Re: setting for keep_wal_segments with replication slots, postgresql 9.4

Then, the second paragraph begins "In lieu of using replication slots... use keep_wal_segments"

This is talking about using keep_wal_segments to always have certain minimum number of WALs even if replication is up-to-date and doesn't need those WALs anymore. 

 Also, if the slot is managing the storage of WAL segments, then isn't there a possibility of filling up a disk in the event of an outage?

Yes, which is why you should have disk alerts set in place and a quick way to redirect WAL files to another machine, possibly one that stores backups.

 And would keep_wal_segments prevent that?

No. If slave is out, slots will keep all WALs irrespective of what the wal_keep_segments is set to. So in a way, wal_keep_segments value is the lowest minimum number of WALs you will have on master at any moment, and with slots it is possible that number will increase if slave goes down. 

Payal Singh,
Database Administrator,

OmniTI Computer Consulting Inc.
Phone: 240.646.0770 x 253

On Wed, Sep 7, 2016 at 9:12 AM, John Scalia <jayknowsunix@gmail.com> wrote:

Hi all,

I've been reading section 25.2.6 Replication Slots in the docs, and I'm just a little confused. This section clearly states that replication slots automates the management of WAL slots so the master will not remove them prior to receipt by all standbys. Then, the second paragraph begins "In lieu of using replication slots... use keep_wal_segments". So, if I have a primary-standby cluster using a replication slot, then what, if anything, should keep_wal_segments be set to? Should I just comment it out?

Also, if the slot is managing the storage of WAL segments, then isn't there a possibility of filling up a disk in the event of an outage? And would keep_wal_segments prevent that? This whole section just seems to be a little vague to me and like it wasn't updated cleanly since slots were envisioned.

Maybe it's just too early in the morning for me :-)

Jay