PostgreSQL 源码解读（112）- WAL#8（XLogCtrl数据结构）

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.

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 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.

三、参考资料

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