From: Bob Lunney <blunney@meetme.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Ondřej Světlík <osvetlik@flexibee.eu>
Cc: pgsql-admin@postgresql.org
Subject: Re: Autovacuum of pg_database
Date: Mon, 9 May 2016 21:21:31 -0400
Message-ID: <0E0FB1FC-41FE-4156-9CE8-535837D6FC81@meetme.com> (raw)
In-Reply-To: <4352.1462833746@sss.pgh.pa.us>
References: <20160509213030.GA496423@alvherre.pgsql>
<4023.1462833047@sss.pgh.pa.us>
<4352.1462833746@sss.pgh.pa.us>
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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@sss.pgh.pa.us> wrote:
>
> I wrote:
>> Alvaro Herrera <alvherre@2ndquadrant.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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To: pgsql-admin@postgresql.org
Cc: blunney@meetme.com, tgl@sss.pgh.pa.us, alvherre@2ndquadrant.com, osvetlik@flexibee.eu
Subject: Re: Autovacuum of pg_database
In-Reply-To: <0E0FB1FC-41FE-4156-9CE8-535837D6FC81@meetme.com>
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