PostgreSQL源码解读（110）-WAL#6（Insert&WAL-XLogRe...
本节简单介绍了XLogRecordAssemble函数的实现逻辑,该函数从已注册的数据和缓冲区中组装XLOG record到XLogRecData链中,为XLOG Record的插入作准备。
成都创新互联公司主要从事网页设计、PC网站建设（电脑版网站建设）、wap网站建设（手机版网站建设）、响应式网站设计、程序开发、网站优化、微网站、小程序设计等，凭借多年来在互联网的打拼，我们在互联网网站建设行业积累了丰富的成都做网站、网站设计、外贸营销网站建设、网站设计、网络营销经验，集策划、开发、设计、营销、管理等多方位专业化运作于一体。
一、数据结构
全局静态变量
XLogRecordAssemble使用的全局变量包括hdr_rdt/hdr_scratch/rdatas等.
/* flags for the in-progress insertion */
//用于插入过程中的标记信息
static uint8 curinsert_flags = 0;
/*
* These are used to hold the record header while constructing a record.
* 'hdr_scratch' is not a plain variable, but is palloc'd at initialization,
* because we want it to be MAXALIGNed and padding bytes zeroed.
* 在构建XLOG Record时通常会存储记录的头部信息.
* 'hdr_scratch'并不是一个普通(plain)变量,而是在初始化时通过palloc初始化,
* 因为我们希望该变量已经是MAXALIGNed并且已被0x00填充.
*
* For simplicity, it's allocated large enough to hold the headers for any
* WAL record.
* 简单起见,该变量预先会分配足够大的空间用于存储所有WAL Record的头部信息.
*/
static XLogRecData hdr_rdt;
static char *hdr_scratch = NULL;
#define SizeOfXlogOrigin (sizeof(RepOriginId) + sizeof(char))
#define HEADER_SCRATCH_SIZE \
(SizeOfXLogRecord + \
MaxSizeOfXLogRecordBlockHeader * (XLR_MAX_BLOCK_ID + 1) + \
SizeOfXLogRecordDataHeaderLong + SizeOfXlogOrigin)
/*
* 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;
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;
static XLogCtlData *XLogCtl = NULL;
/* flags for the in-progress insertion */
static uint8 curinsert_flags = 0;
/*
* A chain of XLogRecDatas to hold the "main data" of a WAL record, registered
* with XLogRegisterData(...).
* 存储WAL Record "main data"的XLogRecDatas数据链
*/
static XLogRecData *mainrdata_head;
static XLogRecData *mainrdata_last = (XLogRecData *) &mainrdata_head;
//链中某个位置的mainrdata大小
static uint32 mainrdata_len; /* total # of bytes in chain */
/*
* ProcLastRecPtr points to the start of the last XLOG record inserted by the
* current backend. It is updated for all inserts. XactLastRecEnd points to
* end+1 of the last record, and is reset when we end a top-level transaction,
* or start a new one; so it can be used to tell if the current transaction has
* created any XLOG records.
* ProcLastRecPtr指向当前后端插入的最后一条XLOG记录的开头。
* 它针对所有插入进行更新。
* XactLastRecEnd指向最后一条记录的末尾位置 + 1，
* 并在结束顶级事务或启动新事务时重置;
* 因此，它可以用来判断当前事务是否创建了任何XLOG记录。
*
* While in parallel mode, this may not be fully up to date. When committing,
* a transaction can assume this covers all xlog records written either by the
* user backend or by any parallel worker which was present at any point during
* the transaction. But when aborting, or when still in parallel mode, other
* parallel backends may have written WAL records at later LSNs than the value
* stored here. The parallel leader advances its own copy, when necessary,
* in WaitForParallelWorkersToFinish.
