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Re: Proposed patch: Smooth replication during VACUUM FULL

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Jaime Casanova <jaime(at)2ndquadrant(dot)com>
Cc: Gabriele Bartolini <gabriele(dot)bartolini(at)2ndquadrant(dot)it>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposed patch: Smooth replication during VACUUM FULL
Date: 2011-04-30 22:48:39
Message-ID: 5139.1304203719@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Jaime Casanova <jaime(at)2ndquadrant(dot)com> writes:
> On Sat, Apr 30, 2011 at 1:19 PM, Gabriele Bartolini
>> I have noticed that during VACUUM FULL on reasonably big tables, replication
>> lag climbs. In order to smooth down the replication lag, I propose the
>> attached patch which enables vacuum delay for VACUUM FULL.

> AFAICS, the problem is that those operations involve the rebuild of
> tables, so we can't simply stop in the middle and wait because we will
> need to hold a strong lock more time... also the patch seems to be
> only doing something for CLUSTER and not for VACUUM FULL.
> or am i missing something?

No, actually it would have no effect on CLUSTER because VacuumCostActive
wouldn't be set.  I think this is basically fixing an oversight in the
patch that changed VACUUM FULL into a variant of CLUSTER.  We used to
use vacuum_delay_point() in the main loops in old-style VACUUM FULL,
but forgot to consider doing so in the CLUSTER-ish implementation.
The argument about holding locks longer doesn't seem relevant to me:
enabling delays during VACUUM FULL would've had that effect in the old
implementation, too, but nobody ever complained about that, and besides
the feature isn't enabled by default.

A bigger objection to this patch is that it seems quite incomplete.
I'm not sure there's much point in adding delays to the first loop of
copy_heap_data() without also providing for delays inside the sorting
code and the eventual index rebuilds; which will make the patch
significantly more complicated and invasive.

Another question is whether this is the right place to be looking
at all.  If Gabriele's setup can't keep up with replication when a
VAC FULL is running, then it can't keep up when under load, period.
This seems like a pretty band-aid-ish response to that sort of problem.

			regards, tom lane

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