Re: questions about wraparound

On Fri, Apr 2, 2021 at 10:29 AM Jehan-Guillaume de Rorthais
<jgdr(at)dalibo(dot)com> wrote:
>
> On Thu, 18 Mar 2021 09:56:16 +0100
> Luca Ferrari <fluca1978(at)gmail(dot)com> wrote:
> [...]
> > Therefore my question is: shouldn't autovacuum be able to freeze other
> > tables/databases? I mean, the wraparound problem in this scenario will
> > cause problems, but I was expecting different numbers for different
> > tables/databases.
>
> In fact, when an autovacuum worker is spawned, here is how it chooses what
> database to process:
>
> 1. look for any database needing a vacuum to prevent a wraparound.
> 2. same with multi-transaction
> 3. other autovacuum considerations
>
> So as long as there's a database in desperate need for a vacuum to prevent a
> wraparound, a worker will try to process it first, again and again.
>
> Because of your long-running transaction, the xid horizon forbid to update the
> rel/datfrozenxid. So next autovacuum round will keep trying to process the same
> database, ignoring others.
>
> Look at the comment in function "do_stat_worker()" in autovacuum.c for more
> details:
> https://git.postgresql.org/cgit/postgresql.git/tree/src/backend/postmaster/autovacuum.c#n1207
>
> When looping over the database list, as soon as "for_xid_wrap" is true, any
> other database is ignored. Then a new worker is popped from the freeWorkers,
> init'ed with the database to freeze and started. So as far as I understand the
> code (I might easily be wrong), all the workers will keep trying to process the
> same database again and again without considering other ones. All because of
> your really-long living xact.

This is a damn good and complete explanation that solved, so far, all my doubts.
Or, ehm, there is one last: why having a TransactionId that is 32 bits
in depth while it is exposed (thru txid_current()) as a 64 bits value?
I mean, having 64 bits would reduce the need for anti-wrap arpund
vacuum. I suspect the usage of 32 bits is both for compatibility and
tuple header size, but I'm just guessing.

Thanks,
Luca