* 在并行模式下，这可能不是完全是最新的。
* 在提交时，事务可以假定覆盖了用户后台进程或在事务期间出现的并行worker进程的所有xlog记录。
* 但是，当中止时，或者仍然处于并行模式时，其他并行后台进程可能在较晚的LSNs中写入了WAL记录，
* 而不是存储在这里的值。
* 当需要时，并行处理进程的leader在WaitForParallelWorkersToFinish中会推进自己的副本。
*/
XLogRecPtr ProcLastRecPtr = InvalidXLogRecPtr;
XLogRecPtr XactLastRecEnd = InvalidXLogRecPtr;
XLogRecPtr XactLastCommitEnd = InvalidXLogRecPtr;
/* For WALInsertLockAcquire/Release functions */
//用于WALInsertLockAcquire/Release函数
static int MyLockNo = 0;
static bool holdingAllLocks = false;
宏定义
XLogRegisterBuffer函数使用的flags
/* flags for XLogRegisterBuffer */
//XLogRegisterBuffer函数使用的flags
#define REGBUF_FORCE_IMAGE 0x01 /* 强制执行full-page-write;force a full-page image */
#define REGBUF_NO_IMAGE 0x02 /* 不需要FPI;don't take a full-page image */
#define REGBUF_WILL_INIT (0x04 | 0x02) /* 在回放时重新初始化page(表示NO_IMAGE);
* page will be re-initialized at
* replay (implies NO_IMAGE) */
#define REGBUF_STANDARD 0x08 /* 标准的page layout(数据在pd_lower和pd_upper之间的数据会被跳过)
* page follows "standard" page layout,
* (data between pd_lower and pd_upper
* will be skipped) */
#define REGBUF_KEEP_DATA 0x10 /* include data even if a full-page image
* is taken */
/*
* Flag bits for the record being inserted, set using XLogSetRecordFlags().
*/
#define XLOG_INCLUDE_ORIGIN 0x01 /* include the replication origin */
#define XLOG_MARK_UNIMPORTANT 0x02 /* record not important for durability */
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
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
XLogRecordAssemble函数从已注册的数据和缓冲区中组装XLOG record到XLogRecData链中,组装完成后可以使用XLogInsertRecord()函数插入到WAL buffer中.
/*
* Assemble a WAL record from the registered data and buffers into an
* XLogRecData chain, ready for insertion with XLogInsertRecord().
* 从已注册的数据和缓冲区中组装XLOG record到XLogRecData链中,
* 组装完成后可以使用XLogInsertRecord()函数插入.
*
* The record header fields are filled in, except for the xl_prev field. The
* calculated CRC does not include the record header yet.
* 除了xl_prev外,XLOG Record的header域已填充完毕.
* 计算的CRC还没有包含header信息.
*
* If there are any registered buffers, and a full-page image was not taken
* of all of them, *fpw_lsn is set to the lowest LSN among such pages. This
* signals that the assembled record is only good for insertion on the
* assumption that the RedoRecPtr and doPageWrites values were up-to-date.
* 如存在已注册的缓冲区,而且full-page-image没有全部包括这些数据,
* *fpw_lsn设置为这些页面中最小的LSN.
* 基于RedoRecPtr和doPageWrites已更新为最新的假设,
* 已组装的XLOG Record对在此假设上的插入是OK的.
*/
static XLogRecData *
XLogRecordAssemble(RmgrId rmid, uint8 info,
XLogRecPtr RedoRecPtr, bool doPageWrites,
XLogRecPtr *fpw_lsn)
{
XLogRecData *rdt;//XLogRecData指针
uint32 total_len = 0;//XLOG Record大小
int block_id;//块ID
pg_crc32c rdata_crc;//CRC
registered_buffer *prev_regbuf = NULL;//已注册的buffer指针
XLogRecData *rdt_datas_last;//
XLogRecord *rechdr;//头部信息
char *scratch = hdr_scratch;
/*
* Note: this function can be called multiple times for the same record.
* All the modifications we do to the rdata chains below must handle that.
* 对于同一个XLOG Record,该函数可以被多次调用.
* 下面我们对rdata链进行的所有更新必须处理这种情况.
*/
/* The record begins with the fixed-size header */
//XLOG Record的头部大小是固定的.
rechdr = (XLogRecord *) scratch;
scratch += SizeOfXLogRecord;//指针移动
hdr_rdt.next = NULL;//hdr_rdt --> static XLogRecData hdr_rdt;
rdt_datas_last = &hdr_rdt;//
hdr_rdt.data = hdr_scratch;//rmgr数据的起始偏移
/*
* Enforce consistency checks for this record if user is looking for it.
