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.

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' <repeats 127 times>, 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' <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)

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 REDOLatest checkpoint's REDO location: 1/48789B30Latest 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.

PostgreSQL 源码解读（4）- 插入数据#3（heap_insert）
PG Source Code