Re: [GENERAL] 9.4.1 -> 9.4.2 problem: could not access status of transaction 1

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, Steve Kehlet <steve(dot)kehlet(at)gmail(dot)com>, Forums postgresql <pgsql-general(at)postgresql(dot)org>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [GENERAL] 9.4.1 -> 9.4.2 problem: could not access status of transaction 1
Date: 2015-06-08 16:37:07
Message-ID: 20150608163707.GD20772@alap3.anarazel.de
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On 2015-06-08 15:15:04 +0200, Andres Freund wrote:
> 1) the autovacuum trigger logic isn't perfect yet. I.e. especially with
> autovacuum=off you can get into situations where emergency vacuums
> aren't started when necessary. This is particularly likely to happen
> if either very large multixacts are used, or if the server has been
> shut down while emergency autovacuum where happening. No corruption
> ensues, but it's not easy to get out of.

A first version to address this problem can be found appended to this
email.

Basically it does:
* Whenever more than MULTIXACT_MEMBER_SAFE_THRESHOLD are used, signal
autovacuum once per members segment
* For both members and offsets, once hitting the hard limits, signal
autovacuum everytime. Otherwise we loose the information when
restarting the database, or when autovac is killed. I ran into this a
bunch of times while testing.

Regards,

Andres

Attachment Content-Type Size
0001-Improve-multixact-emergency-autovacuum-logic.patch text/x-patch 5.8 KB

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