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[Xen-users] Massive iowait with Xen 3.2

To: Xen List <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-users] Massive iowait with Xen 3.2
From: Antoine Benkemoun <antoine.benkemoun@xxxxxxxxx>
Date: Tue, 28 Apr 2009 09:39:41 +0200
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I have gotten my new Xen server working almost perfectly, yes... almost. The server in question is based on Debian Lenny and Xen has been installed from the Debian repos. It works perfectly fine with about 8 domains (it's getting a little cramped on 2GB of RAM but works fine).

The last problem is that when a domain starts to generate some I/O, there is massive iowait. The load that is put on the domain in question is Bit Torrent at approx. 40 Mbits split amongst 6 torrents. Nothing really big it seems. I installed Munin to have a few graphs and be able to see a pattern or something. Here is the graph during the torrenting :


When the torrents stop progessively, the iowait stays the same until all torrents are finished then it goes right back up. I would expect it to change gradually but it doesn't... You can see this with in the following graph :


When I execute the command iostat -x 1 during the high load in question (on the dom0) I get the following :

%user %nice %system %iowait  %steal   %idle
   0,00    0,00     0,00         99,01    0,99    0,00

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda              35,64     0,00      183,17 0,00  7968,32   0,00    43,50    12,02        48,99   5,43    99,41

Obviously during the process, everything becomes quite slow... The domain in question is the only domain really doing something, the others are just idling.

Do you have any idea what is wrong with my setup ?

Thank you in advance for your help,

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