> Something less than a dozen is a small number.  With 3, every write is
> going to 
> require every disk head to seek and you'll wait until the slowest one
> completes 
> before the next operation.

Not to be contrary, but I'm running just fine on a 4-spindle MD-raid5
system under RHEL5 for by backuppc server. The array has 4 400GB WD SATA
disks. I think it helps to tune the array slightly:

     Chunk Size : 256K

Also, make sure native command queueing is enabled and working (hdparm -I
/dev/sdx)

BackupPC_nightly runs in 15 minutes across a pool like:

# Pool is 345.69GB comprising 1212100 files and 4369 directories (as of
5/12 02:07),
# Pool hashing gives 8331 repeated files with longest chain 192,
# Nightly cleanup removed 380 files of size 0.19GB (around 5/12 02:07),
# Pool file system was recently at 37% (5/12 09:54), today's max is 37%
(5/12 02:00) and yesterday's max was 37%.

Also, read performance shouldn't be a problem on a raid-5, but you can
increase the per-spindle read-ahead using

   blockdev --setra 4096 /dev/sdx

This will read-ahead 4096*512k blocks (2MB) from each spindle. I haven't
set this on this machine but I do on others. This is a per-boot setting so
do it in rc.local or equiv.

-Josh

-- 
--------------------------------------------------------
       Joshua Malone       Systems Administrator
     (jmalone AT nrao DOT edu)    NRAO Charlottesville
        434-296-0263         www.cv.nrao.edu
        434-249-5699 (mobile)
BOFH excuse #360:

Your parity check is overdrawn and you're out of cache.
--------------------------------------------------------

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