Disk IO issues

[Xavier Lamien]

On Wed, Dec 31, 2008 at 9:42 PM, Mike McGrath <mmcgrath at redhat.com> wrote:
> Lets pool some knowledge together because at this point, I'm missing
> something.
>
> I've been doing all measurements with sar as bonnie, etc, causes builds to
> timeout.
>
> Problem: We're seeing slower then normal disk IO.  At least I think we
> are.  This is a PERC5/E and MD1000 array.
>
> When I try to do a normal copy "cp -adv /mnt/koji/packages /tmp/" I get
> around 4-6MBytes/s
>
> When I do a cp of a large file "cp /mnt/koji/out /tmp/" I get
> 30-40MBytes/s.
>
> Then I "dd if=/dev/sde of=/dev/null" I get around 60-70 MBytes/s read.
>
> If I "cat /dev/sde > /dev/null" I get between 225-300MBytes/s read.
>
> The above tests are pretty consistent.  /dev/sde is a raid5 array,
> hardware raid.
>
> So my question here is, wtf?  I've been working to do a backup which I
> would think would either cause network utilization to max out, or disk io
> to max out.  I'm not seeing either.  Sar says the disks are 100% utilized
> but I can cause major increases in actual disk reads and writes by just
> running additional commands.  Also, if the disks were 100% utilized I'd
> expect we would see lots more iowait.  We're not though, iowait on the box
> is only %0.06 today.
>
> So, long story short, we're seeing much better performance when just
> reading or writing lots of data (though dd is many times slower then cat).
> But with our real-world traffic, we're just seeing crappy crappy IO.
>
> Thoughts, theories or opinions?  Some of the sysadmin noc guys have access
> to run diagnostic commands, if you want more info about a setting, let me
> know.
>
> I should also mention there's lots going on with this box, for example its
> hardware raid, lvm and I've got xen running on it (though the tests above
> were not in a xen guest).
>

Could you perform an hdparm -tT on that disk ?
Also, output an strace against your cat & dd commands.

if my memory is good enough, cat use mmap() which is faster than
read() (which is used by dd)

-- 
Xavier.t Lamien
--