breakpoint 1 at 0x55b4fb: file xlog.c, line 8668.
(gdb) c
Continuing.
Program received signal SIGINT, Interrupt.
Breakpoint 1, CreateCheckPoint (flags=44) at xlog.c:8668
8668 XLogCtlInsert *Insert = &XLogCtl->Insert;
(gdb)
获取XLOG插入控制器
8668 XLogCtlInsert *Insert = &XLogCtl->Insert;
(gdb) n
8680 if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
(gdb) p XLogCtl
$1 = (XLogCtlData *) 0x7fadf8f6fa80
(gdb) p *XLogCtl
$2 = {Insert = {insertpos_lck = 0 '\000', CurrBytePos = 5505269968, PrevBytePos = 5505269928,
pad = '\000' <repeats 127 times>, RedoRecPtr = 5521450856, forcePageWrites = false, fullPageWrites = true,
exclusiveBackupState = EXCLUSIVE_BACKUP_NONE, nonExclusiveBackups = 0, lastBackupStart = 0,
WALInsertLocks = 0x7fadf8f74100}, LogwrtRqst = {Write = 5521451392, Flush = 5521451392}, RedoRecPtr = 5521450856,
ckptXidEpoch = 0, ckptXid = 2307, asyncXactLSN = 5521363848, replicationSlotMinLSN = 0, lastRemovedSegNo = 0,
unloggedLSN = 1, ulsn_lck = 0 '\000', lastSegSwitchTime = 1546915130, lastSegSwitchLSN = 5521363360, LogwrtResult = {
Write = 5521451392, Flush = 5521451392}, InitializedUpTo = 5538226176, pages = 0x7fadf8f76000 "\230\320\006",
xlblocks = 0x7fadf8f70088, XLogCacheBlck = 2047, ThisTimeLineID = 1, PrevTimeLineID = 1,
archiveCleanupCommand = '\000' <repeats 1023 times>, SharedRecoveryInProgress = false, SharedHotStandbyActive = false,
WalWriterSleeping = true, recoveryWakeupLatch = {is_set = 0, is_shared = true, owner_pid = 0}, lastCheckPointRecPtr = 0,
lastCheckPointEndPtr = 0, lastCheckPoint = {redo = 0, ThisTimeLineID = 0, PrevTimeLineID = 0, fullPageWrites = false,
nextXidEpoch = 0, nextXid = 0, nextOid = 0, nextMulti = 0, nextMultiOffset = 0, oldestXid = 0, oldestXidDB = 0,
oldestMulti = 0, oldestMultiDB = 0, time = 0, oldestCommitTsXid = 0, newestCommitTsXid = 0, oldestActiveXid = 0},
lastReplayedEndRecPtr = 0, lastReplayedTLI = 0, replayEndRecPtr = 0, replayEndTLI = 0, recoveryLastXTime = 0,
currentChunkStartTime = 0, recoveryPause = false, lastFpwDisableRecPtr = 0, info_lck = 0 '\000'}
(gdb) p *Insert
$4 = {insertpos_lck = 0 '\000', CurrBytePos = 5505269968, PrevBytePos = 5505269928, pad = '\000' <repeats 127 times>,
RedoRecPtr = 5521450856, forcePageWrites = false, fullPageWrites = true, exclusiveBackupState = EXCLUSIVE_BACKUP_NONE,
nonExclusiveBackups = 0, lastBackupStart = 0, WALInsertLocks = 0x7fadf8f74100}
(gdb)
RedoRecPtr = 5521450856,这是REDO point,与pg_control文件中的值一致
[xdb@localhost ~]$ echo "obase=16;ibase=10;5521450856"|bc
1491AA768
[xdb@localhost ~]$ pg_controldata|grep REDO
Latest checkpoint's REDO location: 1/491AA768
Latest checkpoint's REDO WAL file: 000000010000000100000049
[xdb@localhost ~]$
在进入critical section前,初始化InitXLogInsert工作空间.
请求CheckpointLock确保在同一时刻只能存在一个checkpoint.
(gdb) n
8683 shutdown = false;
(gdb)
8686 if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
(gdb)
8697 InitXLogInsert();
(gdb)
8705 LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);
(gdb)
8714 MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
(gdb)
8715 CheckpointStats.ckpt_start_t = GetCurrentTimestamp();
(gdb)
进入critical section,让smgr(资源管理器)为checkpoint作准备.
