Re: vacuumdb --jobs deadlock: how to avoid pg_catalog ?

@Adrian thanks again.

I read the postgres docs the same way - that previously used space is marked as available and therefore no need for vacuum full. Previously used = now available space, which gets re-used, in theory.

And yet this same DB is growing at 30-50 GB weekly, despite the fact that 2.4 TB of previously used space is clearly available ("clearly available" as proven by vacuum full shrinking the DB space usage by 2.4 TB). I did verify postgresql.conf has always been properly configured re: autovacuum: 'autovacuum = on'and 'track_counts = on'

I'm not planning on running VACUUM FULL regularly, just "this one time". And I was trying to to parallelize VACUUM FULL and minimize downtime of the production DB caused by table locking. And then I found the option of using "vacuumdb --jobs" which sounded like the perfect solution except for "well you can't actually use --jobs because you'll run into a deadlock and everybody knows that and nobody has a (good) solution for it" :).

--Ed

> On May 14, 2020, at 11:46, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com> wrote:
>
> On 5/14/20 11:27 AM, Eduard Rozenberg wrote:
>> @Adrian thanks.
>> I did a test vacuum full the "normal" non-parallel way (VACUUM FULL sql) previously on a test db copy and saw the DB size (postgres 9.6) shrink from 6.8 TB to 4.4 TB of actual, real disk space usage ($ df -d 1).
>> I don't know the reason so much space was "locked up" (other than there is a lot of data "churn" from daily updates). But I definitely do need to do the vac full on the production db to get down to the smaller size - cannot afford the 2.4 TB of "wasted" space on an ongoing basis.
>
> It may not be wasted space. A regular VACUUM marks space within a table available for reuse(and not returned to OS) when it removes unneeded tuples. It then fills that space up with new tuples, roughly speaking. So if the vacuuming is working properly you will reach a steady state where space is reused and the database on disk size grows slowly as reusable space is occupied. I would monitor the database size on a regular basis. My guess is that the VACUUM FULL is dropping the OS used space and then it fills up again as the database does those updates.
>
>> Based on your email it sounds like the vacuuming of those pg_catalog.* tables is done regardless, as a normal part of doing vac full on my own database.
>> Unfortunately I still don't see an ideal way to run vacuum full in parallel via vacuumdb without running into the expected and documented deadlock. Only method I'm aware of is to list each table individually with "-t table1 -t table2..." to "vacuum db --jobs" which is not pleasant and not exceedingly beautiful.
>> Thanks.
>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com