ons 2008-11-19 klockan 12:26 -0600 skrev Anthony Liguori:
> Ryan Harper wrote:
> > * Henrik Holst <[EMAIL PROTECTED]> [2008-11-18 08:42]:
> >
> >> The bundled qemu in kvm-78 and kvm-79 slows down disk i/o with qcow2
> >> images by an order of 10. If one got 60MB/s before, one gets around
> >> 6MB/s with 78 and 79 (measured with dd)
> >>
> >
> > dd command?
dd if=/dev/zero of=/mnt/zero count=102400000 &
and then "kill -USR1 <pid>" to get the statistics
> >
> > what's your -drive parameters look like, specifically, what if type?
> > ide, scsi, virtio?
It is the same regardless of ide,scsi or virtio. Virtio starts quite
high around 70MB/s but quickly goes down to 6MB/s (after roughly 10
seconds).
The command line (for virtio) is:
qemu-system-x86_64 -M pc -m 771 -smp 1 -vnc
:0,tls,x509verify=/opt/kvm/cert \
-k en-us -monitor pty -serial none -usbdevice tablet \
-drive file=/mnt/d0/drive.root,if=virtio,media=disk,boot=on \
-drive file=/mnt/d0/drive2,if=virtio,media=disk,boot=off \
-net nic,macaddr=DE:AF:00:00:10:43,model=virtio \
-net tap,script=/opt/kvm/share/qemu-ifup \
-uuid 6b7d5438-1309-4cf0-ae08-57234d61d473
I have tested with all kinds of -m settings as well (up to 8GiB) without
any change.
> >
>
> This is probably the change to cache=writethrough. I bet if you set
> cache=writeback then you'll see this go away.
cache=writeback is only slightly faster, like going from 6MB/s to 7MB/s.
The only thing that works is using qemu userland from kvm-77. I have not
tested this with anything other than qcow2 so I'm guessing that it is
related to qcow2 due to the cache change in kvm-78 and your other
comments about qcow2 and these changes when issues about degraded qcow2
performance has come up.
/Henrik Holst
https://witsbits.com
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