From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
Cc: | Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Michael Paesold <mpaesold(at)gmx(dot)at>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>, Guillaume Smet <guillaume(dot)smet(at)gmail(dot)com>, "Matthew T(dot) O'Connor" <matthew(at)zeut(dot)net>, Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: First steps with 8.3 and autovacuum launcher |
Date: | 2007-10-12 14:19:57 |
Message-ID: | 26169.1192198797@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> I think the best way to handle this is to have two limits.
> First limit attempts to autovacuum, but can be cancelled.
> When we hit second limit, sometime later, then autovacuum cannot be
> cancelled.
This seems like uselessly complex overdesign.
Remember that we still have the 8.1-era mechanism of warning messages
and eventual shutdown of normal backends as the wrap point gets closer.
A DBA who persists in cancelling antiwraparound vacuums will start
getting nasty warnings, and if he still persists will eventually be
forced to do the vacuuming manually in a standalone backend (which will
go at full speed btw).
I concur with Simon's original argument that it's a terrible idea to
make a background vacuum cancel-proof. Pretty much the entire argument
for having vacuum at all (as opposed to say Oracle-style rollback) is
that it's good to be able to postpone maintenance overhead to a time of
the DBA's choosing. I don't see a reason why that doesn't apply to
anti-wraparound vacuuming too. If he postpones it too long, the
recovery will get unpleasant, but that's his choice.
An independent reason for minimizing complexity in this area is that
it's a corner case that (I trust) will never be hit by most
installations, which means it'll never get much field testing.
Poorly tested complicated code is dangerous.
In short: put in the automatic lock cancel for regular vacuums, disable
it for antiwraparound vacuums, but don't disable manual cancels; and
definitely don't invent a complicated new set of behaviors around that.
regards, tom lane
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