Re: Logical replication and wal segment retention

When using replication slots with standby nodes, a master node retains the
necessary WAL files in pg_xlog until the standby has received them at the
cost of monitoring the space used by WAL files in pg_xlog as now the disk
space that those filesuse is not strictly controlled by wal_keep_segments
or checkpoint_segments but by elements (perhaps) external to the server
where the master node is running.

In the case of a standby node using streaming replication, the server does
not actually wait for the slave to catch up if it disconnects and simply
deletes the WAL files that are not needed. This has the advantage to
facilitate management of the disk space used by WAL files: use
checkpoint_segments as well in this case. The amount of WAL to keep on
master side can as well be tuned with wal_keep_segments.

Thanks & Regards,
*Shreeyansh DBA Team*
www.shreeyansh.com

On Wed, Feb 27, 2019 at 6:10 PM John Scalia <jayknowsunix(at)gmail(dot)com> wrote:

> Hello, folks,
>
> Yesterday, I had a small file system fill up, due to some logical
> replication testing we had been performing. We had been testing IBM’s IIDR
> system and apparently it had built a logical replication slot on my server.
> When the test was completed, nobody removed the slot, so WAL segments
> stopped being dropped. Now I can understand the difficulty separating what
> physical versus logical replication needs from the WAL segments, but as
> logical replication is database specific not cluster wide, this behavior
> was a little unexpected, since the WAL segments are cluster wide. Are WAL
> segments going to pile up whenever something drops a logical replication
> connection? I’ve seen it, but it seems like this could be a bad thing.
> -
> Jay
>
> Sent from my iPhone
>