Re: Questions about the continuity of WAL archiving - Mailing list pgsql-general

Justin <zzzzz.graf@gmail.com> 于2025年8月13日周三 10:51写道：

On Tue, Aug 12, 2025 at 10:24 PM px shi <spxlyy123@gmail.com> wrote:

How often does your primary node crash, and then not recover due to WALs corruption or WALs not existing?

If it's _ever_ happened, you should _fix that_ instead of rolling your own WAL archival.process.

 I once encountered a case where the recovery process failed to restore to the latest LSN due to missing WAL files in the archive. The root cause was multiple failovers between primary and standby. During one of the switchovers, the primary crashed before completing the archiving of all WAL files. When the standby was promoted to primary, it began archiving WAL files for the new timeline, resulting in a gap between the WAL files of the two timelines. Moreover, no base backup was taken during this period.

I am not sure what the problem is  here either, other than something seriously wrong with configuration with PostgreSQL and PgBackrest.

The replica should be receiving the WAL via a replication slot using Streaming.  Meaning the primary will keep the WAL until the replica is caught up,  if the replica becomes disconnected due to  max_slot_wal_keep_size aka wal_keep_segments  is exceeded the replicas recovery_command can take offer and fetch from the WAL Archive to catch the replica up.  This assumes hot_feedback is on so the WAL replay won't become delayed due to snapshot locks on the replica. 

If  all the above is true the replica should never lag behind unless the disk IO layer is way undersized compared to the Primary.  S3 is being talked about  so it makes me wonder about DISK IO configuration on the primary vs the replica.  I see this causing lag under high load where the replica IO layer is the bottleneck.

If PgBackrest can't keep up with WAL archiving, as others have stated  you need to configure Asynchronous Archiving. The number of workers depends on the load. I have a server running 8 parallel workers to archive 1TB of WAL daily....  And another server  during maintenance tasks generates around 10,000 WAL files in about 2 hours using 6 PgBAckrest workers  All to S3 buckets.

The above statement makes me wonder if there is some kind of High Availability monitor running like pg_autofailover, that is promoting a replica then  converting the former primary to a replica of the recently "promoted replica"  

If the above matches to what is happening, it is very easy to mess up the configuration for WAL archiving and backups. Part of the process of promoting a replica is to make sure WAL archiving is working.  The replica after being promoted immediately kicks of autovacuum to rebuild things like FSM which generates a lot of WAL files.

If you are losing  WAL files the configuration is wrong somewhere..

Just not enough information on the series of events and the configuration to tell what the root cause is other than miss-configuration.


Thanks
Justin

Here’s a scenario: The latest WAL file on the primary node is 0000000100000000000000AF, and the standby node has also received up to 0000000100000000000000AF. However, the latest WAL file that has been successfully archived from the primary is only 0000000100000000000000A1 (WAL files from A2 to AE have not yet been archived). If the primary crashes at this point, triggering a failover, the new primary will start generating and archiving WAL on a new timeline (2), beginning with 0000000200000000000000AF. It will not backfill the missing WAL files from timeline 1 (0000000100000000000000A2 to 0000000100000000000000AE). As a result, while the new primary does not have any local WAL gaps, the archive directory will contain a gap in that WAL range.

I’m not sure if I explained it clearly.