本节介绍了插入数据时与WAL相关的处理逻辑，主要包括heap_insert依赖的函数XLogBeginInsert/XLogRegisterBufData/XLogRegisterData/XLogSetRecordFlags。
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;
registered_buffer
对于每一个使用XLogRegisterBuffer注册的每个数据块,填充到registered_buffer结构体中
/*
* For each block reference registered with XLogRegisterBuffer, we fill in
* a registered_buffer struct.
* 对于每一个使用XLogRegisterBuffer注册的每个数据块,
* 填充到registered_buffer结构体中
*/
typedef struct
{
//slot是否在使用?
bool in_use; /* is this slot in use? */
//REGBUF_* 相关标记
uint8 flags; /* REGBUF_* flags */
//定义关系和数据库的标识符
RelFileNode rnode; /* identifies the relation and block */
//fork进程编号
ForkNumber forkno;
//块编号
BlockNumber block;
//页内容
Page page; /* page content */
//rdata链中的数据总大小
uint32 rdata_len; /* total length of data in rdata chain */
//使用该数据块注册的数据链头
XLogRecData *rdata_head; /* head of the chain of data registered with
* this block */
//使用该数据块注册的数据链尾
XLogRecData *rdata_tail; /* last entry in the chain, or &rdata_head if
* empty */
//临时rdatas数据引用,用于存储XLogRecordAssemble()中使用的备份块数据
XLogRecData bkp_rdatas[2]; /* temporary rdatas used to hold references to
* backup block data in XLogRecordAssemble() */
/* buffer to store a compressed version of backup block image */
//用于存储压缩版本的备份块镜像的缓存
char compressed_page[PGLZ_MAX_BLCKSZ];
} registered_buffer;
//registered_buffer指正
static registered_buffer *registered_buffers;
//已分配的大小
static int max_registered_buffers; /* allocated size */
//最大块号 + 1(当前注册块)
static int max_registered_block_id = 0; /* highest block_id + 1 currently
* registered */
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;
XLogRecData
xloginsert.c中的函数构造一个XLogRecData结构体链用于标识最后的WAL记录
/*
* The functions in xloginsert.c construct a chain of XLogRecData structs
* to represent the final WAL record.
* xloginsert.c中的函数构造一个XLogRecData结构体链用于标识最后的WAL记录
*/
typedef struct XLogRecData
{
//链中的下一个结构体,如无则为NULL
struct XLogRecData *next; /* next struct in chain, or NULL */
//rmgr数据的起始地址
char *data; /* start of rmgr data to include */
//rmgr数据大小
uint32 len; /* length of rmgr data to include */
} XLogRecData;
registered_buffer/registered_buffers
对于每一个使用XLogRegisterBuffer注册的每个数据块,填充到registered_buffer结构体中
/*
* For each block reference registered with XLogRegisterBuffer, we fill in
* a registered_buffer struct.
* 对于每一个使用XLogRegisterBuffer注册的每个数据块,
* 填充到registered_buffer结构体中
*/
typedef struct
{
//slot是否在使用?
bool in_use; /* is this slot in use? */
//REGBUF_* 相关标记
uint8 flags; /* REGBUF_* flags */
//定义关系和数据库的标识符
RelFileNode rnode; /* identifies the relation and block */
//fork进程编号
ForkNumber forkno;
//块编号
BlockNumber block;
//页内容
Page page; /* page content */
//rdata链中的数据总大小
uint32 rdata_len; /* total length of data in rdata chain */
//使用该数据块注册的数据链头
XLogRecData *rdata_head; /* head of the chain of data registered with
* this block */
//使用该数据块注册的数据链尾
XLogRecData *rdata_tail; /* last entry in the chain, or &rdata_head if
* empty */
//临时rdatas数据引用,用于存储XLogRecordAssemble()中使用的备份块数据
XLogRecData bkp_rdatas[2]; /* temporary rdatas used to hold references to
* backup block data in XLogRecordAssemble() */
/* buffer to store a compressed version of backup block image */
//用于存储压缩版本的备份块镜像的缓存
char compressed_page[PGLZ_MAX_BLCKSZ];
} registered_buffer;
//registered_buffer指针(全局变量)
static registered_buffer *registered_buffers;
//已分配的大小
static int max_registered_buffers; /* allocated size */
//最大块号 + 1(当前注册块)
static int max_registered_block_id = 0; /* highest block_id + 1 currently
* registered */
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）
XLogBeginInsert
开始构造WAL记录.
