Re: pgsql: Introduce replication slots.

On Sat, Feb 1, 2014 at 6:09 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> On 2014-02-01 16:47:47 +0900, Fujii Masao wrote:
>> On Sat, Feb 1, 2014 at 12:50 PM, Robert Haas <rhaas@postgresql.org> wrote:
>> > Introduce replication slots.
>> >
>> > Replication slots are a crash-safe data structure which can be created
>> > on either a master or a standby to prevent premature removal of
>> > write-ahead log segments needed by a standby, as well as (with
>> > hot_standby_feedback=on) pruning of tuples whose removal would cause
>> > replication conflicts.  Slots have some advantages over existing
>> > techniques, as explained in the documentation.
>> >
>> > In a few places, we refer to the type of replication slots introduced
>> > by this patch as "physical" slots, because forthcoming patches for
>> > logical decoding will also have slots, but with somewhat different
>> > properties.
>>
>> 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.

BTW, we should add the setting of primary_slotname into recovery.conf.sample?

Regards,

--
Fujii Masao