本节简单介绍了XLOG全局(所有进程之间)共享的数据结构:XLogCtlData和XLogCtlInsert。在这两个结构体中,存储了REDO point/Lock等相关重要的信息.
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一、数据结构
XLogCtlInsert
WAL插入记录时使用的共享数据结构
/*
* Shared state data for WAL insertion.
* WAL插入记录时使用的共享数据结构
*/
typedef struct XLogCtlInsert
{
//包含CurrBytePos和PrevBytePos的lock
slock_t insertpos_lck; /* protects CurrBytePos and PrevBytePos */
/*
* CurrBytePos is the end of reserved WAL. The next record will be
* inserted at that position. PrevBytePos is the start position of the
* previously inserted (or rather, reserved) record - it is copied to the
* prev-link of the next record. These are stored as "usable byte
* positions" rather than XLogRecPtrs (see XLogBytePosToRecPtr()).
* CurrBytePos是保留WAL的结束位置。
* 下一条记录将插入到那个位置。
* PrevBytePos是先前插入(或者保留)记录的起始位置——它被复制到下一条记录的prev-link中。
* 这些存储为“可用字节位置”，而不是XLogRecPtrs(参见XLogBytePosToRecPtr())。
*/
uint64 CurrBytePos;
uint64 PrevBytePos;
/*
* Make sure the above heavily-contended spinlock and byte positions are
* on their own cache line. In particular, the RedoRecPtr and full page
* write variables below should be on a different cache line. They are
* read on every WAL insertion, but updated rarely, and we don't want
* those reads to steal the cache line containing Curr/PrevBytePos.
* 确保以上激烈竞争的自旋锁和字节位置在它们自己的缓存line上。
* 特别是，RedoRecPtr和下面的全页写变量应该位于不同的缓存line上。
* 它们在每次插入WAL时都被读取，但很少更新，
* 我们不希望这些读取窃取包含Curr/PrevBytePos的缓存line。
*/
char pad[PG_CACHE_LINE_SIZE];
/*
* fullPageWrites is the master copy used by all backends to determine
* whether to write full-page to WAL, instead of using process-local one.
* This is required because, when full_page_writes is changed by SIGHUP,
* we must WAL-log it before it actually affects WAL-logging by backends.
* Checkpointer sets at startup or after SIGHUP.
* fullpagewrite是所有后台进程使用的主副本，
* 用于确定是否将整个页面写入WAL，而不是使用process-local副本。
* 这是必需的，因为当SIGHUP更改full_page_write时，
* 我们必须在它通过后台进程实际影响WAL-logging之前对其进行WAL-log记录。
* Checkpointer检查点设置在启动或SIGHUP之后。
*
* To read these fields, you must hold an insertion lock. To modify them,
* you must hold ALL the locks.
* 为了读取这些域,必须持有insertion lock.
* 如需更新,则需要持有所有这些lock.
*/
//插入时的当前redo point
XLogRecPtr RedoRecPtr; /* current redo point for insertions */
//为PITR强制执行full-page写?
bool forcePageWrites; /* forcing full-page writes for PITR? */
//是否全页写?
bool fullPageWrites;
/*
* exclusiveBackupState indicates the state of an exclusive backup (see
* comments of ExclusiveBackupState for more details). nonExclusiveBackups
* is a counter indicating the number of streaming base backups currently
* in progress. forcePageWrites is set to true when either of these is
* non-zero. lastBackupStart is the latest checkpoint redo location used
* as a starting point for an online backup.
* exclusive sivebackupstate表示排他备份的状态
* (有关详细信息，请参阅exclusive sivebackupstate的注释)。
* 非排他性备份是一个计数器，指示当前正在进行的流基础备份的数量。
* forcePageWrites在这两个值都不为零时被设置为true。
* lastBackupStart用作在线备份起点的最新检查点的重做位置。
*/
ExclusiveBackupState exclusiveBackupState;
int nonExclusiveBackups;
XLogRecPtr lastBackupStart;
/*
* WAL insertion locks.
* WAL写入锁
*/
WALInsertLockPadded *WALInsertLocks;
} XLogCtlInsert;
XLogCtl
XLOG的所有共享内存状态信息
/*
* Total shared-memory state for XLOG.