必须在调用XLogRegister*和XLogInsert()函数前调用.
/*
* Begin constructing a WAL record. This must be called before the
* XLogRegister* functions and XLogInsert().
* 开始构造WAL记录.
* 必须在调用XLogRegister*和XLogInsert()函数前调用.
*/
void
XLogBeginInsert(void)
{
//验证逻辑
Assert(max_registered_block_id == 0);
Assert(mainrdata_last == (XLogRecData *) &mainrdata_head);
Assert(mainrdata_len == 0);
/* cross-check on whether we should be here or not */
//交叉校验是否应该在这里还是不应该在这里出现
if (!XLogInsertAllowed())
elog(ERROR, "cannot make new WAL entries during recovery");
if (begininsert_called)
elog(ERROR, "XLogBeginInsert was already called");
//变量赋值
begininsert_called = true;
}
/*
* Is this process allowed to insert new WAL records?
* 判断该进程是否允许插入新的WAL记录
*
* Ordinarily this is essentially equivalent to !RecoveryInProgress().
* But we also have provisions for forcing the result "true" or "false"
* within specific processes regardless of the global state.
* 通常，这本质上等同于! recoverinprogress()。
* 但我们也有规定，无论全局状况如何，都要在特定进程中强制实现“正确”或“错误”的结果。
*/
bool
XLogInsertAllowed(void)
{
/*
* If value is "unconditionally true" or "unconditionally false", just
* return it. This provides the normal fast path once recovery is known
* done.
* 如果值为“无条件为真”或“无条件为假”，则返回。
* 这提供正常的快速判断路径。
*/
if (LocalXLogInsertAllowed >= 0)
return (bool) LocalXLogInsertAllowed;
/*
* Else, must check to see if we're still in recovery.
* 否则,必须检查是否处于恢复状态
*/
if (RecoveryInProgress())
return false;
/*
* On exit from recovery, reset to "unconditionally true", since there is
* no need to keep checking.
* 从恢复中退出,由于不需要继续检查,重置为"无条件为真"
*/
LocalXLogInsertAllowed = 1;
return true;
}
XLogRegisterData
添加数据到正在构造的WAL记录中
/*
* Add data to the WAL record that's being constructed.
* 添加数据到正在构造的WAL记录中
*
* The data is appended to the "main chunk", available at replay with
* XLogRecGetData().
* 数据追加到"main chunk"中,用于XLogRecGetData()函数回放
*/
void
XLogRegisterData(char *data, int len)
{
XLogRecData *rdata;//数据
//验证是否已调用begin
Assert(begininsert_called);
//验证大小
if (num_rdatas >= max_rdatas)
elog(ERROR, "too much WAL data");
rdata = &rdatas[num_rdatas++];
rdata->data = data;
rdata->len = len;
/*
* we use the mainrdata_last pointer to track the end of the chain, so no
* need to clear 'next' here.
* 使用mainrdata_last指针跟踪链条的结束点,在这里不需要清除next变量
*/
mainrdata_last->next = rdata;
mainrdata_last = rdata;
mainrdata_len += len;
}
XLogRegisterBuffer
在缓冲区中注册已构建的WAL记录的依赖,在WAL-logged操作更新每一个page时必须调用此函数
/*
* Register a reference to a buffer with the WAL record being constructed.
* This must be called for every page that the WAL-logged operation modifies.
