Re: qemu-xen qdisk performance

["Roger Pau Monné" <roger.pau@entel.upc.edu>]

2012/2/10 Stefano Stabellini <stefano.stabellini@eu.citrix.com>:
> On Fri, 10 Feb 2012, Roger Pau Monné wrote:
>> Hello,
>>
>> I've recently setup a Linux Dom0 with a 3.0.17 kernel and Xen 4.1.2,
>> and since the 3.x series doesn't have blktap support I'm using qdisk
>> to attach raw images. I've been playing with small images, something
>> like 1GB, and everything seemed fine, speed was not fantastic but it
>> was ok. Today I've set up a bigger machine, with a 20GB raw hdd and
>> the disk write throughput is really slow, inferior to 0.5MB/s. I'm
>> trying to install a Debian PV there, and after more than 3 hours it is
>> still installing the base system.
>>
>> I've looked at the xenstore backend entries, and everything looks fine:
>>
>> /local/domain/0/backend/qdisk/21/51712/frontend =
>> "/local/domain/21/device/vbd/51712"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/params =
>> "aio:/hdd/vm/servlet/servlet.img"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/frontend-id = "21"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/online = "1"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/removable = "0"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/bootable = "1"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/state = "4"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/dev = "xvda"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/type = "tap"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/mode = "w"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/feature-barrier = "1"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/info = "0"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/sector-size = "512"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/sectors = "40960000"   (n0,r21)
>> /local/domain/0/backend/qdisk/21/51712/hotplug-status = "connected"   (n0,r21)
>>
>> Also, the related qemu-dm process doesn't seem to be hung by CPU, in
>> fact it is reporting a CPU usage of 0% almost all the time. I've
>> attached to the qemu-dm process with strace, and it is doing lseeks
>> and writes like crazy, is this normal? Is there any improvement when
>> using qemu-upstream?
>
> Yes, great improvements.
> The old qemu-xen uses threads to simulate async IO so it is very slow;
> upstream QEMU uses Linux AIO and is much faster.

That's great news, so qdisk performance in qemu-upstream should be
similar to blktap?

> I wouldn't expect it to hang completely though, that might be a bug.

No, it doesn't hang completely, just very slow.

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