On Thursday March 29, [EMAIL PROTECTED] wrote:
> Hi,
>
> I have been doing some performance checks on my RAID 5 system.
Good.
>
> The system is
>
> 5 Seagate Cheetahs X15
> Linux 2.4.2
>
> I am using IOtest 3.0 on /dev/md0
> My chunk size is 1M...
>
> When I do random reads of 64K blobs using one process, I get 100
> reads/sec, which is the same as doing random reads on one disk. So I was
> quite happy with that.
>
> My next test was to do random reads using ten processes, I expected 500
> reads/sec, however, I only got 250 reads/sec.
>
> This to me doesn't seem right??? Does anyone know why this is the
> case?
A few possibilities:
1/ you didn't say how fast your SCSI buss is. I guess if it is
reasonably new it would be at least 80Mb/sec which should allow
500 * 64K/s but it wouldn't have to be too old to not allow that,
and I don't like to assume things that aren't stated.
2/ You could be being slowed down by the stripe cache - it only
allows 256 concurrent 4k access. Try increasing NR_STRIPES at the
top of drivers/md/raid5.c - say to 2048. See if that makes a
difference.
3/ Also, try applying
http://www.cse.unsw.edu.au/~neilb/patches/linux/2.4.3-pre6/patch-F-raid5readbypass
This patch speeds up large sequential reads, at a possible small cost
to random read-modify-writes (I haven't measured any problems, but
haven't had the time to explore the performance thoroughly).
What it does is read directly into the filesystems buffer instead
of into the stripe cache and then memcpy into the filesys buffer.
4/ I'm assuming you are doing direct IO to /dev/md0.
Try making a mounting a filesystem of /dev/md0 first. This will
switch the device blocksize to 4K (if you have a 4k block size
filesystem). The larger block size improves performance
substantially. I always do I/O tests to a filesystem, not to the
block device, because it makes a difference and it is a filesystem
that I want to use (though I realise that you may not).
NeilBrown
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