Re: URGENT: Out of disk space pg_xlog

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Bruce Momjian" <bruce(at)momjian(dot)us>
Cc: "Jeremy Haile" <jhaile(at)fastmail(dot)fm>, <pgsql-performance(at)postgresql(dot)org>
Subject: Re: URGENT: Out of disk space pg_xlog
Date: 2006-12-29 18:18:18
Message-ID: 1167416298.3903.230.camel@silverbirch.site
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On Sat, 2006-12-23 at 13:13 -0500, Bruce Momjian wrote:

> The bottom line is that we know of now cases where a long-running
> transaction would delay recycling of the WAL files, so there is
> certainly something not understood here.

We can see from all of this that a checkpoint definitely didn't occur.
Tom's causal chain was just one way that could have happened, there
could well be others.

I've noticed previously that a checkpoint can be starved out when trying
to acquire the CheckpointStartLock. I've witnessed a two minute delay
plus in obtaining the lock in the face of heavy transactions.

If wal_buffers is small enough, WAL write rate high enough and the
transaction rate high enough, a long queue can form for the
WALWriteLock, which ensures that the CheckpointStartLock would queue
indefinitely.

I've tried implementing a queueable shared lock for the
CheckpointStartLock. That helps the checkpoint, but it harms performance
of other transactions waiting to commit, so I let that idea go.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com

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