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I realise the usage of the disks will affect the performance, I’m just looking for a general estimation.

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3 Answers 3

up vote 5 down vote accepted

This Serverfault posting discusses some of the ins-and-outs of storage performance on RAID volumes.

  • If you have a heavy random I/O load then 6 disks in a RAID-5 (RAID-50 for 6 disks is overkill and won't save you any write traffic) will probably outperform a single volume.

  • For a sequential I/O load the throughput will be a maximum of one stripe per revolution of the disk on the RAID-5 volume. For a 64k stripe on a 10,000 RPM disk this is 170 * 64k = approximately 10MB/sec per disk, so your RAID-5 volume will probably support a maximum of 60MB/sec.

    It might even be less than that. For example a Dell PV660 would get one stripe per two revolutions of a diks at best (at least I could never tune it so it did any better), so a 14 disk array would peak at about 70MB/sec. Some controllers support larger stripe sizes up to 1MB or so, so your mileage may vary.

    On a single disk you are not reading stripes, so your theoretical throughput for sequential I/O is the maximum speed of the disk, which tends to be about 100MB/sec or so for modern disks.

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Thanks for the detailed response. Based on what chankster said do you think a raid 10 would be faster than both the options discussed above? –  mrmike Jun 24 '09 at 10:17
    
RAID-10 will be faster for writes than RAID-5 or RAID-50 but it is still subject to the same constraints on sequential throughput from the striping. To some extent it depends on the workload, but for most things a RAID-10 with 6 disks will probably be faster than a single disk. –  ConcernedOfTunbridgeWells Jun 24 '09 at 10:20

The answer, as always is "it depends". What kind of workload? What kind of hard drive controller? Raid 50 will stripe two raid5 2+1 which doesnt seem a very good compromise.

I'd normally say that the raid 50 will be faster but if you're actually considering using 50 on 6 drives, stop right now. Use raid5 for space or raid10 for speed.

Anders

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Odd question - let's assume that you're talking about the same make/model of disks throughout and also let's assume you need a balance of read & write performance. Let's also assume you have a decent controller too.

In that situation you'll see the R50 array provide faster reads, both random and sequential but slower writes, especially random ones.

If I were you I'd go with either R1, R5 or R10.

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