Re: Autovacuum of pg_database

I had the same problem with 9.1 and vacuuming several databases simultaneously. We just lived with it. Other tables in the databases were far larger than pg_sharedepend, so it wasn't the most significant issue we faced.

Bob Lunney

> On May 9, 2016, at 6:42 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> I wrote:
>> Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
>>> Now that I actually tried this, it turned out that this problem is not
>>> so simple. vacuum.c already has logic to use conditional acquire of the
>>> table-level lock, and if not available it skips the table:
>>> LOG: skipping vacuum of "pg_shdepend" --- lock not available
>>> so an autovacuum worker is never "stuck" behind another worker trying to
>>> vacuum the table.
>
>> Hmm ... but then, how do we have the observed symptom of several workers
>> concurrently trying to process the same shared catalog? Seems like all
>> but one should fall out at this point.
>
> Oh, see table_recheck_autovac:
>
> tab->at_vacoptions = VACOPT_SKIPTOAST |
> (dovacuum ? VACOPT_VACUUM : 0) |
> (doanalyze ? VACOPT_ANALYZE : 0) |
> (!wraparound ? VACOPT_NOWAIT : 0);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> I'll bet they're all trying to do anti-wraparound vacuums. This would
> explain why the problem hasn't been observed often enough to have been
> fixed long since.
>
> regards, tom lane
>
>
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