* Do this before at the beginning of this routine to give the possibility
* for callers of XLogInsert() to pass XLR_CHECK_CONSISTENCY directly for
* a record.
* 如正在搜索此记录,则强制检查该记录的一致性.
* 在该处理过程开始前执行此项处理,以便XLogInsert()的调用者
* 可以直接传递XLR_CHECK_CONSISTENCY给XLOG Record.
*/
if (wal_consistency_checking[rmid])
info |= XLR_CHECK_CONSISTENCY;
/*
* Make an rdata chain containing all the data portions of all block
* references. This includes the data for full-page images. Also append
* the headers for the block references in the scratch buffer.
* 构造保存所有块参考的数据部分的rdata链.这包括FPI的数据.
* 同时,在scratch缓冲区中为所有的块引用追加头部信息.
*/
*fpw_lsn = InvalidXLogRecPtr;//初始化变量
for (block_id = 0; block_id < max_registered_block_id; block_id++)//遍历已注册的block
{
registered_buffer *regbuf = ®istered_buffers[block_id];//获取根据block_id获取缓冲区
bool needs_backup;//是否需要backup block
bool needs_data;//是否需要data
XLogRecordBlockHeader bkpb;//XLogRecordBlockHeader
XLogRecordBlockImageHeader bimg;//XLogRecordBlockImageHeader
XLogRecordBlockCompressHeader cbimg = {0};//压缩存储时需要
bool samerel;//是否同一个rel?
bool is_compressed = false;//是否压缩
bool include_image;//是否包括FPI
if (!regbuf->in_use)//未在使用,继续下一个
continue;
/* Determine if this block needs to be backed up */
//确定此block是否需要backup
if (regbuf->flags & REGBUF_FORCE_IMAGE)
needs_backup = true;//强制要求FPI
else if (regbuf->flags & REGBUF_NO_IMAGE)
needs_backup = false;//强制要求不要IMAGE
else if (!doPageWrites)
needs_backup = false;//doPageWrites标记设置为F
else//doPageWrites = T
{
/*
* We assume page LSN is first data on *every* page that can be
* passed to XLogInsert, whether it has the standard page layout
* or not.
* 不管该page是否标准page layout,
* 我们假定在每一个page中最前面的数据是page LSN,
*
*/
XLogRecPtr page_lsn = PageGetLSN(regbuf->page);//获取LSN
needs_backup = (page_lsn <= RedoRecPtr);//是否需要backup
if (!needs_backup)//不需要
{
if (*fpw_lsn == InvalidXLogRecPtr || page_lsn < *fpw_lsn)
*fpw_lsn = page_lsn;//设置LSN
}
}
/* Determine if the buffer data needs to included */
//确定buffer中的data是否需要包括在其中
if (regbuf->rdata_len == 0)//没有数据
needs_data = false;
else if ((regbuf->flags & REGBUF_KEEP_DATA) != 0)//需要包括data
needs_data = true;
else
needs_data = !needs_backup;//needs_backup取反
//BlockHeader设置值
bkpb.id = block_id;//块ID
bkpb.fork_flags = regbuf->forkno;//forkno
bkpb.data_length = 0;//数据长度
if ((regbuf->flags & REGBUF_WILL_INIT) == REGBUF_WILL_INIT)
bkpb.fork_flags |= BKPBLOCK_WILL_INIT;//设置标记
/*
* If needs_backup is true or WAL checking is enabled for current
* resource manager, log a full-page write for the current block.
* 如needs_backup为T,或者当前RM的WAL检查已启用,
* 为当前block执行full-page-write
*/
//需要backup或者要求执行一致性检查
include_image = needs_backup || (info & XLR_CHECK_CONSISTENCY) != 0;
if (include_image)
{
//包含块镜像
Page page = regbuf->page;//获取对应的page
uint16 compressed_len = 0;//压缩后的大小
/*
* The page needs to be backed up, so calculate its hole length
* and offset.
