本节介绍了插入数据时与WAL相关的处理逻辑，主要是heap_insert->XLogInsert函数。
一、数据结构
静态变量
进程中全局共享
/*
* An array of XLogRecData structs, to hold registered data.
* XLogRecData结构体数组,存储已注册的数据
*/
static XLogRecData *rdatas;
//已使用的入口
static int num_rdatas; /* entries currently used */
//已分配的空间大小
static int max_rdatas; /* allocated size */
//是否调用XLogBeginInsert函数
static bool begininsert_called = false;
宏定义
typedef char* Pointer;//指针
typedef Pointer Page;//Page
#define XLOG_HEAP_INSERT 0x00
/*
* Pointer to a location in the XLOG. These pointers are 64 bits wide,
* because we don't want them ever to overflow.
* 指向XLOG中的位置.
* 这些指针大小为64bit,以确保指针不会溢出.
*/
typedef uint64 XLogRecPtr;
/*
* Additional macros for access to page headers. (Beware multiple evaluation
* of the arguments!)
*/
#define PageGetLSN(page) \
PageXLogRecPtrGet(((PageHeader) (page))->pd_lsn)
#define PageSetLSN(page, lsn) \
PageXLogRecPtrSet(((PageHeader) (page))->pd_lsn, lsn)
/* Buffer size required to store a compressed version of backup block image */
//存储压缩会后的块镜像所需要的缓存空间大小
#define PGLZ_MAX_BLCKSZ PGLZ_MAX_OUTPUT(BLCKSZ)
/*
* Fake spinlock implementation using semaphores --- slow and prone
* to fall foul of kernel limits on number of semaphores, so don't use this
* unless you must! The subroutines appear in spin.c.
* 使用信号量的伪自旋锁实现——很慢而且容易与内核对信号量的限制相冲突，
* 所以除非必须，否则不要使用它!
* 相关的子例程出现在spin.c中。
*/
typedef int slock_t;
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、源码解读
heap_insert
主要实现逻辑是插入元组到堆中,其中存在对WAL(XLog)进行处理的部分.
参见PostgreSQL 源码解读（104）- WAL#1（Insert & WAL-heap_insert函数#1）
XLogInsert
插入一个具有指定的RMID和info字节的XLOG记录，该记录的主体是先前通过XLogRegister*调用注册的数据和缓冲区引用。
/*
* Insert an XLOG record having the specified RMID and info bytes, with the
* body of the record being the data and buffer references registered earlier
* with XLogRegister* calls.
* 插入一个具有指定的RMID和info字节的XLOG记录，
* 该记录的主体是先前通过XLogRegister*调用注册的数据和缓冲区引用。
*
* 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".)
* 返回XLOG指针到记录的结束点(下一条记录的开始)。
* 这可以用作受日志操作影响的数据页的LSN。
* (LSN是必须将XLOG刷新到磁盘才能写出数据页的XLOG点。
* 这实现了基本的WAL规则:“在数据之前写日志”。)
*/
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info)
{
XLogRecPtr EndPos;//uint64
/* XLogBeginInsert() must have been called. */
//在此前,XLogBeginInsert()必须已调用
if (!begininsert_called)
elog(ERROR, "XLogBeginInsert was not called");
/*
* The caller can set rmgr bits, XLR_SPECIAL_REL_UPDATE and
* XLR_CHECK_CONSISTENCY; the rest are reserved for use by me.
* 调用方必须设置rmgr位:XLR_SPECIAL_REL_UPDATE & XLR_CHECK_CONSISTENCY.
* 其余在这里保留使用
*/
if ((info & ~(XLR_RMGR_INFO_MASK |
XLR_SPECIAL_REL_UPDATE |
XLR_CHECK_CONSISTENCY)) != 0)
elog(PANIC, "invalid xlog info mask %02X", info);
TRACE_POSTGRESQL_WAL_INSERT(rmid, info);
/*
* In bootstrap mode, we don't actually log anything but XLOG resources;
* return a phony record pointer.
