Postgresql的XLOG累积源码分析-阿里云开发者社区

开发者社区> 数据库> 正文

Postgresql的XLOG累积源码分析

简介: title: PGSQL的XLOG生成和清理逻辑 date: 2018-12-01 08:00:00 categories: Postgresql 总结归纳XLOG清理逻辑 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ch
title: PGSQL的XLOG生成和清理逻辑
date: 2018-12-01 08:00:00
categories: Postgresql

总结归纳XLOG清理逻辑

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 =

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

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。

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。

PG源码分析

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);
}
}
}
  ...
  ...
}

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 */
}
}
}
}

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);
...
}

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)

WAL段累积的原因

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

1 ReplicationSlot

打开流复制槽

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 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='')

4 检查点间隔过长

检查参数配置

版权声明:如果您发现本社区中有涉嫌抄袭的内容,欢迎发送邮件至:developerteam@list.alibaba-inc.com 进行举报,并提供相关证据,一经查实,本社区将立刻删除涉嫌侵权内容。

分享:
数据库
使用钉钉扫一扫加入圈子
+ 订阅

分享数据库前沿,解构实战干货,推动数据库技术变革

其他文章