* page需要备份,计算空闲空间大小和偏移
*/
if (regbuf->flags & REGBUF_STANDARD)
{
//如为标准的REGBUF
/* Assume we can omit data between pd_lower and pd_upper */
//假定我们可以省略pd_lower和pd_upper之间的数据
uint16 lower = ((PageHeader) page)->pd_lower;//获取lower
uint16 upper = ((PageHeader) page)->pd_upper;//获取upper
if (lower >= SizeOfPageHeaderData &&
upper > lower &&
upper <= BLCKSZ)
{
//lower大于Page的头部 && upper大于lower && upper小于块大小
bimg.hole_offset = lower;
cbimg.hole_length = upper - lower;
}
else
{
/* No "hole" to remove */
//没有空闲空间可以移除
bimg.hole_offset = 0;
cbimg.hole_length = 0;
}
}
else
{
//不是标准的REGBUF
/* Not a standard page header, don't try to eliminate "hole" */
//不是标准的page header,不要尝试估算"hole"
bimg.hole_offset = 0;
cbimg.hole_length = 0;
}
/*
* Try to compress a block image if wal_compression is enabled
* 如果wal_compression启用,则尝试压缩
*/
if (wal_compression)
{
is_compressed =
XLogCompressBackupBlock(page, bimg.hole_offset,
cbimg.hole_length,
regbuf->compressed_page,
&compressed_len);//调用XLogCompressBackupBlock压缩
}
/*
* Fill in the remaining fields in the XLogRecordBlockHeader
* struct
* 填充XLogRecordBlockHeader结构体的剩余域字段
*/
bkpb.fork_flags |= BKPBLOCK_HAS_IMAGE;
/*
* Construct XLogRecData entries for the page content.
* 为page内容构造XLogRecData入口
*/
rdt_datas_last->next = ®buf->bkp_rdatas[0];
rdt_datas_last = rdt_datas_last->next;
//设置标记
bimg.bimg_info = (cbimg.hole_length == 0) ? 0 : BKPIMAGE_HAS_HOLE;
/*
* If WAL consistency checking is enabled for the resource manager
* of this WAL record, a full-page image is included in the record
* for the block modified. During redo, the full-page is replayed
* only if BKPIMAGE_APPLY is set.
* 如WAL一致性检查已启用,被更新的block已在XLOG Record中包含了FPI.
* 在redo期间,在设置了BKPIMAGE_APPLY标记的情况下full-page才会回放.
*/
if (needs_backup)
bimg.bimg_info |= BKPIMAGE_APPLY;//设置标记
if (is_compressed)//是否压缩?
{
bimg.length = compressed_len;//压缩后的空间
bimg.bimg_info |= BKPIMAGE_IS_COMPRESSED;//压缩标记
rdt_datas_last->data = regbuf->compressed_page;//放在registered_buffer中
rdt_datas_last->len = compressed_len;//长度
}
else
{
//没有压缩
//image的大小
bimg.length = BLCKSZ - cbimg.hole_length;
if (cbimg.hole_length == 0)
{
rdt_datas_last->data = page;//数据指针直接指向page
rdt_datas_last->len = BLCKSZ;//大小为block size
}
else
{
/* must skip the hole */
//跳过hole
rdt_datas_last->data = page;//数据指针
rdt_datas_last->len = bimg.hole_offset;//获取hole的偏移
rdt_datas_last->next = ®buf->bkp_rdatas[1];//第2部分
rdt_datas_last = rdt_datas_last->next;//
rdt_datas_last->data =
page + (bimg.hole_offset + cbimg.hole_length);//指针指向第二部分
rdt_datas_last->len =
BLCKSZ - (bimg.hole_offset + cbimg.hole_length);//设置长度
}
}
total_len += bimg.length;//调整总长度
}
if (needs_data)//需要包含数据
{
/*
* Link the caller-supplied rdata chain for this buffer to the
* overall list.
