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[Xen-users] Domain U very slow performance on disk write

Hi everyone.

I have an Ubuntu 10.04 domain U that has very poor performance on disk
write, i.e.:

domain 0:

time sh -c "dd if=/dev/zero of=bigfile bs=8k count=1000 && sync"

real    0m0.870s
user    0m0.000s
sys     0m0.012s

domain U:

time sh -c "dd if=/dev/zero of=bigfile bs=8k count=1000 && sync"

real    0m38.944s
user    0m0.004s
sys     0m0.020s

i.e. 38s vs 0.87s

Of course, all the guest becomes unusable.

The backend device is tap2:aio managing a raw img file.

The domain 0 is ubuntu 12.04 (development branch) with 3.2.0-8 kernel but I
also tried with ubuntu 10.04 

The issue happens both in HVM and in PV guest mode

While in PV mode I tried (on the PV) the following kernels:

but I had no luck

I also tried disabling irqbalance on domU, setting elevator = noop on both
dom0 and domU, settings the raid speed limit from 200000 to 50000, lowering
the vcpus from 4 to 1, setting dedicated cpu to dom0

The issue is still there.

Any ideas?

Thanks to all.

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