Re: Connection slots reserved for replication

> On 7. Feb 2019, at 07:51, Michael Paquier <michael@paquier.xyz> wrote:
>
> On Sat, Feb 02, 2019 at 10:35:20AM +0900, Michael Paquier wrote:
>> Oh, I have just noticed this patch when doing my vacuum homework.  I
>> have just added my name as committer, just wait a bit until the CF is
>> closed before I begin looking at it..
>
> So, I have looked at this thread and the latest patch, and the thing
> looks in good shape.  Nice job by the authors and the reviewers.  It
> is a bit of a pain for users to hope that max_connections would be
> able to handle replication connections when needed, which can cause
> backups to not happen.  Using a separate GUC while we have
> max_wal_senders here to do the job is also not a good idea, so the
> approach of the patch is sound.  And on top of that, dependencies
> between GUCs get reduced.

Thank you for picking it up!

>
> I have spotted one error though: the patch does not check if changing
> max_wal_senders overflows MAX_BACKEND, and we need to reflect a proper
> calculation into check_autovacuum_max_workers,
> check_max_worker_processes and check_maxconnections.

Good catch, thank you.

I’ve noticed you returned the 'see max_connections’ parameter there. As noticed
previously in this thread by Kyotaro Horiguchi, it’s not clear what exactly we
are supposed to see there, perhaps it makes sense to elaborate (besides, the
comment on max_wal_senders at guc.c:382 hints that max_wal_senders depends on
max_connections, which is not true anymore).

>
> One thing is that the slot position calculation gets a bit more
> complicated with the new slot set for WAL senders, still the code is
> straight-forward to follow so that's not really an issue in my
> opinion.  The potential backward-incompatible issue issue of creating
> a standby with lower max_wal_senders value than the primary is also
> something we can live with as that's simple enough to address, and I'd
> like to think that most users prefer inheriting the parameter from the
> primary to ease failover flows.

+1

>
> It seems to me that it would be a good idea to have an
> autovacuum-worker-related message in the context of InitProcess() for
> a failed backend creation, but we can deal with that later if
> necessary.

Hm.. I am wondering how the autovacuum workers can run out of slots there?

>
> With all that reviewed, I finish with the attached that I am
> comfortable enough to commit.  XLOG_PAGE_MAGIC is bumped as well, as a
> reminder because xl_parameter_change gets an upgrade, and I am most
> likely going to forget it.
>
> Please let me know if you have comments.  I am still planning to do an
> extra pass on it.

Apart from the comment-related notes above your changes look good to me, thank
you.

Cheers,
Oleksii