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;
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