Re: [Xen-devel] qemu-xen qdisk performance

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 10 Feb 2012, Roger Pau Monnà 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.
I wouldn't expect it to hang completely though, that might be a bug.

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