0 前言
1、2部分是对XLOG生成和清理逻辑的分析,XLOG暴涨的处理直接看第3部分。
1 WAL归档
# 在自动的WAL检查点之间的日志文件段的最大数量 checkpoint_segments = # 在自动WAL检查点之间的最长时间 checkpoint_timeout = # 缓解io压力 checkpoint_completion_target = # 日志文件段的保存最小数量,为了备库保留更多段 wal_keep_segments = # 已完成的WAL段通过archive_command发送到归档存储 archive_mode = # 强制timeout切换到新的wal段文件 archive_timeout = max_wal_size = min_wal_size =
1.1 不开启归档时
文件数量受下面几个参数控制,通常不超过
(2 + checkpoint_completion_target) * checkpoint_segments + 1
或
checkpoint_segments + wal_keep_segments + 1
个文件。
如果一个旧段文件不再需要了会重命名然后继续覆盖使用,如果由于短期的日志输出高峰导致了超过
3 * checkpoint_segments + 1
个文件,直接删除文件。
1.2 开启归档时
文件数量:删除归档成功的段文件
抽象来看一个运行的PG生成一个无限长的WAL日志序列。每段16M,这些段文件的名字是数值命名的,反映在WAL序列中的位置。在不用WAL归档的时候,系统通常只是创建几个段文件然后循环使用,方法是把不再使用的段文件重命名为更高的段编号。
当且仅当归档命令成功时,归档命令返回零。 在得到一个零值结果之后,PostgreSQL将假设该WAL段文件已经成功归档,稍后将删除段文件。一个非零值告诉PostgreSQL该文件没有被归档,会周期性的重试直到成功。
2 PG源码分析
2.1 删除逻辑
触发删除动作
RemoveOldXlogFiles > CreateCheckPoint > CreateRestartPoint
wal_keep_segments判断(调用这个函数修改_logSegNo,然后再传入RemoveOldXlogFiles)
static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo) { XLogSegNo segno; XLogRecPtr keep; XLByteToSeg(recptr, segno); keep = XLogGetReplicationSlotMinimumLSN(); /* compute limit for wal_keep_segments first */ if (wal_keep_segments > 0) { /* avoid underflow, don't go below 1 */ if (segno <= wal_keep_segments) segno = 1; else segno = segno - wal_keep_segments; } /* then check whether slots limit removal further */ if (max_replication_slots > 0 && keep != InvalidXLogRecPtr) { XLogSegNo slotSegNo; 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; }
删除逻辑
static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr) { ... ... while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL) { /* Ignore files that are not XLOG segments */ if (strlen(xlde->d_name) != 24 || strspn(xlde->d_name, "0123456789ABCDEF") != 24) continue; /* * We ignore the timeline part of the XLOG segment identifiers in * deciding whether a segment is still needed. This ensures that we * won't prematurely remove a segment from a parent timeline. We could * probably be a little more proactive about removing segments of * non-parent timelines, but that would be a whole lot more * complicated. * * We use the alphanumeric sorting property of the filenames to decide * which ones are earlier than the lastoff segment. */ if (strcmp(xlde->d_name + 8, lastoff + 8) <= 0) { if (XLogArchiveCheckDone(xlde->d_name)) # 归档关闭返回真 # 存在done文件返回真 # 存在.ready返回假 # recheck存在done文件返回真 # 重建.ready文件返回假 { /* Update the last removed location in shared memory first */ UpdateLastRemovedPtr(xlde->d_name); # 回收 或者 直接删除,清理.done和.ready文件 RemoveXlogFile(xlde->d_name, endptr); } } } ... ... }
2.2 归档逻辑
static void pgarch_ArchiverCopyLoop(void) { char xlog[MAX_XFN_CHARS + 1]; # 拿到最老那个没有被归档的xlog文件名 while (pgarch_readyXlog(xlog)) { int failures = 0; for (;;) { /* * Do not initiate any more archive commands after receiving * SIGTERM, nor after the postmaster has died unexpectedly. The * first condition is to try to keep from having init SIGKILL the * command, and the second is to avoid conflicts with another * archiver spawned by a newer postmaster. */ if (got_SIGTERM || !PostmasterIsAlive()) return; /* * Check for config update. This is so that we'll adopt a new * setting for archive_command as soon as possible, even if there * is a backlog of files to be archived. */ if (got_SIGHUP) { got_SIGHUP = false; ProcessConfigFile(PGC_SIGHUP); } # archive_command没设的话不再执行 # 我们的command没有设置,走的是这个分支 if (!XLogArchiveCommandSet()) { /* * Change WARNING to DEBUG1, since we will left archive_command empty to * let external tools to manage archive */ ereport(DEBUG1, (errmsg("archive_mode enabled, yet archive_command is not set"))); return; } # 执行归档命令! if (pgarch_archiveXlog(xlog)) { # 成功了,把.ready改名为.done pgarch_archiveDone(xlog); /* * Tell the collector about the WAL file that we successfully * archived */ pgstat_send_archiver(xlog, false); break; /* out of inner retry loop */ } else { /* * Tell the collector about the WAL file that we failed to * archive */ pgstat_send_archiver(xlog, true); if (++failures >= NUM_ARCHIVE_RETRIES) { ereport(WARNING, (errmsg("archiving transaction log file \"%s\" failed too many times, will try again later", xlog))); return; /* give up archiving for now */ } pg_usleep(1000000L); /* wait a bit before retrying */ } } } }
2.3 ready生成逻辑
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible) { ... if (finishing_seg) { issue_xlog_fsync(openLogFile, openLogSegNo); /* signal that we need to wakeup walsenders later */ WalSndWakeupRequest(); LogwrtResult.Flush = LogwrtResult.Write; /* end of page */ # 归档打开 && wal_level >= archive if (XLogArchivingActive()) # 生成ready文件 XLogArchiveNotifySeg(openLogSegNo); XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL); ...
2.4 总结
ready文件只要满足archive_mode=on和wal_lever>=archive,就总会生成(XLogWrite函数调用生成)
因为archive_command设置空,所以ready文件的消费完全由外部程序控制
done文件的处理由PG完成,两个地方会触发done文件处理,检查点和重启点
处理多少done文件受wal_keep_segments和replication_slot控制(KeepLogSeg函数)
3 WAL段累积的原因(长求总?)
注意:无论如何注意不要手动删除xlog文件
注意:checkpoint产生的日志回不立即生成ready文件,是在下一个xlog后一块生成的
3.1 ReplicationSlot
打开流了复制槽
-- 流复制插槽 -- 如果restart_lsn和当前XLOG相差非常大的字节数, 需要排查slot的订阅者是否能正常接收XLOG, -- 或者订阅者是否正常. 长时间不将slot的数据取走, pg_xlog目录可能会撑爆 select pg_xlog_location_diff(pg_current_xlog_location(),restart_lsn), * from pg_replication_slots;
删除
select pg_drop_replication_slot('xxx');
删除后PG会在下一个checkpoint清理xlog
3.2 较大的wal_keep_segments
检查参数配置,注意打开这个参数会使xlog和ready有一定延迟
3.3 回收出现问题
如果不使用PG自动回收机制,数据库依赖外部程序修改.ready文件,需要检测回收进程
(archive_mode=on archive_command='')
3.4 检查点间隔过长
检查参数配置
以上为个人经验,希望能给大家一个参考,也希望大家多多支持脚本之家。如有错误或未考虑完全的地方,望不吝赐教。