8720 START_CRIT_SECTION();
(gdb)
(gdb)
8722 if (shutdown)
(gdb)
8736 smgrpreckpt();
(gdb)
8739 MemSet(&checkPoint, 0, sizeof(checkPoint));
(gdb)
开始填充Checkpoint XLOG Record
(gdb)
8740 checkPoint.time = (pg_time_t) time(NULL);
(gdb) p checkPoint
$5 = {redo = 0, ThisTimeLineID = 0, PrevTimeLineID = 0, fullPageWrites = false, nextXidEpoch = 0, nextXid = 0, nextOid = 0,
nextMulti = 0, nextMultiOffset = 0, oldestXid = 0, oldestXidDB = 0, oldestMulti = 0, oldestMultiDB = 0, time = 0,
oldestCommitTsXid = 0, newestCommitTsXid = 0, oldestActiveXid = 0}
(gdb) n
8747 if (!shutdown && XLogStandbyInfoActive())
(gdb)
8750 checkPoint.oldestActiveXid = InvalidTransactionId;
在请求插入locks前,获取最后一个重要的XLOG Record的位置.
(gdb)
8756 last_important_lsn = GetLastImportantRecPtr();
(gdb)
8762 WALInsertLockAcquireExclusive();
(gdb)
(gdb) p last_important_lsn
$6 = 5521451352 --> 0x1491AA958
在检查插入状态确定checkpoint的REDO pointer时,必须阻塞同步插入操作.
(gdb) n
8763 curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);
(gdb)
8770 if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
(gdb) p curInsert
$7 = 5521451392 --> 0x1491AA980
(gdb)
继续填充Checkpoint XLOG Record
(gdb) n
8790 if (flags & CHECKPOINT_END_OF_RECOVERY)
(gdb)
8793 checkPoint.ThisTimeLineID = ThisTimeLineID;
(gdb)
8794 if (flags & CHECKPOINT_END_OF_RECOVERY)
(gdb)
8797 checkPoint.PrevTimeLineID = ThisTimeLineID;
(gdb) p ThisTimeLineID
$8 = 1
(gdb) n
8799 checkPoint.fullPageWrites = Insert->fullPageWrites;
(gdb)
8809 freespace = INSERT_FREESPACE(curInsert);
(gdb)
8810 if (freespace == 0)
(gdb) p freespace
$9 = 5760
(gdb) n
8817 checkPoint.redo = curInsert;
(gdb)
8830 RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;
(gdb)
(gdb) p checkPoint
$10 = {redo = 5521451392, ThisTimeLineID = 1, PrevTimeLineID = 1, fullPageWrites = true, nextXidEpoch = 0, nextXid = 0,
nextOid = 0, nextMulti = 0, nextMultiOffset = 0, oldestXid = 0, oldestXidDB = 0, oldestMulti = 0, oldestMultiDB = 0,
time = 1546933255, oldestCommitTsXid = 0, newestCommitTsXid = 0, oldestActiveXid = 0}
(gdb)
更新共享的RedoRecPtr以备将来的XLogInsert调用,必须在持有所有插入锁才能完成。
(gdb) n
8836 WALInsertLockRelease();
(gdb)
8839 SpinLockAcquire(&XLogCtl->info_lck);
(gdb)
8840 XLogCtl->RedoRecPtr = checkPoint.redo;
(gdb)
8841 SpinLockRelease(&XLogCtl->info_lck);
(gdb)
8847 if (log_checkpoints)
(gdb)
(gdb) p XLogCtl->RedoRecPtr
$11 = 5521451392
获取其他组装checkpoint记录的信息.
