NiftyFedora Mitch wrote:
On Mon, Jan 19, 2009 at 12:10 AM, <thomas62186218@xxxxxxx> wrote:
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?
Read bandwidth will always be quicker than writes with parity.
Data and parity both need to be written in an atomic way so the completion
will be gated by the last write to be "posted as done" back to the
system.
It can pay to locate the journal on a physically different, smaller
and faster resource.
One informative experiment might be to mount the file system as ext2 and compare
and contrast with the same FS mounted as ext3. I am not recommending ext2
over ext3 other than as an experiment to see what the impact of the
journal activity is...
Also see noatime, relatime, etc.
--
Bill Davidsen <davidsen@xxxxxxx>
"Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark
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