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Re: [Xen-users] Massive iowait with Xen 3.2

On Tue, Apr 28, 2009 at 6:51 PM, Antoine Benkemoun
<antoine.benkemoun@xxxxxxxxx> wrote:
> The dom0 is a pretty minimal install of Debian with nothing else then Xen,
> SSH and fail2ban running. It can easily fit on 256MB of RAM. In fact, it
> averagely uses 100MB of RAM.

Hmm, interesting. I should test Debian or Ubuntu for firewall domUs sometimes.

> I have reproduced the problem by generating IOs with 3 torrents and 1 file
> copy.
> XM top shows :
>     commun ------       2959    3.5     262144   12.7     262144
> 12.7     1    1  1828352  1567395    3    16267  3411443  1848929   32
>   Domain-0 -----r       2617    2.7     262240   12.7   no limit
> n/a     1    0        0        0    0        0        0        0   32

Since you ommit the headers, the output is quite confusing :P
What are vbd stats for the domU, does it have lots of reads/writes?

> Load average for "commun", the domain being loaded, is above 5. Memory is
> only used at about 50% and only 440k of swap is used. Along goes the usual
> symptoms, really slow web page loads and really slow reactivity of about
> everything in the domain. Reactivity in the other domains is rather normal.

So it's not swap problem. However looking at your previous iostat
output %util is over 99 (which means your disk is busy), r/s over 180,
and rsec/s almost 8000 (meaning about 4 MB/s). Based on this numbers
I'm GUESSING you only have one lowend disk (not raid), and it has
reached its IOPS caps. In other words, get more disks :P

A suggestion, it would probably be easier to manage if you use LVM and
put dom0 and domU each on its own LV. That way iostat can show per-LV
staticstic so you can easily determine who's using most disk I/O.



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