Re: pgsql: Introduce replication slots.

On 2014-02-02 14:44:17 +0900, Fujii Masao wrote:
> On Sat, Feb 1, 2014 at 6:37 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > On 2014-02-01 18:33:08 +0900, Fujii Masao wrote:
> >> On Sat, Feb 1, 2014 at 6:09 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> >> > On 2014-02-01 16:47:47 +0900, Fujii Masao wrote:
> >> >> This patch changed basebackup.c so that it skips pg_replslot. It's OK
> >> >> to skip all files in that directory, but an empty pg_replslot must be
> >> >> included in the backup. Otherwise we cannot start PostgreSQL from
> >> >> the backup taken via pg_basebackup. Attached patch fixes this problem.
> >> >
> >> > That's a pretty fair point. Not sure how that could escape my
> >> > notice. The patch does look sane to me.
> >> >
> >> > I wonder if we additionally should add code to recreate pg_replslot on
> >> > startup, similar to pg_xlog?
> >>
> >> Similar to pg_xlog/archive_status, not pg_xlog? That might be an option.
> >> But I'm not inclined to do that for now. The fact that the essential
> >> directory like pg_replslot doesn't exist indicates the symptom of
> >> something strange. And, changing that way might prevent us from
> >> detecting such symptom.
> >
> > The reason I am wondering is that it makes a fair bit of sense to
> > exclude it in open-coded base backups as well, and excluding the
> > entire directory might be the easiest way there. But I guess people
> > manage for pg_xlog/, so it's really not something that would reduce pain
> > measurably.
>
> On second thought, we should always delete all files in pg_replslot
> when starting recovery from the backup?

Are you suggesting to always delete them when in standby_mode? If so,
no, that'd be bad, we intentially *do* want to support situations in
which we stream from the standby, i.e. cascading types of setups.

If you mean doing so when initially starting with a backup label,
hm. Maybe. That would make it impossible to keep replication slots when
moving to a new server with a short downtime, which seems a bit
annoying.

> The users who are
> using their own backup script instead of pg_basebackup might
> include pg_replslot files in the backup. Currently, in this case,
> the replication slots which were created before would be available
> even after the recovery. Is this OK? If not, all files in pg_replslot
> should be removed at the beginning of the PITR. OTOH, if that's OK,
> I think that pg_basebackup should not skip pg_replslot files.
> Thought?

Robert raised this previously in
http://archives.postgresql.org/message-id/CA%2BTgmoar6BLb%2B7BQUYEmkmdFSE1f8khCZCDP-aCojOrESiNLBg%40mail.gmail.com :
> - Exclude pg_replslot from base backups. This might need more thought
> and documentation; people who use the filesystem method to perform
> backups might need to be advised to remove this directory in some
> cases also, or people who use pg_basebackup might want to keep it in
> some cases (not sure).

I can see usecases for removing and keeping them. Removing them has the
big advantage that the user won't be surprised by a slot's existance
which prevents resources (WAL, xmin horizon/VACUUM) from being
reclaimed.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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