SUMMARY [SDS RAID performance]

From: Philip Plane (
Date: Mon Jul 28 1997 - 00:11:53 CDT

My query was:

> On a Enterprise 2/200 with a Multipack I set up a RAID of 11 2.1Gb disks using
> SDS 4.1. How should this perform compared to a normal disk?
> Here is what I am seeing when copying a 11Mbyte file:
> from RAID to RAID 8.5 seconds (same file system)
> from RAID to disk 1.3 seconds
> from disk to RAID 7.5 seconds
> from disk to disk 1.5 seconds (different file systems)
> To me this looks like the RAID is three time slower to write.
> Read performance looks similar for RAID and normal disk. Dump reports 2363
> KB/Sec from RAID and between 1168 to 2785 KB/Sec from the various disks.
> Is this normal? What can I do to improve write performance?

The short answer is:

- Because Solstice Disk Suite does RAID 5 in software, there is a small overhead
in calculating the parity on write. This doesn't affect reads. Opinions vary on
how much of a performance hit you get with writes from RAID 5, but most say

- The optimum number of disks in the RAID is 5. More disks reduce performance. I
am using 11. :-(

- Having 12 disks (11 in the RAID + 1 hot spare) on 1 controller hurts

The unusually slow performance I am seeing is a result of the combination of
these things.

To improve performance I could:

- Use a hardware RAID solution. (costs money)

- Make two smaller RAIDs and concatenate them. (lose a little disk space)

- Put in another controller and put half the disks on it. (costs money)

- Use striping + mirroring. (lose lotsa disk space or costs money)

- Alter the interleave.

Thanks to:

Rick von Richter <>
Ed Romascan <ed@magma.COM>
Rich Kulawiec <> (Sanjay Patel) (Jason Noorman) (Marc S. Gibian)
Karl von Jena <> (John Malick)
Brion Leary <>

Philip Plane
Senior Systems Administrator
Unix Network Services
Client Operational Support
Information Technology
Inland Revenue

This messages represents the views and opinions of the author only and not those of the Inland Revenue Department.

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