* 把该缓冲区链接到调用者提供的rdata链中构成一个整体的链表
*/
bkpb.fork_flags |= BKPBLOCK_HAS_DATA;//设置标记
bkpb.data_length = regbuf->rdata_len;//长度
total_len += regbuf->rdata_len;//总大小
rdt_datas_last->next = regbuf->rdata_head;//调整指针
rdt_datas_last = regbuf->rdata_tail;
}
//存在上一个regbuf 而且是同一个RefFileNode(关系一样/表空间一样/block一样)
if (prev_regbuf && RelFileNodeEquals(regbuf->rnode, prev_regbuf->rnode))
{
samerel = true;//设置标记
bkpb.fork_flags |= BKPBLOCK_SAME_REL;//同一个REL
}
else
samerel = false;
prev_regbuf = regbuf;//切换为当前的regbuf
/* Ok, copy the header to the scratch buffer */
//已OK,拷贝头部信息到scratch缓冲区中
memcpy(scratch, &bkpb, SizeOfXLogRecordBlockHeader);
scratch += SizeOfXLogRecordBlockHeader;//调整偏移
if (include_image)
{
//包含FPI,追加SizeOfXLogRecordBlockImageHeader
memcpy(scratch, &bimg, SizeOfXLogRecordBlockImageHeader);
scratch += SizeOfXLogRecordBlockImageHeader;//调整偏移
if (cbimg.hole_length != 0 && is_compressed)
{
//压缩存储,追加SizeOfXLogRecordBlockCompressHeader
memcpy(scratch, &cbimg,
SizeOfXLogRecordBlockCompressHeader);
scratch += SizeOfXLogRecordBlockCompressHeader;//调整偏移
}
}
if (!samerel)
{
//不是同一个REL,追加RelFileNode
memcpy(scratch, ®buf->rnode, sizeof(RelFileNode));
scratch += sizeof(RelFileNode);//调整偏移
}
//后跟BlockNumber
memcpy(scratch, ®buf->block, sizeof(BlockNumber));
scratch += sizeof(BlockNumber);//调整偏移
}
/* followed by the record's origin, if any */
//接下来,是XLOG Record origin
if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&
replorigin_session_origin != InvalidRepOriginId)
{
//
*(scratch++) = (char) XLR_BLOCK_ID_ORIGIN;
memcpy(scratch, &replorigin_session_origin, sizeof(replorigin_session_origin));
scratch += sizeof(replorigin_session_origin);
}
/* followed by main data, if any */
//接下来是main data
if (mainrdata_len > 0)//main data大小 > 0
{
if (mainrdata_len > 255)//超过255,则使用Long格式
{
*(scratch++) = (char) XLR_BLOCK_ID_DATA_LONG;
memcpy(scratch, &mainrdata_len, sizeof(uint32));
scratch += sizeof(uint32);
}
else//否则使用Short格式
{
*(scratch++) = (char) XLR_BLOCK_ID_DATA_SHORT;
*(scratch++) = (uint8) mainrdata_len;
}
rdt_datas_last->next = mainrdata_head;
rdt_datas_last = mainrdata_last;
total_len += mainrdata_len;
}
rdt_datas_last->next = NULL;
hdr_rdt.len = (scratch - hdr_scratch);//头部大小
total_len += hdr_rdt.len;//总大小
/*
* Calculate CRC of the data
* 计算数据的CRC
*
* Note that the record header isn't added into the CRC initially since we
* don't know the prev-link yet. Thus, the CRC will represent the CRC of
* the whole record in the order: rdata, then backup blocks, then record
* header.
* 由于我们还不知道prev-link的数值,因此头部不在最初的CRC中.
* 因此，CRC将按照以下顺序表示整个记录的CRC: rdata，然后是backup blocks，然后是record header。
*/
INIT_CRC32C(rdata_crc);
COMP_CRC32C(rdata_crc, hdr_scratch + SizeOfXLogRecord, hdr_rdt.len - SizeOfXLogRecord);
for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
/*
* Fill in the fields in the record header. Prev-link is filled in later,
* once we know where in the WAL the record will be inserted. The CRC does
* not include the record header yet.
* 填充记录头部信息的其他域字段.
* Prev-link将在该记录插入在哪里的时候再填充.
* CRC还不包括记录的头部信息.
*/
rechdr->xl_xid = GetCurrentTransactionIdIfAny();
rechdr->xl_tot_len = total_len;
rechdr->xl_info = info;
rechdr->xl_rmid = rmid;
rechdr->xl_prev = InvalidXLogRecPtr;
rechdr->xl_crc = rdata_crc;
return &hdr_rdt;
}
三、跟踪分析
场景一:清除数据后,执行checkpoint后的第一次插入
测试脚本如下:
testdb=# truncate table t_wal_partition;
TRUNCATE TABLE
testdb=# checkpoint;
CHECKPOINT
testdb=# insert into t_wal_partition(c1,c2,c3) VALUES(1,'checkpoint','checkpoint');
设置断点,进入XLogRecordAssemble
(gdb) b XLogRecordAssemble
Breakpoint 1 at 0x565411: file xloginsert.c, line 488.
