Xen users - Mar 2008 - different read i/o performance for equal guests

Hello,

I''m using Xen 3.0 in a Debian Linux Etch / Dell PowerEdge 860 / 4GB
RAM / Pentium 4 Dual Core 3Ghz. The machine is using a RAID Controller
SAS 5iR, configured with two 500GB disks in RAID-1 (mirroring). I was
getting I/O throughput problems, but then I''ve searched the Internet
and find a solution saying that I needed to enable the write cache on
the RAID controller. Well, my I/O problems have gone in the Dom0 and
some DomU, but in one DomU I still have a low read I/O. The write it''s
ok. I have Dom0 with almost 512MB RAM, two DomU with 1GB RAM and two
DomU with 768MB RAM. All of them are Debian Linux Etch, with the
kernel provided by Xen, I didn''t do any modifications. Here''s
xm list
output:

Name                                      ID Mem(MiB) VCPUs State   Time(s)
Domain-0                                 0      467          2
r-----    81656.7
DomU-1                                   9     1024         2
-b----    5554.6
DomU-2                                   2     1024         2
-b----    1068746.3
DomU-3                                   8      768          2
-b----    64660.8
DomU-4                                   3      768          2
-b----    120196.4

To test the I/O I''m using the tool iozone. I''ve ran
"iozone -i 0 -i 1
-o -s 40m" in Dom0 and the results are:

KB        reclen   write    rewrite     read       reread
40960       4       5849   20200   1857762  1877786

Then in the DomU-2:

 KB       reclen   write    rewrite    read      reread
40960       4       4159   16464  1744170  1821813

Besides the lost performance, the results seem acceptable.

Now in DomU-1:

  KB      reclen   write    rewrite    read      reread
40960       4       4543   16265   709607   708969

The writes performance seems ok, but the reads are much worse than
DomU-2. The test results on DomU-3, DomU-4 are similar to DomU-2. So,
DomU-1 its the only that have poor read i/o.

The load averages are:
Dom0: load average: 0.00, 0.00, 0.00
DomU-1: load average: 0.00, 0.01, 0.00
DomU-2: load average: 0.10, 0.13, 0.13
DomU-3: load average: 0.02, 0.12, 0.12
DomU-4: load average: 0.00, 0.01, 0.00

So, the worse i/o performance of DomU-1 doesn''t seem to be a
consequence of the machine load.

As I said, all the DomU''s had the same installation and have the same
kernel (2.6.18-5-xen-686 with SMP).

Anyone have any ideia why the DomU-1 have poor read i/o performance?
Thanks,
Pedro Simões

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