* 在bootstrap模式,除了XLOG资源外,不需要实际记录内容.
* 返回一个伪记录指针.
*/
if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
{
XLogResetInsertion();
EndPos = SizeOfXLogLongPHD; /* 返回伪记录指针;start of 1st chkpt record */
return EndPos;
}
do
{
//循环
XLogRecPtr RedoRecPtr;
bool doPageWrites;
XLogRecPtr fpw_lsn;
XLogRecData *rdt;
/*
* Get values needed to decide whether to do full-page writes. Since
* we don't yet have an insertion lock, these could change under us,
* but XLogInsertRecord will recheck them once it has a lock.
* 获取决定是否执行全页写入所需的值。
* 由于我们还没有插入锁，所以这些可能会在我们的操作期间被更改，
* 但是XLogInsertRecord一旦有了锁，就会重新检查它们。
*/
GetFullPageWriteInfo(&RedoRecPtr, &doPageWrites);
rdt = XLogRecordAssemble(rmid, info, RedoRecPtr, doPageWrites,
&fpw_lsn);
//curinsert_flags类型为uint8
EndPos = XLogInsertRecord(rdt, fpw_lsn, curinsert_flags);
} while (EndPos == InvalidXLogRecPtr);
XLogResetInsertion();
return EndPos;
}
XLogInsertRecord
插入一个由已经构造的数据chunks链表示的XLOG记录。
/*
* Insert an XLOG record represented by an already-constructed chain of data
* chunks. This is a low-level routine; to construct the WAL record header
* and data, use the higher-level routines in xloginsert.c.
* 插入一个由已经构造的数据chunks链表示的XLOG记录。
* 这是一个比较底层的处理逻辑实现,
* 使用xloginsert.c中高层的子程序构造WAL记录的头部和数据
*
* If 'fpw_lsn' is valid, it is the oldest LSN among the pages that this
* WAL record applies to, that were not included in the record as full page
* images. If fpw_lsn <= RedoRecPtr, the function does not perform the
* insertion and returns InvalidXLogRecPtr. The caller can then recalculate
* which pages need a full-page image, and retry. If fpw_lsn is invalid, the
* record is always inserted.
* 如"fpw_lsn"是有效的,那么该值为在所有的WAL记录应用到pages中最小的LSN,
* 但该值不包括全页镜像的记录.
* 如fpw_lsn <= RedoRecPtr,该函数不会执行插入同时会返回InvalidXLogRecPtr.
* 调用者可以重新计算哪些pages需要full-page image以及记录入口.
* 如果fpw_lsn无效,那么记录已被插入.
*
* 'flags' gives more in-depth control on the record being inserted. See
* XLogSetRecordFlags() for details.
* "flags"在即将插入的记录上给定了更多的深层次的控制.
* 查看函数XLogSetRecordFlags()获取更多的细节信息.
*
* The first XLogRecData in the chain must be for the record header, and its
* data must be MAXALIGNed. XLogInsertRecord fills in the xl_prev and
* xl_crc fields in the header, the rest of the header must already be filled
* by the caller.
* 链中的第一个XLogRecData必须是吉林的头部,数据必须已被MAXALIGNed.
* XLogInsertRecord填充在头部的xl_prev和xl_crc域中,
* 头部的其他域已通过调用者提供.
*
* 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".)
* 返回XLOG指针,指向记录结束的位置(下一记录的起始点).
