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Re: [HACKERS] wal_checksum = on (default) | off

From: Florian Weimer <fw(at)deneb(dot)enyo(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Simon Riggs" <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Re: [HACKERS] wal_checksum = on (default) | off
Date: 2007-01-04 21:48:56
Message-ID: 87k602v4xj.fsf@mid.deneb.enyo.de (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
* Tom Lane:

> I think short burst errors are fairly likely: the kind of scenario I'm
> worried about is a wild store corrupting a word of a WAL entry while
> it's waiting around to be written in the WAL buffers.

Ah, does this mean that each WAL entry gets its own checksum?  In this
case, Adler32 is indeed suboptimal because it doesn't use the full 32
bits for short inputs.  It might still catch many wild stores, but the
statistics are worse than for CRC32.

(I had assumed that PostgreSQLs WAL checksumming was justified by the
partial write issue.  The wild store could easily occur with a heap
page, too, and AFAIK, tuples, aren't checksummed.  Which would be an
interesting option, I guess.)

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