(gdb) c
Continuing.
Breakpoint 1, XLogRecordAssemble (rmid=10 '\n', info=128 '\200', RedoRecPtr=5507633240, doPageWrites=true,
fpw_lsn=0x7fff05cfe378) at xloginsert.c:488
488 uint32 total_len = 0;
输入参数:
rmid=10即0x0A --> Heap
RedoRecPtr=5507633240
doPageWrites=true,需要full-page-write
fpw_lsn=0x7fff05cfe378
接下来是变量赋值,
其中hdr_scratch的定义为:static char *hdr_scratch = NULL;
hdr_rdt的定义为:static XLogRecData hdr_rdt;
(gdb) n
491 registered_buffer *prev_regbuf = NULL;
(gdb)
494 char *scratch = hdr_scratch;
(gdb)
502 rechdr = (XLogRecord *) scratch;
(gdb)
503 scratch += SizeOfXLogRecord;
(gdb)
XLOG Record的头部信息
(gdb) p *(XLogRecord *)rechdr
$11 = {xl_tot_len = 114, xl_xid = 1997, xl_prev = 5507669824, xl_info = 128 '\200', xl_rmid = 1 '\001', xl_crc = 3794462175}
scratch指针指向Header之后的地址
(gdb) p hdr_scratch
$12 = 0x18a24c0 "r"
为全局变量hdr_rdt赋值
505 hdr_rdt.next = NULL;
(gdb)
506 rdt_datas_last = &hdr_rdt;
(gdb)
507 hdr_rdt.data = hdr_scratch;
(gdb) p hdr_rdt
$5 = {next = 0x0, data = 0x18a24c0 "r", len = 26}
(gdb) p *(XLogRecord *)hdr_rdt.data
$7 = {xl_tot_len = 114, xl_xid = 1997, xl_prev = 5507669824, xl_info = 128 '\200', xl_rmid = 1 '\001', xl_crc = 3794462175}
不执行一致性检查
(gdb) n
515 if (wal_consistency_checking[rmid])
(gdb)
523 *fpw_lsn = InvalidXLogRecPtr;
(gdb)
初始化fpw_lsn,开始循环.
已注册的block id只有1个.
(gdb) n
524 for (block_id = 0; block_id < max_registered_block_id; block_id++)
(gdb) p max_registered_block_id
$13 = 1
获取已注册的buffer.
其中:
rnode->RelFilenode结构体,spcNode->表空间/dbNode->数据库/relNode->关系
block->块ID
page->数据页指针(char *)
rdata_len->rdata链中的数据总大小
rdata_head->使用该数据块注册的数据链头
rdata_tail->使用该数据块注册的数据链尾
bkp_rdatas->临时rdatas数据引用,用于存储XLogRecordAssemble()中使用的备份块数据.
bkp_rdatas用于组装block image,bkp_rdatas[0]存储空闲空间(hole)前的数据,bkp_rdatas[1]存储空闲空间后的数据.
(gdb) n
526 registered_buffer *regbuf = ®istered_buffers[block_id];
(gdb)
531 XLogRecordBlockCompressHeader cbimg = {0};
(gdb) p *regbuf
$14 = {in_use = true, flags = 14 '\016', rnode = {spcNode = 1663, dbNode = 16402, relNode = 25258}, forkno = MAIN_FORKNUM,
block = 0, page = 0x7fb8539e7380 "", rdata_len = 32, rdata_head = 0x18a22c0, rdata_tail = 0x18a22d8, bkp_rdatas = {{
next = 0x18a4230, data = 0x7fb85390f380 "\001", len = 252}, {next = 0x18a22a8, data = 0x7fb85390fe28 "\315\a",
len = 5464}}, compressed_page = '\000' }
注意:
在内存中,main data已由函数XLogRegisterData注册,由mainrdata_head和mainrdata_last指针维护,本例中,填充了xl_heap_insert结构体.
block data由XLogRegisterBuffer初始化,由XLogRegisterBufData填充数据,在本例中,通过XLogRegisterBufData注册了两次数据,第一次是xl_heap_header结构体,第二次是实际的数据(实质上只是数据指针,最终需要什么数据,由组装器确定).