* 这可以用作受日志操作影响的数据页的LSN。
* (LSN是必须将XLOG刷新到磁盘上才能写出数据页的XLOG点。
* 这实现了WAL的基本规则"在写数据前写日志")
*/
XLogRecPtr
XLogInsertRecord(XLogRecData *rdata,
XLogRecPtr fpw_lsn,
uint8 flags)
{
XLogCtlInsert *Insert = &XLogCtl->Insert;//XLOG写入控制器
pg_crc32c rdata_crc;//uint32
bool inserted;
XLogRecord *rechdr = (XLogRecord *) rdata->data;
uint8 info = rechdr->xl_info & ~XLR_INFO_MASK;
bool isLogSwitch = (rechdr->xl_rmid == RM_XLOG_ID &&
info == XLOG_SWITCH);
XLogRecPtr StartPos;
XLogRecPtr EndPos;
bool prevDoPageWrites = doPageWrites;
/* we assume that all of the record header is in the first chunk */
//假定所有的记录头部数据都处于第一个chunk中
Assert(rdata->len >= SizeOfXLogRecord);
/* cross-check on whether we should be here or not */
//交叉检查
if (!XLogInsertAllowed())
elog(ERROR, "cannot make new WAL entries during recovery");
/*----------
*
* We have now done all the preparatory work we can without holding a
* lock or modifying shared state. From here on, inserting the new WAL
* record to the shared WAL buffer cache is a two-step process:
* 现在，我们已经完成了所有的准备工作，无需持有锁或修改共享状态。
* 从这里开始，将新的WAL记录插入到共享的WAL缓冲区缓存需要两个步骤:
*
* 1. Reserve the right amount of space from the WAL. The current head of
* reserved space is kept in Insert->CurrBytePos, and is protected by
* insertpos_lck.
* 1. 从WAL中预留合适的空间.预留空间的头部保存在Insert->CurrBytePos中,
* 通过insertpos_lck锁保护
*
* 2. Copy the record to the reserved WAL space. This involves finding the
* correct WAL buffer containing the reserved space, and copying the
* record in place. This can be done concurrently in multiple processes.
* 2. 拷贝记录到保留的WAL空间中.这会涉及到寻找持有保留空间的正确的WAL缓冲区,
* 以及拷贝记录到合适的位置上.
* 在多进程间必须同步完成.
*
* To keep track of which insertions are still in-progress, each concurrent
* inserter acquires an insertion lock. In addition to just indicating that
* an insertion is in progress, the lock tells others how far the inserter
* has progressed. There is a small fixed number of insertion locks,
* determined by NUM_XLOGINSERT_LOCKS. When an inserter crosses a page
* boundary, it updates the value stored in the lock to the how far it has
* inserted, to allow the previous buffer to be flushed.
* 为了跟踪那个插入操作仍处于进行当中,每一个当前的插入器需要insertion锁.
* 除了用于标识那个insertion处于进行当中,锁同时会告知其他插入器可以处理的边界界限.
* 系统有少数几个固定数量的insertion所,通过参数NUM_XLOGINSERT_LOCKS定义.
* 如果某个插入器跨越了page的边界,该插入器会更新存储在锁中的值以表示它已插入的大小,
* 这样方便刷新先前的缓存.
*
* Holding onto an insertion lock also protects RedoRecPtr and
* fullPageWrites from changing until the insertion is finished.
* 持有插入锁还可以保护RedoRecPtr和fullpagewrite在插入完成之前不受更改。
*
* Step 2 can usually be done completely in parallel. If the required WAL
* page is not initialized yet, you have to grab WALBufMappingLock to
* initialize it, but the WAL writer tries to do that ahead of insertions
* to avoid that from happening in the critical path.
* 步骤2通常可以完全并行完成。
* 如果所需的WAL页面还没有初始化，您必须获取WALBufMappingLock来初始化它，
* 但是WAL writer进程会在插入之前尝试这样做，以避免在关键路径中发生这种情况。
*
*----------
*/
START_CRIT_SECTION();
if (isLogSwitch)
WALInsertLockAcquireExclusive();
else
WALInsertLockAcquire();
/*
* Check to see if my copy of RedoRecPtr is out of date. If so, may have
* to go back and have the caller recompute everything. This can only
* happen just after a checkpoint, so it's better to be slow in this case
* and fast otherwise.
