Postgresql中xlog生成和清理逻辑

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;elsesegno = 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.donepgarch_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 >= archiveif (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