Date: Sun, 25 Dec 2005 12:37:55 -0500
From: Michael Stone <mstone+postgres@mathom.us>
Subject: Re: What's the best hardver for PostgreSQL 8.1?
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On Sat, Dec 24, 2005 at 05:45:20PM -0500, Ron wrote:
>Caches help, and the bigger the cache the better, but once you are 
>doing enough writes fast enough (and that doesn't take much even with 
>a few GBs of cache) the recalculate-checksums-and-write-new-ones 
>overhead will decrease the write speed of real data.  Bear in mind 
>that the HD's _raw_ write speed hasn't been decreased.  Those HD's 
>are pounding away as fast as they can for you.  Your _effective_ or 
>_data level_ write speed is what decreases due to overhead.

You're overgeneralizing. Assuming a large cache and a sequential write,
there's need be no penalty for raid 5. (For random writes you may
need to read unrelated blocks in order to calculate parity, but for
large sequential writes the parity blocks should all be read from
cache.) A modern cpu can calculate parity for raid 5 on the order of
gigabytes per second, and even crummy embedded processors can do
hundreds of megabytes per second. You may have run into some lousy
implementations, but you should be much more specific about what
hardware you're talking about instead of making sweeping
generalizations.

>Side Note: people often forget the other big reason to use RAID 10 
>over RAID 5.  RAID 5 is always only 2 HD failures from data 
>loss.  RAID 10 can lose up to 1/2 the HD's in the array w/o data loss 
>unless you get unlucky and lose both members of a RAID 1 set.

IOW, your RAID 10 is only 2 HD failures from data loss also. If that's
an issue you need to go with RAID 6 or add another disk to each mirror.

Mike Stone