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Date: Wed, 2 May 2001 16:54:14 -0400
From: "Brad L. Chisholm" <blc@bsdwins.com>
To: freebsd-questions@FreeBSD.org
Subject: Optimal setup for large raid?
Message-ID: <20010502165413.B9113@bsdone.bsdwins.com>
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We are planning to create a large software raid volume,
and I am interested in input about what might make the 
best configuration.

We have 52 identical 9Gb drives (Seagate ST19171W) spread
across 4 SCSI controllers (Adaptec AHA 2944UW), with 13
drives per controller.  We want fault-tolerance, but cannot
afford to "waste" 50% of our space for a mirrored (raid1)
configuration.  Thus, we are considering some sort of
raid5 setup using vinum (possibly in combination with ccd).
We are running FreeBSD 4.3-RELEASE, on a 550Mz P3 with
384Mb of memory.

Possible configurations:

   Configuration #1: A single raid5 vinum volume consisting of all 52 drives.

   Questions:

      A) Is there a performance penalty for this many drives in
         a raid5 array?

      B) Should the plex be configured with sequential drives on
         different controllers?  (i.e. if drives 1-13 are on controller 1,
         14-27 on controller 2, 27-39 on controller 3, and 40-52 on
         controller 4, should the drive ordering be:

             1,14,27,40,2,15,28,41,...
         or  1,2,3,4,5,6,7,8,...


    Configuration #2: Multiple raid5 vinum volumes (perhaps 1 per controller),
                      combined into a single volume by striping the raid5
                      volumes.  (Basically a "raid50" setup.)

    Questions:

       A) Is this possible with vinum?  From the documentation, it didn't
          appear to be, so we were considering using 'ccd' to stripe the
          raid5 volumes together.

       B) Would this perform better, worse, or about the same as #1?


Any other configurations that might prove superior?

The final volume will be used as an online backup area, and will contain
a relatively few, large tar files.  Write performance will likely be
more important that read, although I realize using raid5 will impact 
write performance.

Any suggestions on what might be the best stripe size to use?

Thanks in advance for any suggestions you might have.

-Brad

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