(gdb) p *mainrdata_head
$18 = {next = 0x18a22c0, data = 0x7fff05cfe3f0 "\001", len = 3}
(gdb) p *(xl_heap_insert *)mainrdata_head->data
$20 = {offnum = 1, flags = 0 '\000'}
(gdb) p *regbuf->rdata_head
$32 = {next = 0x18a22d8, data = 0x7fff05cfe3e0 "\003", len = 5}
(gdb) p *(xl_heap_header *)regbuf->rdata_head->data
$28 = {t_infomask2 = 3, t_infomask = 2050, t_hoff = 24 '\030'}
(gdb) p *regbuf->rdata_head->next
$34 = {next = 0x18a22f0, data = 0x18edaef "", len = 27}
以字符格式显示地址0x18edaef之后的27个字节(tuple data)
(gdb) x/27bc 0x18edaef
0x18edaef: 0 '\000' 1 '\001' 0 '\000' 0 '\000' 0 '\000' 23 '\027' 99 'c' 104 'h'
0x18edaf7: 101 'e' 99 'c' 107 'k' 112 'p' 111 'o' 105 'i' 110 'n' 116 't'
0x18edaff: 23 '\027' 99 'c' 104 'h' 101 'e' 99 'c' 107 'k' 112 'p' 111 'o'
0x18edb07: 105 'i' 110 'n' 116 't'
继续往下执行,由于该记录是第一条记录,因此无需执行full-page-image
(gdb) n
533 bool is_compressed = false;
(gdb)
536 if (!regbuf->in_use)
(gdb)
540 if (regbuf->flags & REGBUF_FORCE_IMAGE)
(gdb) p regbuf->flags
$36 = 14 '\016'
(gdb) n
542 else if (regbuf->flags & REGBUF_NO_IMAGE)
(gdb)
543 needs_backup = false;
needs_data为T,事务日志中仅写入tuple data
(gdb) n
564 if (regbuf->rdata_len == 0)
(gdb) p regbuf->rdata_len
$37 = 32
(gdb) n
566 else if ((regbuf->flags & REGBUF_KEEP_DATA) != 0)
(gdb)
569 needs_data = !needs_backup;
(gdb)
571 bkpb.id = block_id;
(gdb) p needs_data
$38 = true
设置XLogRecordBlockHeader字段值,page中第一个tuple,标记设置为BKPBLOCK_WILL_INIT
(gdb) n
572 bkpb.fork_flags = regbuf->forkno;
(gdb)
573 bkpb.data_length = 0;
(gdb)
575 if ((regbuf->flags & REGBUF_WILL_INIT) == REGBUF_WILL_INIT)
(gdb) n
576 bkpb.fork_flags |= BKPBLOCK_WILL_INIT;
(gdb)
582 include_image = needs_backup || (info & XLR_CHECK_CONSISTENCY) != 0;
(gdb) p bkpb
$40 = {id = 0 '\000', fork_flags = 64 '@', data_length = 0}
(gdb)
不需要执行FPI
(gdb) p info
$41 = 128 '\200'
(gdb) n
584 if (include_image)
(gdb) p include_image
$42 = false
(gdb)
需要包含数据
(gdb) n
691 if (needs_data)
(gdb)
697 bkpb.fork_flags |= BKPBLOCK_HAS_DATA;
(gdb)
698 bkpb.data_length = regbuf->rdata_len;
(gdb)
699 total_len += regbuf->rdata_len;
(gdb)
701 rdt_datas_last->next = regbuf->rdata_head;
(gdb)
702 rdt_datas_last = regbuf->rdata_tail;
(gdb) p bkpb
$43 = {id = 0 '\000', fork_flags = 96 '`', data_length = 32}
(gdb) p total_len
$44 = 32
(gdb) p *rdt_datas_last
$45 = {next = 0x18a22c0, data = 0x18a24c0 "r", len = 26}
已OK,拷贝头部信息到scratch缓冲区中
(gdb) n
705 if (prev_regbuf && RelFileNodeEquals(regbuf->rnode, prev_regbuf->rnode))
(gdb) p prev_regbuf
$46 = (registered_buffer *) 0x0
(gdb) n
711 samerel = false;
(gdb)
712 prev_regbuf = regbuf;
(gdb)
715 memcpy(scratch, &bkpb, SizeOfXLogRecordBlockHeader);
后面是RefFileNode + BlockNumber
(gdb)