* XLOG的所有共享内存状态信息
*/
typedef struct XLogCtlData
{
XLogCtlInsert Insert;//插入控制器
/* Protected by info_lck: */
//------ 通过info_lck锁保护
XLogwrtRqst LogwrtRqst;
//Insert->RedoRecPtr最近的拷贝
XLogRecPtr RedoRecPtr; /* a recent copy of Insert->RedoRecPtr */
//最后的checkpoint的nextXID & epoch
uint32 ckptXidEpoch; /* nextXID & epoch of latest checkpoint */
TransactionId ckptXid;
//最新异步提交/回滚的LSN
XLogRecPtr asyncXactLSN; /* LSN of newest async commit/abort */
//slot需要的最"老"的LSN
XLogRecPtr replicationSlotMinLSN; /* oldest LSN needed by any slot */
//最后移除/回收的XLOG段
XLogSegNo lastRemovedSegNo; /* latest removed/recycled XLOG segment */
/* Fake LSN counter, for unlogged relations. Protected by ulsn_lck. */
//---- "伪装"的LSN计数器,用于不需要记录日志的关系.通过ulsn_lck锁保护
XLogRecPtr unloggedLSN;
slock_t ulsn_lck;
/* Time and LSN of last xlog segment switch. Protected by WALWriteLock. */
//---- 切换后最新的xlog段的时间线和LSN,通过WALWriteLock锁保护
pg_time_t lastSegSwitchTime;
XLogRecPtr lastSegSwitchLSN;
/*
* Protected by info_lck and WALWriteLock (you must hold either lock to
* read it, but both to update)
* 通过info_lck和WALWriteLock保护
* (必须持有其中之一才能读取,必须全部持有才能更新)
*/
XLogwrtResult LogwrtResult;
/*
* Latest initialized page in the cache (last byte position + 1).
* 在缓存中最后初始化的page(最后一个字节位置 + 1)
*
* To change the identity of a buffer (and InitializedUpTo), you need to
* hold WALBufMappingLock. To change the identity of a buffer that's
* still dirty, the old page needs to be written out first, and for that
* you need WALWriteLock, and you need to ensure that there are no
* in-progress insertions to the page by calling
* WaitXLogInsertionsToFinish().
* 如需改变缓冲区的标识(以及InitializedUpTo),需要持有WALBufMappingLock锁.
* 改变标记为dirty的缓冲区的标识符,旧的page需要先行写出,因此必须持有WALWriteLock锁,
* 而且必须确保没有正在通过调用WaitXLogInsertionsToFinish()进行执行中的插入page操作
*/
XLogRecPtr InitializedUpTo;
/*
* These values do not change after startup, although the pointed-to pages
* and xlblocks values certainly do. xlblock values are protected by
* WALBufMappingLock.
* 在启动后这些值不会修改,虽然pointed-to pages和xlblocks值通常会更改.
* xlblock的值通过WALBufMappingLock锁保护.
*/
//未写入的XLOG pages的缓存
char *pages; /* buffers for unwritten XLOG pages */
//ptr-s的第一个字节 + XLOG_BLCKSZ
XLogRecPtr *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
//已分配的xlog缓冲的索引最高值
int XLogCacheBlck; /* highest allocated xlog buffer index */
/*
* Shared copy of ThisTimeLineID. Does not change after end-of-recovery.
* If we created a new timeline when the system was started up,
* PrevTimeLineID is the old timeline's ID that we forked off from.
* Otherwise it's equal to ThisTimeLineID.
* ThisTimeLineID的共享拷贝.
* 在完成恢复后不要修改.
* 如果在系统启动后创建了一个新的时间线,PrevTimeLineID是从旧时间线分叉的ID.
* 否则,PrevTimeLineID = ThisTimeLineID
*/
TimeLineID ThisTimeLineID;
TimeLineID PrevTimeLineID;
/*
* SharedRecoveryInProgress indicates if we're still in crash or archive
* recovery. Protected by info_lck.
* SharedRecoveryInProgress标记是否处于宕机或者归档恢复中,通过info_lck锁保护.
*/
bool SharedRecoveryInProgress;
/*
* SharedHotStandbyActive indicates if we're still in crash or archive
* recovery. Protected by info_lck.
* SharedHotStandbyActive标记是否处于宕机或者归档恢复中,通过info_lck锁保护.
*/
bool SharedHotStandbyActive;
/*
* WalWriterSleeping indicates whether the WAL writer is currently in
* low-power mode (and hence should be nudged if an async commit occurs).
* Protected by info_lck.
* WalWriterSleeping标记WAL writer进程是否处于"节能"模式
* (因此，如果发生异步提交，应该对其进行微操作).
* 通过info_lck锁保护.
*/
bool WalWriterSleeping;
/*
* recoveryWakeupLatch is used to wake up the startup process to continue
* WAL replay, if it is waiting for WAL to arrive or failover trigger file
* to appear.
* recoveryWakeupLatch等待WAL arrive或者failover触发文件出现,
* 如出现则唤醒启动进程继续执行WAL回放.
*
*/
Latch recoveryWakeupLatch;
/*
* During recovery, we keep a copy of the latest checkpoint record here.
* lastCheckPointRecPtr points to start of checkpoint record and
* lastCheckPointEndPtr points to end+1 of checkpoint record. Used by the
* checkpointer when it wants to create a restartpoint.
* 在恢复期间,我们保存最后检查点记录的一个拷贝在这里.
* lastCheckPointRecPtr指向检查点的起始位置
* lastCheckPointEndPtr指向执行检查点的结束点+1位置
* 在checkpointer进程希望创建一个重新启动的点时使用.
*
* Protected by info_lck.
* 使用info_lck锁保护.