* 在缓冲区中注册已构建的WAL记录的依赖
* 在WAL-logged操作更新每一个page时必须调用此函数
*/
void
XLogRegisterBuffer(uint8 block_id, Buffer buffer, uint8 flags)
{
registered_buffer *regbuf;//缓冲
/* NO_IMAGE doesn't make sense with FORCE_IMAGE */
//NO_IMAGE不能与REGBUF_NO_IMAGE同时使用
Assert(!((flags & REGBUF_FORCE_IMAGE) && (flags & (REGBUF_NO_IMAGE))));
Assert(begininsert_called);
//块ID > 最大已注册的缓冲区,报错
if (block_id >= max_registered_block_id)
{
if (block_id >= max_registered_buffers)
elog(ERROR, "too many registered buffers");
max_registered_block_id = block_id + 1;
}
//赋值
regbuf = ®istered_buffers[block_id];
//获取Tag
BufferGetTag(buffer, ®buf->rnode, ®buf->forkno, ®buf->block);
regbuf->page = BufferGetPage(buffer);
regbuf->flags = flags;
regbuf->rdata_tail = (XLogRecData *) ®buf->rdata_head;
regbuf->rdata_len = 0;
/*
* Check that this page hasn't already been registered with some other
* block_id.
* 检查该page是否已被其他block_id注册
*/
#ifdef USE_ASSERT_CHECKING
{
int i;
for (i = 0; i < max_registered_block_id; i++)//循环检查
{
registered_buffer *regbuf_old = ®istered_buffers[i];
if (i == block_id || !regbuf_old->in_use)
continue;
Assert(!RelFileNodeEquals(regbuf_old->rnode, regbuf->rnode) ||
regbuf_old->forkno != regbuf->forkno ||
regbuf_old->block != regbuf->block);
}
}
#endif
regbuf->in_use = true;//标记为使用
}
/*
* BufferGetTag
* Returns the relfilenode, fork number and block number associated with
* a buffer.
* 返回与缓冲区相关的relfilenode,fork编号和块号
*/
void
BufferGetTag(Buffer buffer, RelFileNode *rnode, ForkNumber *forknum,
BlockNumber *blknum)
{
BufferDesc *bufHdr;
/* Do the same checks as BufferGetBlockNumber. */
//验证buffer已被pinned
Assert(BufferIsPinned(buffer));
if (BufferIsLocal(buffer))
bufHdr = GetLocalBufferDescriptor(-buffer - 1);
else
bufHdr = GetBufferDescriptor(buffer - 1);
/* pinned, so OK to read tag without spinlock */
//pinned,不需要spinlock读取tage
*rnode = bufHdr->tag.rnode;
*forknum = bufHdr->tag.forkNum;
*blknum = bufHdr->tag.blockNum;
}
/*
* BufferIsLocal
* True iff the buffer is local (not visible to other backends).
* 如缓冲区对其他后台进程不不可见,则为本地buffer
*/
#define BufferIsLocal(buffer) ((buffer) < 0)
#define GetBufferDescriptor(id) (&BufferDescriptors[(id)].bufferdesc)
#define GetLocalBufferDescriptor(id) (&LocalBufferDescriptors[(id)])
BufferDesc *LocalBufferDescriptors = NULL;
BufferDescPadded *BufferDescriptors;
XLogRegisterBufData
在正在构造的WAL记录中添加buffer相关的数据.
/*
* Add buffer-specific data to the WAL record that's being constructed.
* 在正在构造的WAL记录中添加buffer相关的数据.
*
* Block_id must reference a block previously registered with
* XLogRegisterBuffer(). If this is called more than once for the same
* block_id, the data is appended.
* Block_id必须引用先前注册到XLogRegisterBuffer()中的数据块。
* 如果对同一个block_id不止一次调用，那么数据将会追加。
*
* The maximum amount of data that can be registered per block is 65535
* bytes. That should be plenty; if you need more than BLCKSZ bytes to
* reconstruct the changes to the page, you might as well just log a full
* copy of it. (the "main data" that's not associated with a block is not
* limited)
* 每个块可注册的最大大小是65535Bytes.