716 scratch += SizeOfXLogRecordBlockHeader;
(gdb)
717 if (include_image)
(gdb)
728 if (!samerel)
(gdb)
730 memcpy(scratch, ®buf->rnode, sizeof(RelFileNode));
(gdb)
731 scratch += sizeof(RelFileNode);
(gdb)
733 memcpy(scratch, ®buf->block, sizeof(BlockNumber));
(gdb)
734 scratch += sizeof(BlockNumber);
(gdb)
524 for (block_id = 0; block_id < max_registered_block_id; block_id++)
结束循环
524 for (block_id = 0; block_id < max_registered_block_id; block_id++)
(gdb)
接下来是replorigin_session_origin(实际并不需要)
738 if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&
(gdb) p curinsert_flags
$47 = 1 '\001'
(gdb)
$48 = 1 '\001'
(gdb) n
739 replorigin_session_origin != InvalidRepOriginId)
(gdb)
738 if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&
接下来是main data
(gdb)
747 if (mainrdata_len > 0)
(gdb)
749 if (mainrdata_len > 255)
(gdb)
757 *(scratch++) = (char) XLR_BLOCK_ID_DATA_SHORT;
(gdb)
(gdb)
758 *(scratch++) = (uint8) mainrdata_len;
(gdb)
760 rdt_datas_last->next = mainrdata_head;
(gdb)
761 rdt_datas_last = mainrdata_last;
(gdb)
762 total_len += mainrdata_len;
(gdb)
计算大小
764 rdt_datas_last->next = NULL;
(gdb)
766 hdr_rdt.len = (scratch - hdr_scratch);
(gdb) p scratch
$49 = 0x18a24ee ""
(gdb) p hdr_scratch
$50 = 0x18a24c0 "r"
(gdb) p hdr_rdt.len
$51 = 26
(gdb) p total_len
$52 = 35
(gdb)
(gdb) n
767 total_len += hdr_rdt.len;
(gdb)
计算CRC
(gdb)
777 INIT_CRC32C(rdata_crc);
(gdb)
778 COMP_CRC32C(rdata_crc, hdr_scratch + SizeOfXLogRecord, hdr_rdt.len - SizeOfXLogRecord);
(gdb)
779 for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb) n
780 COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
(gdb)
779 for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb)
780 COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
(gdb)
779 for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb)
780 COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
(gdb)
779 for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb)
787 rechdr->xl_xid = GetCurrentTransactionIdIfAny();
填充记录头部信息的其他域字段.
(gdb) n
788 rechdr->xl_tot_len = total_len;
(gdb)
789 rechdr->xl_info = info;
(gdb)
790 rechdr->xl_rmid = rmid;
(gdb)
791 rechdr->xl_prev = InvalidXLogRecPtr;
(gdb)
792 rechdr->xl_crc = rdata_crc;
(gdb)
794 return &hdr_rdt;
(gdb)
795 }
(gdb) p rechdr
$62 = (XLogRecord *) 0x18a24c0
(gdb) p *rechdr
$63 = {xl_tot_len = 81, xl_xid = 1998, xl_prev = 0, xl_info = 128 '\200', xl_rmid = 10 '\n', xl_crc = 1852971194}
(gdb)
full-page-write场景后续再行分析
四、参考资料
Write Ahead Logging — WAL
PostgreSQL 源码解读（4）- 插入数据#3（heap_insert）
PostgreSQL 事务日志WAL结构浅析
PG Source Code
分享标题：PostgreSQL源码解读（110）-WAL#6（Insert&WAL-XLogRe...
文章源于：http://ybzwz.com/article/gopjei.html