*/
XLogRecPtr lastCheckPointRecPtr;
XLogRecPtr lastCheckPointEndPtr;
CheckPoint lastCheckPoint;
/*
* lastReplayedEndRecPtr points to end+1 of the last record successfully
* replayed. When we're currently replaying a record, ie. in a redo
* function, replayEndRecPtr points to the end+1 of the record being
* replayed, otherwise it's equal to lastReplayedEndRecPtr.
* lastReplayedEndRecPtr指向最后一个成功回放的记录的结束点 + 1的位置.
* 如果正处于redo函数回放记录期间,那么replayEndRecPtr指向正在恢复的记录的结束点 + 1的位置,
* 否则replayEndRecPtr = lastReplayedEndRecPtr
*/
XLogRecPtr lastReplayedEndRecPtr;
TimeLineID lastReplayedTLI;
XLogRecPtr replayEndRecPtr;
TimeLineID replayEndTLI;
/* timestamp of last COMMIT/ABORT record replayed (or being replayed) */
//最后的COMMIT/ABORT回放(或正在回放)记录的时间戳
TimestampTz recoveryLastXTime;
/*
* timestamp of when we started replaying the current chunk of WAL data,
* only relevant for replication or archive recovery
* 我们开始回放当前的WAL chunk的时间戳(仅与复制或存档恢复相关)
*/
TimestampTz currentChunkStartTime;
/* Are we requested to pause recovery? */
//是否请求暂停恢复
bool recoveryPause;
/*
* lastFpwDisableRecPtr points to the start of the last replayed
* XLOG_FPW_CHANGE record that instructs full_page_writes is disabled.
* lastFpwDisableRecPtr指向最后已回放的XLOG_FPW_CHANGE记录(禁用对整个页面的写指令)的起始点.
*/
XLogRecPtr lastFpwDisableRecPtr;
//锁结构
slock_t info_lck; /* locks shared variables shown above */
} XLogCtlData;
static XLogCtlData *XLogCtl = NULL;
二、跟踪分析
跟踪任意一个后台进程,打印全局变量XLogCtl.
(gdb) p XLogCtl
$6 = (XLogCtlData *) 0x7f391e00ea80
(gdb) p *XLogCtl
$7 = {Insert = {insertpos_lck = 0 '\000', CurrBytePos = 5494680728, PrevBytePos = 5494680616,
pad = '\000' , RedoRecPtr = 5510830896, forcePageWrites = false, fullPageWrites = true,
exclusiveBackupState = EXCLUSIVE_BACKUP_NONE, nonExclusiveBackups = 0, lastBackupStart = 0,
WALInsertLocks = 0x7f391e013100}, LogwrtRqst = {Write = 5510831008, Flush = 5510831008}, RedoRecPtr = 5510830896,
ckptXidEpoch = 0, ckptXid = 2036, asyncXactLSN = 5510830896, replicationSlotMinLSN = 0, lastRemovedSegNo = 0,
unloggedLSN = 1, ulsn_lck = 0 '\000', lastSegSwitchTime = 1545962218, lastSegSwitchLSN = 5507670464, LogwrtResult = {
Write = 5510831008, Flush = 5510831008}, InitializedUpTo = 5527601152, pages = 0x7f391e014000 "\230\320\006",
xlblocks = 0x7f391e00f088, XLogCacheBlck = 2047, ThisTimeLineID = 1, PrevTimeLineID = 1,
archiveCleanupCommand = '\000' , SharedRecoveryInProgress = false, SharedHotStandbyActive = false,
WalWriterSleeping = true, recoveryWakeupLatch = {is_set = 0, is_shared = true, owner_pid = 0}, lastCheckPointRecPtr = 0,
lastCheckPointEndPtr = 0, lastCheckPoint = {redo = 0, ThisTimeLineID = 0, PrevTimeLineID = 0, fullPageWrites = false,
nextXidEpoch = 0, nextXid = 0, nextOid = 0, nextMulti = 0, nextMultiOffset = 0, oldestXid = 0, oldestXidDB = 0,
oldestMulti = 0, oldestMultiDB = 0, time = 0, oldestCommitTsXid = 0, newestCommitTsXid = 0, oldestActiveXid = 0},
lastReplayedEndRecPtr = 0, lastReplayedTLI = 0, replayEndRecPtr = 0, replayEndTLI = 0, recoveryLastXTime = 0,
currentChunkStartTime = 0, recoveryPause = false, lastFpwDisableRecPtr = 0, info_lck = 0 '\000'}
(gdb)
其中:
1.XLogCtl->Insert是XLogCtlInsert结构体变量.
2.RedoRecPtr为5510830896 -> 1/48789B30,该值与pg_control文件中的REDO location相对应.
[xdb@localhost ~]$ pg_controldata|grep REDO
Latest checkpoint's REDO location: 1/48789B30
Latest checkpoint's REDO WAL file: 000000010000000100000048
3.ThisTimeLineID&PrevTimeLineID,时间线ID,值为1.
其他相关信息可对照结构体定义阅读.
三、参考资料
PostgreSQL 源码解读（4）- 插入数据#3（heap_insert）
PG Source Code