(gdb) n
8850 TRACE_POSTGRESQL_CHECKPOINT_START(flags);
(gdb)
8860 LWLockAcquire(XidGenLock, LW_SHARED);
(gdb)
8861 checkPoint.nextXid = ShmemVariableCache->nextXid;
(gdb)
8862 checkPoint.oldestXid = ShmemVariableCache->oldestXid;
(gdb)
8863 checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
(gdb)
8864 LWLockRelease(XidGenLock);
(gdb)
8866 LWLockAcquire(CommitTsLock, LW_SHARED);
(gdb)
8867 checkPoint.oldestCommitTsXid = ShmemVariableCache->oldestCommitTsXid;
(gdb)
8868 checkPoint.newestCommitTsXid = ShmemVariableCache->newestCommitTsXid;
(gdb)
8869 LWLockRelease(CommitTsLock);
(gdb)
8872 checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
(gdb) n
8873 if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
(gdb)
8876 LWLockAcquire(OidGenLock, LW_SHARED);
(gdb)
8877 checkPoint.nextOid = ShmemVariableCache->nextOid;
(gdb) p checkPoint
$13 = {redo = 5521451392, ThisTimeLineID = 1, PrevTimeLineID = 1, fullPageWrites = true, nextXidEpoch = 0, nextXid = 2308,
nextOid = 0, nextMulti = 0, nextMultiOffset = 0, oldestXid = 561, oldestXidDB = 16400, oldestMulti = 0,
oldestMultiDB = 0, time = 1546933255, oldestCommitTsXid = 0, newestCommitTsXid = 0, oldestActiveXid = 0}
(gdb) n
8878 if (!shutdown)
(gdb)
8879 checkPoint.nextOid += ShmemVariableCache->oidCount;
(gdb)
8880 LWLockRelease(OidGenLock);
(gdb) p *ShmemVariableCache
$14 = {nextOid = 42575, oidCount = 8189, nextXid = 2308, oldestXid = 561, xidVacLimit = 200000561,
xidWarnLimit = 2136484208, xidStopLimit = 2146484208, xidWrapLimit = 2147484208, oldestXidDB = 16400,
oldestCommitTsXid = 0, newestCommitTsXid = 0, latestCompletedXid = 2307, oldestClogXid = 561}
(gdb) n
8882 MultiXactGetCheckptMulti(shutdown,
(gdb)
再次查看checkpoint结构体
(gdb) p checkPoint
$15 = {redo = 5521451392, ThisTimeLineID = 1, PrevTimeLineID = 1, fullPageWrites = true, nextXidEpoch = 0, nextXid = 2308,
nextOid = 50764, nextMulti = 1, nextMultiOffset = 0, oldestXid = 561, oldestXidDB = 16400, oldestMulti = 1,
oldestMultiDB = 16402, time = 1546933255, oldestCommitTsXid = 0, newestCommitTsXid = 0, oldestActiveXid = 0}
(gdb)
结束CRIT_SECTION
(gdb)
8896 END_CRIT_SECTION();
获取虚拟事务ID(无效的信息)
(gdb) n
8927 vxids = GetVirtualXIDsDelayingChkpt(&nvxids);
(gdb)
8928 if (nvxids > 0)
(gdb) p vxids
$16 = (VirtualTransactionId *) 0x2f4eb20
(gdb) p *vxids
$17 = {backendId = 2139062143, localTransactionId = 2139062143}
(gdb) p nvxids
$18 = 0
(gdb)
(gdb) n
8935 pfree(vxids);
(gdb)
把共享内存中的数据刷到磁盘上,并执行fsync
(gdb)
8937 CheckPointGuts(checkPoint.redo, flags);
(gdb) p flags
$19 = 44
(gdb) n
8947 if (!shutdown && XLogStandbyInfoActive())
(gdb)
进入critical section.
(gdb) n
8950 START_CRIT_SECTION();
(gdb)
现在可以插入checkpoint record到XLOG中了.
