PostgreSQL如何删除不使用的xlog文件
一、问题
经常会在复制的时候遇到这样的问题,需要复制的xlog文件找不到了。那么xlog文件什么时候删除?又会删除多少保留多少个xlog文件?都有哪些xlog文件需要保留?本文将从原理上对这些问题进行解读。 二、原理 4、遍历pg_wal目录下的所有xlog文件,进行删除:RemoveOldXlogFiles 三、代码流程 CreateCheckPoint: XLogCtlInsert *Insert = &XLogCtl->Insert;//标识插入的位置 curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);//添加页头大小后的位置 //(((curInsert) % XLOG_BLCKSZ == 0) ? 0 : (XLOG_BLCKSZ - (curInsert) % XLOG_BLCKSZ)) freespace = INSERT_FREESPACE(curInsert);//curInsert所在页是否有空闲空间 if (freespace == 0){ if (curInsert % XLogSegSize == 0)//正好一个xlog段文件用完,即将使用下一个段文件,则跳过36字节 curInsert += SizeOfXLogLongPHD;//36字节 else//xlog段文件中正好一页用完,即将使用下一页,则跳过20字节 curInsert += SizeOfXLogShortPHD;//20字节 } checkPoint.redo = curInsert;//xlog文件上,实际的即将插入位置 RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo; ... //插入checkpoint记录后末尾位置,即下一个xlog开始的位置 recptr = XLogInsert(RM_XLOG_ID,shutdown ? XLOG_CHECKPOINT_SHUTDOWN :XLOG_CHECKPOINT_ONLINE); ... PriorRedoPtr = ControlFile->checkPointCopy.redo;//上一次checkpoint的起始位置 ... if (PriorRedoPtr != InvalidXLogRecPtr){//上一次checkpoint开始到这一次checkpoint开始,产生的XLOG大小为入参 /* CheckPointDistanceEstimate: 1、CheckPointDistanceEstimate<RedoRecPtr - PriorRedoPtr时:RedoRecPtr - PriorRedoPtr 2、CheckPointDistanceEstimate>=RedoRecPtr - PriorRedoPtr时:0.9*CheckPointDistanceEstimate+0.1*(RedoRecPtr - PriorRedoPtr) */ UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr); //_logSegNo = (PriorRedoPtr) / XLogSegSize XLByteToSeg(PriorRedoPtr,_logSegNo); KeepLogSeg(recptr,&_logSegNo); _logSegNo--; RemoveOldXlogFiles(_logSegNo,PriorRedoPtr,recptr); 2、两个宏定义 #define UsableBytesInPage (XLOG_BLCKSZ - SizeOfXLogShortPHD)//注意:不是文件第一页 #define UsableBytesInSegment ((XLOG_SEG_SIZE / XLOG_BLCKSZ) * UsableBytesInPage - (SizeOfXLogLongPHD - SizeOfXLogShortPHD)) 3、函数XLogBytePosToRecPtr static XLogRecPtr XLogBytePosToRecPtr(uint64 bytepos) { //bytepos:不包括xlog页的页头等额外字节占用的大小 fullsegs = bytepos / UsableBytesInSegment; bytesleft = bytepos % UsableBytesInSegment; /* 1、如果bytesleft < XLOG_BLCKSZ-32,则表示定位到第一页上,则文件偏移值跳过第一页页头大小 2、如果bytesleft >= XLOG_BLCKSZ-32,则表示定位不是第一页 */ if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD){ /* fits on first page of segment */ seg_offset = bytesleft + SizeOfXLogLongPHD; }else{ /* account for the first page on segment with long header */ seg_offset = XLOG_BLCKSZ;//先跳过第一页 bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;//去掉第一页存放XLOG的大小 fullpages = bytesleft / UsableBytesInPage;//剩下的需要几个页 bytesleft = bytesleft % UsableBytesInPage;//剩下的偏移 // 文件偏移=第一页大小+剩下的几个页大小+剩下的偏移+最后一页的页头 seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD; } //result=(fullsegs) * XLOG_SEG_SIZE + seg_offset XLogSegNoOffsetToRecPtr(fullsegs,seg_offset,result); return result; } 4、函数KeepLogSeg static void KeepLogSeg(XLogRecPtr recptr,XLogSegNo *logSegNo) { //segno为当前xlog即将插入位置在第几个文件上 XLByteToSeg(recptr,segno); //XLogCtl->replicationSlotMinLSN;备机上请求预留的最小值? keep = XLogGetReplicationSlotMinimumLSN(); /* compute limit for wal_keep_segments first */ if (wal_keep_segments > 0){ /* 首先计算wal_keep_segments得到的限制: 1、比如wal_keep_segments值是10,若当前insert的位置的文件号segno为5,那么向前推进到1 2、否则向前推进wal_keep_segments后的segno前的可删除 */ if (segno <= wal_keep_segments) segno = 1; else segno = segno - wal_keep_segments; } /* then check whether slots limit removal further */ //计算slots限制,如果其算出的值小于wal_keep_segments计算出的值,则需要使用slotSegNo,slots还有用,不能删除 if (max_replication_slots > 0 && keep != InvalidXLogRecPtr){ XLByteToSeg(keep,slotSegNo); if (slotSegNo <= 0) segno = 1; else if (slotSegNo < segno) segno = slotSegNo; } /* don‘t delete WAL segments newer than the calculated segment */ if (segno < *logSegNo) *logSegNo = segno; //note: //如果计算出的segno比上次checkpoint时的文件号logSegNo还有小,则取这次计算的segno //如果计算出的segno比上次checkpoint时的文件号logSegNo大,则取上次checkpoint时的文件号。 //因为恢复时如果是主机,读取最新checkpoint记录失败后,会读取上一次checkpoint记录,如果上次checkpoint的文件被删除,这里就读取不到记录了 } 5、函数RemoveOldXlogFiles static void /* 构建一个log文件名,用于判断,该文件之前的xlog可以删除。用不到时间线,所以可以使用0 */ XLogFileName(lastoff,segno); while ((xlde = ReadDir(xldir,XLOGDIR)) != NULL){ /* 忽略非xlog文件 */ if (!IsXLogFileName(xlde->d_name) && !IsPartialXLogFileName(xlde->d_name)) continue; /* 1、跳过时间线进行比较 */ if (strcmp(xlde->d_name + 8,lastoff + 8) <= 0){ if (XLogArchiveCheckDone(xlde->d_name)){//如果没有开启归档:总是TRUE;否则,归档完成后才为TRUE /* Update the last removed location in shared memory first */ //XLogCtl->lastRemovedSegNo = segno; UpdateLastRemovedPtr(xlde->d_name); RemoveXlogFile(xlde->d_name,endptr); } } } } 6、函数RemoveXlogFile RemoveXlogFile(const char *segname,XLogRecPtr endptr) XLByteToSeg(endptr,endlogSegNo); if (PriorRedoPtr == InvalidXLogRecPtr) recycleSegNo = endlogSegNo + 10; else recycleSegNo = XLOGfileslop(PriorRedoPtr); snprintf(path,MAXPGPATH,XLOGDIR "/%s",segname); if (endlogSegNo <= recycleSegNo && lstat(path,&statbuf) == 0 && S_ISREG(statbuf.st_mode) && InstallXLogFileSegment(&endlogSegNo,path,true,recycleSegNo,true)) { endlogSegNo++; }else{ rc = durable_unlink(path,LOG); } } 7、函数InstallXLogFileSegment static bool
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