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。

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 = &registered_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 = &regbuf->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 = &regbuf->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, &regbuf->rnode, sizeof(RelFileNode)); scratch += sizeof(RelFileNode);//调整偏移 } //后跟BlockNumber memcpy(scratch, &regbuf->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 TABLEtestdb=# checkpoint;CHECKPOINTtestdb=# insert into t_wal_partition(c1,c2,c3) VALUES(1,'checkpoint','checkpoint');

设置断点,进入XLogRecordAssemble

(gdb) b XLogRecordAssembleBreakpoint 1 at 0x565411: file xloginsert.c, line 488.(gdb) cContinuing.Breakpoint 1, XLogRecordAssemble (rmid=10 '\n', info=128 '\200', RedoRecPtr=5507633240, doPageWrites=true, fpw_lsn=0x7fff05cfe378) at xloginsert.c:488488 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) n491 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) n515 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) n524 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) n526 registered_buffer *regbuf = &registered_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' <repeats 8195 times>}

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 0x18edaef0x18edaef: 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) n533 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) n542 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) n564 if (regbuf->rdata_len == 0)(gdb) p regbuf->rdata_len$37 = 32(gdb) n566 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) n572 bkpb.fork_flags = regbuf->forkno;(gdb) 573 bkpb.data_length = 0;(gdb) 575 if ((regbuf->flags & REGBUF_WILL_INIT) == REGBUF_WILL_INIT)(gdb) n576 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) n584 if (include_image)(gdb) p include_image$42 = false(gdb)

需要包含数据

(gdb) n691 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) n705 if (prev_regbuf && RelFileNodeEquals(regbuf->rnode, prev_regbuf->rnode))(gdb) p prev_regbuf$46 = (registered_buffer *) 0x0(gdb) n711 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, &regbuf->rnode, sizeof(RelFileNode));(gdb) 731 scratch += sizeof(RelFileNode);(gdb) 733 memcpy(scratch, &regbuf->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) n739 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) n767 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) n780 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) n788 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

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