(gdb)
8955 XLogBeginInsert();
(gdb) n
8956 XLogRegisterData((char *) (&checkPoint), sizeof(checkPoint));
(gdb)
8957 recptr = XLogInsert(RM_XLOG_ID,
(gdb)
8961 XLogFlush(recptr);
(gdb)
8970 if (shutdown)
(gdb)
更新控制文件(pg_control),首先为UpdateCheckPointDistanceEstimate()记录上一个checkpoint的REDO ptr
(gdb)
8982 if (shutdown && checkPoint.redo != ProcLastRecPtr)
(gdb)
8990 PriorRedoPtr = ControlFile->checkPointCopy.redo;
(gdb)
8995 LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
(gdb) p ControlFile->checkPointCopy.redo
$20 = 5521450856
(gdb) n
8996 if (shutdown)
(gdb)
8998 ControlFile->checkPoint = ProcLastRecPtr;
(gdb)
8999 ControlFile->checkPointCopy = checkPoint;
(gdb)
9000 ControlFile->time = (pg_time_t) time(NULL);
(gdb)
9002 ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
(gdb)
9003 ControlFile->minRecoveryPointTLI = 0;
(gdb)
9010 SpinLockAcquire(&XLogCtl->ulsn_lck);
(gdb)
9011 ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
(gdb)
9012 SpinLockRelease(&XLogCtl->ulsn_lck);
(gdb)
9014 UpdateControlFile();
(gdb)
9015 LWLockRelease(ControlFileLock);
(gdb)
9018 SpinLockAcquire(&XLogCtl->info_lck);
(gdb) p *ControlFile
$21 = {system_identifier = 6624362124887945794, pg_control_version = 1100, catalog_version_no = 201809051,
state = DB_IN_PRODUCTION, time = 1546934255, checkPoint = 5521451392, checkPointCopy = {redo = 5521451392,
ThisTimeLineID = 1, PrevTimeLineID = 1, fullPageWrites = true, nextXidEpoch = 0, nextXid = 2308, nextOid = 50764,
nextMulti = 1, nextMultiOffset = 0, oldestXid = 561, oldestXidDB = 16400, oldestMulti = 1, oldestMultiDB = 16402,
time = 1546933255, oldestCommitTsXid = 0, newestCommitTsXid = 0, oldestActiveXid = 0}, unloggedLSN = 1,
minRecoveryPoint = 0, minRecoveryPointTLI = 0, backupStartPoint = 0, backupEndPoint = 0, backupEndRequired = false,
wal_level = 0, wal_log_hints = false, MaxConnections = 100, max_worker_processes = 8, max_prepared_xacts = 0,
max_locks_per_xact = 64, track_commit_timestamp = false, maxAlign = 8, floatFormat = 1234567, blcksz = 8192,
relseg_size = 131072, xlog_blcksz = 8192, xlog_seg_size = 16777216, nameDataLen = 64, indexMaxKeys = 32,
toast_max_chunk_size = 1996, loblksize = 2048, float4ByVal = true, float8ByVal = true, data_checksum_version = 0,
mock_authentication_nonce = "\220\277\067Vg\003\205\232U{\177 h\216\271D\266\063[\\=6\365S\tA\353\361ߧw\301",
crc = 930305687}
(gdb)
更新checkpoint XID/epoch的共享内存拷贝,退出critical section,并让smgr执行checkpoint收尾工作(比如删除旧文件等).
(gdb) n
9019 XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
(gdb)
9020 XLogCtl->ckptXid = checkPoint.nextXid;
(gdb)
9021 SpinLockRelease(&XLogCtl->info_lck);
(gdb)
9027 END_CRIT_SECTION();
(gdb)
9032 smgrpostckpt();
(gdb)
删除旧的日志文件，这些文件自最后一个检查点后已不再需要，以防止保存xlog的磁盘撑满。
(gdb) n
9038 if (PriorRedoPtr != InvalidXLogRecPtr)
(gdb) p PriorRedoPtr
$23 = 5521450856
(gdb) n
9039 UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);
(gdb)
9045 XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);
(gdb)
9046 KeepLogSeg(recptr, &_logSegNo);
(gdb) p RedoRecPtr
$24 = 5521451392
(gdb) p _logSegNo
$25 = 329
(gdb) p wal_segment_size
$26 = 16777216
(gdb) n
9047 _logSegNo--;
(gdb)
9048 RemoveOldXlogFiles(_logSegNo, RedoRecPtr, recptr);
(gdb)
9054 if (!shutdown)
(gdb) p recptr
$27 = 5521451504
(gdb)
执行其他相关收尾工作
(gdb) n
9055 PreallocXlogFiles(recptr);
(gdb)
9064 if (!RecoveryInProgress())
(gdb)
9065 TruncateSUBTRANS(GetOldestXmin(NULL, PROCARRAY_FLAGS_DEFAULT));
(gdb)
9068 LogCheckpointEnd(false);
(gdb)
9070 TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
(gdb)
9076 LWLockRelease(CheckpointLock);
(gdb)
9077 }
(gdb)
完成调用
(gdb)
CheckpointerMain () at checkpointer.c:488
488 ckpt_performed = true;
(gdb)
DONE!
四、参考资料
checkpointer.c
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PostgreSQL 源码解读（115）- 后台进程#3（checkpointer进程#2）
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