* 通常来说这已经足够了;如果需要大小比BLCKSZ字节更大的数据用于重建页面的变化,
* 那么需要整页进行拷贝.
* (与数据块相关的"main data"是不受限的)
*/
void
XLogRegisterBufData(uint8 block_id, char *data, int len)
{
registered_buffer *regbuf;//注册的缓冲区
XLogRecData *rdata;//数据
Assert(begininsert_called);//XLogBeginInsert函数已调用
/* find the registered buffer struct */
//寻找已注册的缓存结构体
regbuf = ®istered_buffers[block_id];
if (!regbuf->in_use)
elog(ERROR, "no block with id %d registered with WAL insertion",
block_id);
if (num_rdatas >= max_rdatas)
elog(ERROR, "too much WAL data");
rdata = &rdatas[num_rdatas++];
rdata->data = data;
rdata->len = len;
regbuf->rdata_tail->next = rdata;
regbuf->rdata_tail = rdata;
regbuf->rdata_len += len;
}
XLogSetRecordFlags
为即将"到来"的WAL记录设置插入状态标记
XLOG_INCLUDE_ORIGIN 确定复制起点是否应该包含在记录中
XLOG_MARK_UNIMPORTANT 表示记录对于持久性并不重要，这可以避免触发WAL归档和其他后台活动
/*
* Set insert status flags for the upcoming WAL record.
* 为即将"到来"的WAL记录设置插入状态标记
*
* The flags that can be used here are:
* - XLOG_INCLUDE_ORIGIN, to determine if the replication origin should be
* included in the record.
* - XLOG_MARK_UNIMPORTANT, to signal that the record is not important for
* durability, which allows to avoid triggering WAL archiving and other
* background activity.
* 标记用于:
* - XLOG_INCLUDE_ORIGIN 确定复制起点是否应该包含在记录中
* - XLOG_MARK_UNIMPORTANT 表示记录对于持久性并不重要，这可以避免触发WAL归档和其他后台活动。
*/
void
XLogSetRecordFlags(uint8 flags)
{
Assert(begininsert_called);
curinsert_flags = flags;
}
三、跟踪分析
测试脚本如下
insert into t_wal_partition(c1,c2,c3) VALUES(0,'HASH0','HAHS0');
XLogBeginInsert
启动gdb,设置断点,进入XLogBeginInsert
(gdb) b XLogBeginInsert
Breakpoint 1 at 0x564897: file xloginsert.c, line 122.
(gdb) c
Continuing.
Breakpoint 1, XLogBeginInsert () at xloginsert.c:122
122 Assert(max_registered_block_id == 0);
校验,调用XLogInsertAllowed
122 Assert(max_registered_block_id == 0);
(gdb) n
123 Assert(mainrdata_last == (XLogRecData *) &mainrdata_head);
(gdb)
124 Assert(mainrdata_len == 0);
(gdb)
127 if (!XLogInsertAllowed())
(gdb) step
XLogInsertAllowed () at xlog.c:8126
8126 if (LocalXLogInsertAllowed >= 0)
(gdb) n
8132 if (RecoveryInProgress())
(gdb)
8139 LocalXLogInsertAllowed = 1;
(gdb)
8140 return true;
(gdb)
8141 }
(gdb)
赋值,设置begininsert_called为T,返回
(gdb)
XLogBeginInsert () at xloginsert.c:130
130 if (begininsert_called)
(gdb) p begininsert_called
$1 = false
(gdb) n
133 begininsert_called = true;
(gdb)
134 }
(gdb)
heap_insert (relation=0x7f5cc0338228, tup=0x29b2440, cid=0, options=0, bistate=0x0) at heapam.c:2567
2567 XLogRegisterData((char *) &xlrec, SizeOfHeapInsert);
(gdb)
XLogRegisterData
进入XLogRegisterData函数
(gdb) step
XLogRegisterData (data=0x7fff03ba99e0 "\002", len=3) at xloginsert.c:327
327 Assert(begininsert_called);