* 看看进程的RedoRecPtr是不是过期了。
* 如果是，可能需要返回并让调用方重新计算所有内容。
* 这只会在检查点之后才会发生，所以在这种情况下最好慢一点，否则最好快一点。
*
* Also check to see if fullPageWrites or forcePageWrites was just turned
* on; if we weren't already doing full-page writes then go back and
* recompute.
* 还要检查是否打开了fullpagewrite或forcepagewrite;
* 如果我们还没有完成整页的写操作，那么返回并重新计算。
*
* If we aren't doing full-page writes then RedoRecPtr doesn't actually
* affect the contents of the XLOG record, so we'll update our local copy
* but not force a recomputation. (If doPageWrites was just turned off,
* we could recompute the record without full pages, but we choose not to
* bother.)
* 如果我们并没有在执行全页写操作，那么RedoRecPtr实际上不会影响XLOG记录的内容，
* 因此我们将更新本地副本，但不会强制进行重新计算。
* (如果doPageWrites关闭，可以在没有完整页面的情况下重新计算记录，但我们没有这种麻烦的做法。)
*
*/
if (RedoRecPtr != Insert->RedoRecPtr)
{
Assert(RedoRecPtr < Insert->RedoRecPtr);
RedoRecPtr = Insert->RedoRecPtr;
}
doPageWrites = (Insert->fullPageWrites || Insert->forcePageWrites);
if (doPageWrites &&
(!prevDoPageWrites ||
(fpw_lsn != InvalidXLogRecPtr && fpw_lsn <= RedoRecPtr)))
{
/*
* Oops, some buffer now needs to be backed up that the caller didn't
* back up. Start over.
* 糟糕，现在需要备份一些调用者没有备份的缓冲区。
* 让我们重新开始吧。
*/
WALInsertLockRelease();
END_CRIT_SECTION();
return InvalidXLogRecPtr;
}
/*
* Reserve space for the record in the WAL. This also sets the xl_prev
* pointer.
* 在WAL预留记录空间.同时会设置xl_prev指针.
*
*/
if (isLogSwitch)
inserted = ReserveXLogSwitch(&StartPos, &EndPos, &rechdr->xl_prev);
else
{
ReserveXLogInsertLocation(rechdr->xl_tot_len, &StartPos, &EndPos,
&rechdr->xl_prev);
inserted = true;
}
if (inserted)
{
/*
* Now that xl_prev has been filled in, calculate CRC of the record
* header.
* 现在xl_prev指针已填充,计算记录头部的CRC
*/
rdata_crc = rechdr->xl_crc;
COMP_CRC32C(rdata_crc, rechdr, offsetof(XLogRecord, xl_crc));
FIN_CRC32C(rdata_crc);
rechdr->xl_crc = rdata_crc;
/*
* All the record data, including the header, is now ready to be
* inserted. Copy the record in the space reserved.
* 所有的记录数据,包括头部数据,准备插入!
* 拷贝记录到保留空间中.
*/
CopyXLogRecordToWAL(rechdr->xl_tot_len, isLogSwitch, rdata,
StartPos, EndPos);
/*
* Unless record is flagged as not important, update LSN of last
* important record in the current slot. When holding all locks, just
* update the first one.
* 除非记录被标记为不重要，否则更新当前slot中最后一条重要记录的LSN。
* 如持有所有锁，只需更新第一个。
*/
if ((flags & XLOG_MARK_UNIMPORTANT) == 0)
{
int lockno = holdingAllLocks ? 0 : MyLockNo;
WALInsertLocks[lockno].l.lastImportantAt = StartPos;
}
}
else
{
/*
* This was an xlog-switch record, but the current insert location was
* already exactly at the beginning of a segment, so there was no need
* to do anything.
* 这是一个xlog-switch记录，但是当前插入位置已经确切地位于段的开头，所以不需要做任何事情。
*/
}
/*
* Done! Let others know that we're finished.
* 全部完成!让其他插入器知道我们已经完成了!
