Re: Why does PostgresNode.pm set such a low value of max_wal_senders?

Michael Paquier <michael(at)paquier(dot)xyz> writes:
> On Wed, Sep 30, 2020 at 10:38:59PM -0700, Noah Misch wrote:
>> In favor of minimal values, we've had semaphore-starved buildfarm members in
>> the past. Perhaps those days are over, seeing that this commit has not yet
>> broken a buildfarm member in that particular way. Keeping max_wal_senders=10
>> seems fine.

> Indeed, I am not spotting anything suspicious here.

Yeah, so far so good. Note that PostgresNode.pm does attempt to cater for
semaphore-starved machines, by cutting max_connections as much as it can.
In practice the total semaphore usage of a subscription test is probably
still less than that of one postmaster with default max_connections.

>> No, PostgreSQL commit 54c2ecb changed that. I recommend an explicit
>> max_wal_senders=10 in PostgresNode, which makes it easy to test
>> wal_level=minimal:

> Ah, thanks, I have missed this piece. So we really need to have a
> value set in this module after all.

Agreed, I'll go put it back.

On the other point, I think that we should continue to complain
about max_wal_senders > 0 with wal_level = minimal. If we reduce
that to a LOG message, which'd be the net effect of trying to be
laxer, people wouldn't see it and would then wonder why they can't
start replication.

regards, tom lane