Re: PATCH: track last known XLOG segment in control file

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: PATCH: track last known XLOG segment in control file
Date: 2015-12-12 22:20:23
Message-ID: 20151212222023.GR14789@awork2.anarazel.de
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Hi,

On 2015-12-12 22:14:13 +0100, Tomas Vondra wrote:
> this is the second improvement proposed in the thread [1] about ext4 data
> loss issue. It adds another field to control file, tracking the last known
> WAL segment. This does not eliminate the data loss, just the silent part of
> it when the last segment gets lost (due to forgetting the rename, deleting
> it by mistake or whatever). The patch makes sure the cluster refuses to
> start if that happens.

Uh, that's fairly expensive. In many cases it'll significantly increase
the number of fsyncs. I've a bit of a hard time believing this'll be
worthwhile. Additionally this doesn't seem to take WAL replay into
account?

Andres

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