PG中有日志归档功能，主要目的就是备份恢复，PITR，为啥要做日志归档呢？因为在做检查点时会清理WAL日志，清理了之后，就没法实现恢复到任意时刻数据库状态了，而有了日志归档，我们可以保存从数据库初始状态到当前时刻的所有日志，相当于给数据库做了一个备份。当发生故障或者误操作时，可以恢复到指定时刻数据库的状态。
打开日志归档
在配置文件中配置archive_mode=on打开日志归档，启动时会创建归档进程archiver，通过archive_command中配置的命令进行归档。
# - Archiving -
archive_mode = on # enables archiving; off, on, or always (change requires restart)
archive_command = 'cp %p /home/postgres/pgsql/archive/%f' # command to use to archive a logfile segment
# placeholders: %p = path of file to archive
# %f = file name only
# e.g. 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f'
archive_timeout = 1800 # force a logfile segment switch after this
# number of seconds; 0 disables
归档进程源码
我们看一下归档进程的源码，在src/backend/postmaster/pgarch.c中：
PgArchiverMain(void)
--> pgarch_MainLoop(); // 进入归档主循环
--> pgarch_ArchiverCopyLoop();
--> pgarch_readyXlog
日志归档的逻辑，主要是什么时候进行归档？核心要点是发生日志段切换时会触发，那我们看一下那些情况会触发日志切换
- 当WAL日志中的一个日志段（日志文件）已满，需要切换到下一个日志段时，就可以通知archiver进程将这个日志归档。产生日志切换的进程会在通知Postmaster之前先在
pg_wal/archive_status下生成一个.ready文件，这个文件和待归档日志同名。 - 如果长时间没有归档，触发
archive_timeout超时，则强制进行日志切换，强制归档 - 调用
pg_switch_wal()函数手动触发
我们看一下归档进程主循环的实现逻辑，就是等待归档通知信号，拷贝日志：
static void pgarch_MainLoop(void)
{
pg_time_t last_copy_time = 0;
bool time_to_stop;
// 进入主循环, 等待收到日志归档通知
do {
ResetLatch(MyLatch);
/* When we get SIGUSR2, we do one more archive cycle, then exit */
time_to_stop = ready_to_stop;
/* Check for barrier events and config update */
HandlePgArchInterrupts();
// ...
/* Do what we're here for */
pgarch_ArchiverCopyLoop(); // 进行日志归档，拷贝WAL日志
last_copy_time = time(NULL);
/* Sleep until a signal is received, or until a poll is forced by
* PGARCH_AUTOWAKE_INTERVAL having passed since last_copy_time, or until postmaster dies. */
if (!time_to_stop) /* Don't wait during last iteration */
{
pg_time_t curtime = (pg_time_t) time(NULL);
int timeout;
timeout = PGARCH_AUTOWAKE_INTERVAL - (curtime - last_copy_time);
if (timeout > 0) {
int rc;
rc = WaitLatch(MyLatch, WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,timeout * 1000L, WAIT_EVENT_ARCHIVER_MAIN);
if (rc & WL_POSTMASTER_DEATH)
time_to_stop = true;
}
}
} while (!time_to_stop);
}
触发归档的时机1
其中最重要的就是什么时候发信号，通知可以归档，是在切换日志段的时候，什么时候会切换日志段呢？用户可以通过调用pg_switch_wal函数强制切换日志段，正常情况下是不断插入日志的过程中，如果超出了日志段的大小，会触发切换日志段。我们看一下这块的处理逻辑。
具体的
XLogWrite调用过程可参考文章PostgreSQL源码分析——WAL日志（二）
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible)
{
// ...
/*
* If we just wrote the whole last page of a logfile segment,
* fsync the segment immediately. This avoids having to go back
* and re-open prior segments when an fsync request comes along
* later. Doing it here ensures that one and only one backend will
* perform this fsync.
*
* This is also the right place to notify the Archiver that the
* segment is ready to copy to archival storage, and to update the
* timer for archive_timeout, and to signal for a checkpoint if
* too many logfile segments have been used since the last checkpoint. */
if (finishing_seg) // 一个段已满
{
// 将该段刷入磁盘，保证归档日志的数据完整性
issue_xlog_fsync(openLogFile, openLogSegNo);
// 通知walsender进程发送日志给standby
/* signal that we need to wakeup walsenders later */
WalSndWakeupRequest();
LogwrtResult.Flush = LogwrtResult.Write; /* end of page */
if (XLogArchivingActive())
XLogArchiveNotifySeg(openLogSegNo); // 发送日志归档通知信息
// 更新日志切换时间，计算archive_timeout用
XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);
XLogCtl->lastSegSwitchLSN = LogwrtResult.Flush;
/*
* Request a checkpoint if we've consumed too much xlog since
* the last one. For speed, we first check using the local
* copy of RedoRecPtr, which might be out of date; if it looks
* like a checkpoint is needed, forcibly update RedoRecPtr and
* recheck.
*/
if (IsUnderPostmaster && XLogCheckpointNeeded(openLogSegNo))
{
(void) GetRedoRecPtr();
if (XLogCheckpointNeeded(openLogSegNo))
RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
}
}
}
我们看一下这个XLogArchiveNotify函数实现，日志归档通知，创建一个.ready文件，表示可以进行归档。当归档完成时，将对应的.ready文件重命名为.done文件
void XLogArchiveNotifySeg