Re: Page Checksums

From: Jim Nasby <jim(at)nasby(dot)net>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, David Fetter <david(at)fetter(dot)org>, Stephen Frost <sfrost(at)snowman(dot)net>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org, Greg Smith <greg(at)2ndQuadrant(dot)com>, Koichi Suzuki <koichi(dot)szk(at)gmail(dot)com>
Subject: Re: Page Checksums
Date: 2012-01-10 00:12:06
Message-ID: B81291FE-2ABD-4208-81FB-D5C65581D22A@nasby.net
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On Jan 8, 2012, at 5:25 PM, Simon Riggs wrote:
> On Mon, Dec 19, 2011 at 8:18 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>
>> Double-writes would be a useful option also to reduce the size of WAL that
>> needs to be shipped in replication.
>>
>> Or you could just use a filesystem that does CRCs...
>
> Double writes would reduce the size of WAL and we discussed many times
> we want that.
>
> Using a filesystem that does CRCs is basically saying "let the
> filesystem cope". If that is an option, why not just turn full page
> writes off and let the filesystem cope?

I don't think that just because a filesystem CRC's that you can't have a torn write.

Filesystem CRCs very likely will not happen to data that's in the cache. For some users, that's a huge amount of data to leave un-protected.

Filesystem bugs do happen... though presumably most of those would be caught by the filesystem's CRC check... but you never know!
--
Jim C. Nasby, Database Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net

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