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Re: RAID stripe size question

From: "Alex Turner" <armtuk(at)gmail(dot)com>
To: "Mikael Carneholm" <Mikael(dot)Carneholm(at)wirelesscar(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: RAID stripe size question
Date: 2006-07-17 06:13:04
Message-ID: 33c6269f0607162313k1770ac16ld5e90e106bf5e8dc@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
With 18 disks dedicated to  data, you could make 100/7*9 seeks/second (7ms
av seeks time, 9 independant units) which is 128seeks/second writing on
average 64kb of data, which is 4.1MB/sec throughput worst case, probably 10x
best case so 40Mb/sec - you might want to take more disks for your data and
less for your WAL.

Someone check my math here...

And as always - run benchmarks with your app to verify

Alex.

On 7/16/06, Mikael Carneholm <Mikael(dot)Carneholm(at)wirelesscar(dot)com> wrote:
>
>  I have finally gotten my hands on the MSA1500 that we ordered some time
> ago. It has 28 x 10K 146Gb drives, currently grouped as 10 (for wal) + 18
> (for data). There's only one controller (an emulex), but I hope performance
> won't suffer too much from that. Raid level is 0+1, filesystem is ext3.
>
> Now to the interesting part: would it make sense to use different stripe
> sizes on the separate disk arrays? In theory, a smaller stripe size (8-32K)
> should increase sequential write throughput at the cost of decreased
> positioning performance, which sounds good for WAL (assuming WAL is never
> "searched" during normal operation). And for disks holding the data, a
> larger stripe size (>32K) should provide for more concurrent (small)
> reads/writes at the cost of decreased raw throughput. This is with an OLTP
> type application in mind, so I'd rather have high transaction throughput
> than high sequential read speed. The interface is a 2Gb FC so I'm throttled
> to (theoretically) 192Mb/s, anyway.
>
> So, does this make sense? Has anyone tried it and seen any performance
> gains from it?
>
> Regards,
> Mikael.
>

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