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[Xen-users] Poor disk performance in xen 3.1

Hi List,

I`m getting extremely poor disk performance with xen 3.1, here are some 
results from hdparam -tT /dev/sda in Dom0:

20 Vms running (for few weeks):
 Timing cached reads:   1472 MB in  2.00 seconds = 735.98 MB/sec
 Timing buffered disk reads:   28 MB in  3.24 seconds =   8.65 MB/sec

All Vms stopped, xend stopped:
 Timing cached reads:   1872 MB in  2.00 seconds = 936.35 MB/sec
 Timing buffered disk reads:   38 MB in  3.47 seconds =  10.94 MB/sec

after reboot (same kernel, no VMs started, xend running)
 Timing cached reads:   1804 MB in  2.00 seconds = 902.54 MB/sec
 Timing buffered disk reads:  216 MB in  3.01 seconds =  71.82 MB/sec

after reboot (non-xen kernel)
 Timing cached reads:   1810 MB in  2.00 seconds = 905.01 MB/sec
 Timing buffered disk reads:  216 MB in  3.04 seconds =  71.08 MB/sec

after reboot in xen kernel +  VMs start
 Timing cached reads:   1876 MB in  2.00 seconds = 937.98 MB/sec
 Timing buffered disk reads:   64 MB in  3.04 seconds =  21.04 MB/sec

After VM stop and xend stop
 Timing cached reads:   1900 MB in  2.00 seconds = 950.28 MB/sec
 Timing buffered disk reads:   60 MB in  3.09 seconds =  19.40 MB/sec

After 12 hours work of the VMs:
Timing cached reads:   1094 MB in  2.00 seconds = 546.26 MB/sec
Timing buffered disk reads:   68 MB in  3.64 seconds =  18.70 MB/sec

I could live with the poor performance in tests when VMs are running as 20 VMs 
could make a lot of I/O, but whats with the performance when I stop all 
DomUs? I get ~1/3 of the performance I get before starting DomUs and there 
are no other services running on Dom0 that could make I/O.

Has anyone had such issues, what might cause this?

The server is:
2x xeon @2.8
6GB ECC ram
4x 500GB sata disks on a hpt 2210 raid controller in raid 5.
Xen 3.1 built from source.
Ubuntu feisty in dom0.

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