*/
WALInsertLockRelease();
MarkCurrentTransactionIdLoggedIfAny();
END_CRIT_SECTION();
/*
* Update shared LogwrtRqst.Write, if we crossed page boundary.
* 如跨越了page边界,更新共享的LogwrtRqst.Write变量
*/
if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
{
SpinLockAcquire(&XLogCtl->info_lck);
/* advance global request to include new block(s) */
//预先请求包含新块(s)
if (XLogCtl->LogwrtRqst.Write < EndPos)
XLogCtl->LogwrtRqst.Write = EndPos;
/* update local result copy while I have the chance */
//如有机会,更新本地的结果拷贝
LogwrtResult = XLogCtl->LogwrtResult;
SpinLockRelease(&XLogCtl->info_lck);
}
/*
* If this was an XLOG_SWITCH record, flush the record and the empty
* padding space that fills the rest of the segment, and perform
* end-of-segment actions (eg, notifying archiver).
* 如果这是一条XLOG_SWITCH记录，
* 刷新记录和填充该段其余部分的空白填充空间，
* 并执行段结束操作(例如，通知归档器)。
*/
if (isLogSwitch)
{
TRACE_POSTGRESQL_WAL_SWITCH();
XLogFlush(EndPos);
/*
* Even though we reserved the rest of the segment for us, which is
* reflected in EndPos, we return a pointer to just the end of the
* xlog-switch record.
* 即使我们为自己保留了段的其余部分(这反映在EndPos中)，
* 我们也只返回一个指向xlog-switch记录末尾的指针。
*/
if (inserted)
{
EndPos = StartPos + SizeOfXLogRecord;
if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
{
uint64 offset = XLogSegmentOffset(EndPos, wal_segment_size);
if (offset == EndPos % XLOG_BLCKSZ)
EndPos += SizeOfXLogLongPHD;
else
EndPos += SizeOfXLogShortPHD;
}
}
}
#ifdef WAL_DEBUG//DEBUG代码
if (XLOG_DEBUG)
{
static XLogReaderState *debug_reader = NULL;
StringInfoData buf;
StringInfoData recordBuf;
char *errormsg = NULL;
MemoryContext oldCxt;
oldCxt = MemoryContextSwitchTo(walDebugCxt);
initStringInfo(&buf);
appendStringInfo(&buf, "INSERT @ %X/%X: ",
(uint32) (EndPos >> 32), (uint32) EndPos);
/*
* We have to piece together the WAL record data from the XLogRecData
* entries, so that we can pass it to the rm_desc function as one
* contiguous chunk.
*/
initStringInfo(&recordBuf);
for (; rdata != NULL; rdata = rdata->next)
appendBinaryStringInfo(&recordBuf, rdata->data, rdata->len);
if (!debug_reader)
debug_reader = XLogReaderAllocate(wal_segment_size, NULL, NULL);
if (!debug_reader)
{
appendStringInfoString(&buf, "error decoding record: out of memory");
}
else if (!DecodeXLogRecord(debug_reader, (XLogRecord *) recordBuf.data,
&errormsg))
{
appendStringInfo(&buf, "error decoding record: %s",
errormsg ? errormsg : "no error message");
}
else
{
appendStringInfoString(&buf, " - ");
xlog_outdesc(&buf, debug_reader);
}
elog(LOG, "%s", buf.data);
pfree(buf.data);
pfree(recordBuf.data);
MemoryContextSwitchTo(oldCxt);
}
#endif
/*
* Update our global variables
* 更新全局变量
*/
ProcLastRecPtr = StartPos;
XactLastRecEnd = EndPos;
return EndPos;
}
三、跟踪分析
测试脚本如下
insert into t_wal_partition(c1,c2,c3) VALUES(0,'HASH0','HAHS0');
启动gdb,设置断点,进入XLogInsert
(gdb) b XLogInsert
Breakpoint 1 at 0x5652d6: file xloginsert.c, line 420.
(gdb) c
Continuing.
