qemu-xen qdisk performance

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?

Thanks, Roger.