(gdb) p *data
$2 = 2 '\002'
(gdb) p *(xl_heap_insert *)data
$3 = {offnum = 2, flags = 0 '\000'}
执行相关判断,并赋值
rdatas是XLogRecData结构体指针,全局静态变量:
static XLogRecData *rdatas;
(gdb) n
329 if (num_rdatas >= max_rdatas)
(gdb) p num_rdatas
$4 = 0
(gdb) p max_rdatas
$5 = 20
(gdb) n
331 rdata = &rdatas[num_rdatas++];
(gdb) p rdatas[0]
$6 = {next = 0x0, data = 0x0, len = 0}
(gdb) p rdatas[1]
$7 = {next = 0x0, data = 0x0, len = 0}
相关结构体赋值
其中mainrdata_last是mainrdata_head的地址:
static XLogRecData *mainrdata_head;
static XLogRecData *mainrdata_last = (XLogRecData *) &mainrdata_head;
(gdb) n
333 rdata->data = data;
(gdb)
334 rdata->len = len;
(gdb)
341 mainrdata_last->next = rdata;
(gdb)
342 mainrdata_last = rdata;
(gdb)
344 mainrdata_len += len;
(gdb)
345 }
完成调用,回到heap_insert
(gdb) n
heap_insert (relation=0x7f5cc0338228, tup=0x29b2440, cid=0, options=0, bistate=0x0) at heapam.c:2569
2569 xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
XLogRegisterBuffer
进入XLogRegisterBuffer
(gdb) step
XLogRegisterBuffer (block_id=0 '\000', buffer=99, flags=8 '\b') at xloginsert.c:218
218 Assert(!((flags & REGBUF_FORCE_IMAGE) && (flags & (REGBUF_NO_IMAGE))));
判断block_id,设置max_registered_block_id变量等.
注:max_registered_buffers初始化为5
(gdb) n
219 Assert(begininsert_called);
(gdb)
221 if (block_id >= max_registered_block_id)
(gdb) p max_registered_block_id
$14 = 0
(gdb) n
223 if (block_id >= max_registered_buffers)
(gdb) p max_registered_buffers
$15 = 5
(gdb) n
225 max_registered_block_id = block_id + 1;
(gdb)
228 regbuf = ®istered_buffers[block_id];
(gdb) p max_registered_buffers
$16 = 5
(gdb) p max_registered_block_id
$17 = 1
(gdb) n
230 BufferGetTag(buffer, ®buf->rnode, ®buf->forkno, ®buf->block);
(gdb) p *regbuf
$18 = {in_use = false, flags = 0 '\000', rnode = {spcNode = 0, dbNode = 0, relNode = 0}, forkno = MAIN_FORKNUM, block = 0,
page = 0x0, rdata_len = 0, rdata_head = 0x0, rdata_tail = 0x0, bkp_rdatas = {{next = 0x0, data = 0x0, len = 0}, {
next = 0x0, data = 0x0, len = 0}}, compressed_page = '\000' <repeats 8195 times>}
获取buffer的tag
rnode/forkno/block
(gdb) n
231 regbuf->page = BufferGetPage(buffer);
(gdb) p *regbuf
$19 = {in_use = false, flags = 0 '\000', rnode = {spcNode = 1663, dbNode = 16402, relNode = 17034}, forkno = MAIN_FORKNUM,
block = 0, page = 0x0, rdata_len = 0, rdata_head = 0x0, rdata_tail = 0x0, bkp_rdatas = {{next = 0x0, data = 0x0,
len = 0}, {next = 0x0, data = 0x0, len = 0}}, compressed_page = '\000' <repeats 8195 times>}
设置flags等其他变量
(gdb) n
232 regbuf->flags = flags;
(gdb)
233 regbuf->rdata_tail = (XLogRecData *) ®buf->rdata_head;
(gdb)
234 regbuf->rdata_len = 0;
(gdb)
244 for (i = 0; i < max_registered_block_id; i++)
