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PostPosted: Mon Apr 13, 2009 8:01 pm 
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Joined: Sun Sep 25, 2005 3:50 pm
Posts: 1013
Location: Los Angeles
I came across this post on the Gentoo forums and found it helped my software RAID5 write performance. I saw ~100% increase in my write performance over the default settings.

Check your write cache stripe size (Substitute for md0 if your array is something other than /dev/md0):
Code:
# cat /sys/block/md0/md/stripe_cache_size
256


Then try increasing it to 512, 1024, 2048, 2096, 8192:
Code:
# echo 512 > /sys/block/md0/md/stripe_cache_size


cd to your raid array (i.e. cd /path/to/mounted/raid5/array) and then benchmark performance. It's best to do this when your array is otherwise idle. I used an empty 15GB file to test.
Code:
# dd if=/dev/zero of=bigfile bs=1M count=15000 ; rm -f bigfile

and note the MB/s when it finishes. I found that a seting of 8192 was optimal for my array. Anything bigger saw diminishing returns.

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