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Message-ID: <dcc563d10803030722i19fd2789o3c151e5d89b81bf4@mail.gmail.com>
Date: Mon, 3 Mar 2008 09:22:12 -0600
From: "Scott Marlowe" <scott.marlowe@gmail.com>
To: "Mark Mielke" <mark@mark.mielke.cc>
Subject: Re: How to allocate 8 disks
Cc: Matthew <matthew@flymine.org>,
	pgsql-performance <pgsql-performance@postgresql.org>
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References: <1204285894.11773.33.camel@franck-gusty>
	<dcc563d10802290613v482f52dr43ea95b52886de31@mail.gmail.com>
	<1204296705.11773.51.camel@franck-gusty>
	<Pine.GSO.4.64.0802291019570.28452@westnet.com>
	<47C88826.4040209@paradise.net.nz> <47C99B3E.4010606@emolecules.com>
	<20080301130347.0832b323@commandprompt.com>
	<47C9D05C.3040908@emolecules.com>
	<Pine.LNX.4.64.0803031206340.20402@aragorn.flymine.org>
	<47CC0FD1.7090404@mark.mielke.cc>

On Mon, Mar 3, 2008 at 8:48 AM, Mark Mielke <mark@mark.mielke.cc> wrote:
> Matthew wrote:
>  > On Sat, 1 Mar 2008, Craig James wrote:
>  >> Right, I do understand that, but reliability is not a top priority in
>  >> this system.  The database will be replicated, and can be reproduced
>  >> from the raw data.
>  >
>  > So what you're saying is:
>  >
>  > 1. Reliability is not important.
>  > 2. There's zero write traffic once the database is set up.
>  >
>  > If this is true, then RAID-0 is the way to go. I think Greg's options
>  > are good. Either:
>  >
>  > 2 discs RAID 1: OS
>  > 6 discs RAID 0: database + WAL
>  >
>  > which is what we're using here (except with more discs), or:
>  >
>  > 8 discs RAID 10: everything
>
>  Has anybody been able to prove to themselves that RAID 0 vs RAID 1+0 is
>  faster for these sorts of loads? My understanding is that RAID 1+0 *can*
>  reduce latency for reads, but that it relies on random access, whereas
>  RAID 0 performs best for sequential scans? Does PostgreSQL ever do
>  enough random access to make RAID 1+0 shine?

RAID 1+0 has certain theoretical advantages in parallel access
scenarios that straight RAID-0 wouldn't have.  I.e. if you used n>2
disks in a mirror and built a RAID-0 out of those types of mirrors,
then you could theoretically have n users reading data on the same
"drive"  (the raid-1 underneath the raid-0) at the same time where
RAID-0 would only have the one disk to read from.  The effects of this
advantage are dulled by caching, depending on how much of the data set
you can cache.  With a system that can cache it's whole data set in
memory (not uncommon for transactional systems) or at least a large
percentage, the n>2 RAID-1 sets aren't that big of an advantage.

RAID-0 of n drives should behave pretty similarly to RAID-10 with 2n
drives for most types of access.  I.e. no better or worse for
sequential or random access, if the number of drives is equivalent.