I tested RAID 5 and RAID 6 with 12 x 15K SAS drives on Ubuntu 8.04
64-bit and found their performance to be about the same. I used 256 K
chunk size, v1.0 superblocks, stripecachesize of 16384, and readahead
of 65536.
RAID 5 reads: 774 MB/sec
RAID 5 writes: 585 MB/sec
RAID 6 reads: 742 MB/sec
RAID 6 writes: 559 MB/sec
My CPU utilization remains under 10% though during writes, and I'm
wondering what can be done to get write performance closer to read
performance. I have dual quad-core CPUs so there's plenty of CPU to go
around. Any ideas on that front?
-Thomas
-----Original Message-----
From: Peter Rabbitson <rabbit+list@xxxxxxxxx>
To: linux-raid@xxxxxxxxxxxxxxx
Sent: Sun, 18 Jan 2009 11:40 pm
Subject: Raid6 write performance
Hi,
I am experimenting with raid6 on 4 drives on 2.6.27.11. The problem I am
having is that no matter what chunk size I use, the write benchmark
always comes out at single drive speed, although I should be seeing
double drive speed (read speed is at near 4x as expected). Is there some
hidden setting that I am overlooking, or is this a current known
limitation of raid6? In contrast if I make a raid5 on these 4 drives, I
get the expected 3xdrive write speed, and occasionally 4xdrive linear
read speed.
When the write test is running, I get about 14% of system cpu a sporadic
40% of iowait and the rest idle at all times (machine is in runlevel 1
so not to screw with results). Anyone has any ideas?
Thank you.
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