Breakpoint 1, XLogInsert (rmid=10 '\n', info=0 '\000') at xloginsert.c:420
420 if (!begininsert_called)
在此前,XLogBeginInsert()必须已调用
420 if (!begininsert_called)
(gdb) n
调用方必须设置rmgr位:XLR_SPECIAL_REL_UPDATE & XLR_CHECK_CONSISTENCY.其余在这里保留使用
427 if ((info & ~(XLR_RMGR_INFO_MASK |
(gdb) n
432 TRACE_POSTGRESQL_WAL_INSERT(rmid, info);
进入循环
(gdb) n
438 if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
(gdb)
457 GetFullPageWriteInfo(&RedoRecPtr, &doPageWrites);
获取决定是否执行全页写入所需的值
(gdb) p *RedoRecPtr
$1 = 1166604425
(gdb) p doPageWrites
$2 = false
(gdb) n
459 rdt = XLogRecordAssemble(rmid, info, RedoRecPtr, doPageWrites,
(gdb) p RedoRecPtr
$3 = 5411227832
(gdb) p doPageWrites
$4 = true
获取rdt
(gdb) n
462 EndPos = XLogInsertRecord(rdt, fpw_lsn, curinsert_flags);
(gdb) p *rdt
$5 = {next = 0x2a911b8, data = 0x2a8f460 , len = 51}
XLogInsertRecord->调用XLogInsertRecord,进入XLogInsertRecord函数
fpw_lsn=0, flags=1 '\001'
(gdb) step
XLogInsertRecord (rdata=0xf9cc70 , fpw_lsn=0, flags=1 '\001') at xlog.c:970
970 XLogCtlInsert *Insert = &XLogCtl->Insert;
XLogInsertRecord->获取插入管理器
(gdb) n
973 XLogRecord *rechdr = (XLogRecord *) rdata->data;
(gdb) p *Insert
$6 = {insertpos_lck = 0 '\000', CurrBytePos = 5395369608, PrevBytePos = 5395369552, pad = '\000' ,
RedoRecPtr = 5411227832, forcePageWrites = false, fullPageWrites = true, exclusiveBackupState = EXCLUSIVE_BACKUP_NONE,
nonExclusiveBackups = 0, lastBackupStart = 0, WALInsertLocks = 0x7fa2523d4100}
XLogInsertRecord->变量赋值
(gdb) n
974 uint8 info = rechdr->xl_info & ~XLR_INFO_MASK;
(gdb)
975 bool isLogSwitch = (rechdr->xl_rmid == RM_XLOG_ID &&
(gdb)
979 bool prevDoPageWrites = doPageWrites;
(gdb)
982 Assert(rdata->len >= SizeOfXLogRecord);
(gdb)
(gdb) p *rechdr
$7 = {xl_tot_len = 210, xl_xid = 1948, xl_prev = 0, xl_info = 0 '\000', xl_rmid = 10 '\n', xl_crc = 3212449170}
(gdb) p info
$8 = 0 '\000'
(gdb) p isLogSwitch
$9 = false
(gdb) p prevDoPageWrites
$10 = true
XLogInsertRecord->执行相关判断,开启CRIT_SECTION,并获取WAL插入锁
(gdb) n
985 if (!XLogInsertAllowed())
(gdb)
1020 START_CRIT_SECTION();
(gdb)
1021 if (isLogSwitch)
(gdb)
1024 WALInsertLockAcquire();
(gdb)
1042 if (RedoRecPtr != Insert->RedoRecPtr)
(gdb)
XLogInsertRecord->执行相关判断,更新doPageWrites
(gdb) p RedoRecPtr
$11 = 5411227832
(gdb) p Insert->RedoRecPtr
$12 = 5411227832
(gdb) n
1047 doPageWrites = (Insert->fullPageWrites || Insert->forcePageWrites);
(gdb)
1049 if (doPageWrites &&
(gdb) p doPageWrites
$13 = true
(gdb) n
1050 (!prevDoPageWrites ||
(gdb)
1049 if (doPageWrites &&
XLogInsertRecord->在WAL预留记录空间.同时会设置xl_prev指针.
