Re: Testing autovacuum wraparound (including failsafe)

On Fri, Jun 11, 2021 at 10:19 AM Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2021-06-10 16:42:01 +0300, Anastasia Lubennikova wrote:
> > Cool. Thank you for working on that!
> > Could you please share a WIP patch for the $subj? I'd be happy to help with
> > it.
>
> I've attached the current WIP state, which hasn't evolved much since
> this message... I put the test in src/backend/access/heap/t/001_emergency_vacuum.pl
> but I'm not sure that's the best place. But I didn't think
> src/test/recovery is great either.
>

Thank you for sharing the WIP patch.

Regarding point (1) you mentioned (StartupSUBTRANS() takes a long time
for zeroing out all pages), how about using single-user mode instead
of preparing the transaction? That is, after pg_resetwal we check the
ages of datfrozenxid by executing a query in single-user mode. That
way, we don’t need to worry about autovacuum concurrently running
while checking the ages of frozenxids. I’ve attached a PoC patch that
does the scenario like:

1. start cluster with autovacuum=off and create tables with a few data
and make garbage on them
2. stop cluster and do pg_resetwal
3. start cluster in single-user mode
4. check age(datfrozenxid)
5. stop cluster
6. start cluster and wait for autovacuums to increase template0,
template1, and postgres datfrozenxids

I put new tests in src/test/module/heap since we already have tests
for brin in src/test/module/brin.

I think that tap test facility to run queries in single-user mode will
also be helpful for testing a new vacuum option/command that is
intended to use in emergency cases and proposed here[1].

Regards,

[1]  https://www.postgresql.org/message-id/flat/20220128012842.GZ23027%40telsasoft.com#b76c13554f90d1c8bb5532d6f3e5cbf8


--
Masahiko Sawada
EDB:  https://www.enterprisedb.com/