(gdb) p regbuf->flags
$21 = 8 '\b'
(gdb) p *regbuf->rdata_tail
$23 = {next = 0x0, data = 0x292e1a8 "", len = 0}
(gdb) p regbuf->rdata_len
$24 = 0
检查该page是否已被其他block_id注册
最后设置in_use为T,返回XLogRegisterBufData
(gdb) n
246 registered_buffer *regbuf_old = ®istered_buffers[i];
(gdb)
248 if (i == block_id || !regbuf_old->in_use)
(gdb)
249 continue;
(gdb)
244 for (i = 0; i < max_registered_block_id; i++)
(gdb)
258 regbuf->in_use = true;
(gdb)
259 }
(gdb)
heap_insert (relation=0x7f5cc0338228, tup=0x29b2440, cid=0, options=0, bistate=0x0) at heapam.c:2579
2579 XLogRegisterBufData(0, (char *) &xlhdr, SizeOfHeapHeader);
XLogRegisterBufData
进入XLogRegisterBufData函数
(gdb) step
XLogRegisterBufData (block_id=0 '\000', data=0x7fff03ba99d0 "\003", len=5) at xloginsert.c:366
366 Assert(begininsert_called);
寻找已注册的缓存结构体
(gdb) n
369 regbuf = ®istered_buffers[block_id];
(gdb)
370 if (!regbuf->in_use)
(gdb) p *regbuf
$25 = {in_use = true, flags = 8 '\b', rnode = {spcNode = 1663, dbNode = 16402, relNode = 17034}, forkno = MAIN_FORKNUM,
block = 0, page = 0x7f5c93854380 "\001", rdata_len = 0, rdata_head = 0x0, rdata_tail = 0x292e1a8, bkp_rdatas = {{
next = 0x0, data = 0x0, len = 0}, {next = 0x0, data = 0x0, len = 0}}, compressed_page = '\000' <repeats 8195 times>}
(gdb) p *regbuf->page
$26 = 1 '\001'
(gdb) n
374 if (num_rdatas >= max_rdatas)
(gdb)
在正在构造的WAL记录中添加buffer相关的数据.
(gdb) n
376 rdata = &rdatas[num_rdatas++];
(gdb) p num_rdatas
$27 = 1
(gdb) p max_rdatas
$28 = 20
(gdb) n
378 rdata->data = data;
(gdb)
379 rdata->len = len;
(gdb)
381 regbuf->rdata_tail->next = rdata;
(gdb)
382 regbuf->rdata_tail = rdata;
(gdb)
383 regbuf->rdata_len += len;
(gdb)
384 }
(gdb) p *rdata
$29 = {next = 0x0, data = 0x7fff03ba99d0 "\003", len = 5}
(gdb)
完成调用,回到heap_insert
(gdb) n
heap_insert (relation=0x7f5cc0338228, tup=0x29b2440, cid=0, options=0, bistate=0x0) at heapam.c:2583
2583 heaptup->t_len - SizeofHeapTupleHeader);
继续调用XLogRegisterBufData函数注册tuple实际数据
2583 heaptup->t_len - SizeofHeapTupleHeader);
(gdb) n
2581 XLogRegisterBufData(0,
(gdb)
XLogSetRecordFlags
为即将"到来"的WAL记录设置插入状态标记
(gdb)
2586 XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);
逻辑很简单,设置标记位curinsert_flags
(gdb) step
XLogSetRecordFlags (flags=1 '\001') at xloginsert.c:399
399 Assert(begininsert_called);
(gdb) n
400 curinsert_flags = flags;
(gdb)
401 }
(gdb)
heap_insert (relation=0x7f5cc0338228, tup=0x29b2440, cid=0, options=0, bistate=0x0) at heapam.c:2588
2588 recptr = XLogInsert(RM_HEAP_ID, info);
(gdb)
调用XLogInsert,插入WAL
(gdb)
2590 PageSetLSN(page, recptr);
...
XLogInsert函数下节再行介绍.
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
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