(gdb)
1050 (!prevDoPageWrites ||
(gdb)
1066 if (isLogSwitch)
(gdb)
1070 ReserveXLogInsertLocation(rechdr->xl_tot_len, &StartPos, &EndPos,
(gdb)
1072 inserted = true;
(gdb) p rechdr->xl_tot_len
$14 = 210
(gdb) p StartPos
$15 = 5411228000
(gdb) p EndPos
$16 = 5411228216
(gdb) p *rechdr->xl_prev
Cannot access memory at address 0x14288c928
(gdb) p rechdr->xl_prev
$17 = 5411227944
(gdb)
XLogInsertRecord->现在xl_prev指针已填充,计算记录头部的CRC
(gdb) n
1075 if (inserted)
(gdb)
1081 rdata_crc = rechdr->xl_crc;
(gdb)
1082 COMP_CRC32C(rdata_crc, rechdr, offsetof(XLogRecord, xl_crc));
(gdb)
1083 FIN_CRC32C(rdata_crc);
(gdb)
1084 rechdr->xl_crc = rdata_crc;
(gdb)
1090 CopyXLogRecordToWAL(rechdr->xl_tot_len, isLogSwitch, rdata,
(gdb) p rdata_crc
$18 = 2310972234
(gdb) p *rechdr
$19 = {xl_tot_len = 210, xl_xid = 1948, xl_prev = 5411227944, xl_info = 0 '\000', xl_rmid = 10 '\n', xl_crc = 2310972234}
(gdb)
XLogInsertRecord->所有的记录数据,包括头部数据已OK,准备插入!拷贝记录到保留空间中.
除非记录被标记为不重要，否则更新当前slot中最后一条重要记录的LSN.
(gdb) n
1098 if ((flags & XLOG_MARK_UNIMPORTANT) == 0)
(gdb)
1100 int lockno = holdingAllLocks ? 0 : MyLockNo;
(gdb)
(gdb) n
1102 WALInsertLocks[lockno].l.lastImportantAt = StartPos;
(gdb)
1117 WALInsertLockRelease();
XLogInsertRecord->全部完成!让其他插入器知道我们已经完成了!
如跨越了page边界,更新共享的LogwrtRqst.Write变量
(gdb)
1117 WALInsertLockRelease();
(gdb) n
1119 MarkCurrentTransactionIdLoggedIfAny();
(gdb)
1121 END_CRIT_SECTION();
(gdb)
1126 if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
(gdb)
1142 if (isLogSwitch)
XLogInsertRecord->更新全局变量,函数返回
(gdb)
1220 ProcLastRecPtr = StartPos;
(gdb)
1221 XactLastRecEnd = EndPos;
(gdb)
1223 return EndPos;
(gdb)
1224 }
返回XLogInsert,重置insertion,返回EndPos,结束
(gdb)
XLogInsert (rmid=10 '\n', info=0 '\000') at xloginsert.c:463
463 } while (EndPos == InvalidXLogRecPtr);
(gdb) n
465 XLogResetInsertion();
(gdb)
467 return EndPos;
(gdb)
468 }
(gdb) p EndPos
$20 = 5411228216
(gdb)
$21 = 5411228216
(gdb) n
heap_insert (relation=0x7fa280616228, tup=0x2b15440, cid=0, options=0, bistate=0x0) at heapam.c:2590
2590 PageSetLSN(page, recptr);
(gdb)
DONE!
四、参考资料
Write Ahead Logging — WAL
PostgreSQL 源码解读（4）- 插入数据#3（heap_insert）
PgSQL · 特性分析 · 数据库崩溃恢复（上）
PgSQL · 特性分析 · 数据库崩溃恢复（下）
PgSQL · 特性分析 · Write-Ahead Logging机制浅析
PostgreSQL WAL Buffers, Clog Buffers Deep Dive