qcow2 slowdown in kvm-78,79

qcow2 slowdown in kvm-78,79

[Henrik Holst]

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)

Replacing qemu with the one bundled in kvm-77 solves this regression.

I have tested all the cache= options to -drive without success.
cache=writeback and cache=off was a little bit better than the default
cache=writethrough but not with any useful improvement.

/Henrik Holst
https://Witsbits.com - Utility Computing

Re: qcow2 slowdown in kvm-78,79

[Thomas Mueller]

On Tue, 18 Nov 2008 15:39:52 +0100, Henrik Holst wrote:

> 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)

so i'm not the only one facing this problem. 

- Thomas

Re: qcow2 slowdown in kvm-78,79

[Ryan Harper]

* Henrik Holst <henrik@witsbits.com> [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?

what's your -drive parameters look like, specifically, what if type?
ide, scsi, virtio?


-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@us.ibm.com

Re: qcow2 slowdown in kvm-78,79

[Anthony Liguori]

Ryan Harper wrote:
> * Henrik Holst <henrik@witsbits.com> [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?
>
> what's your -drive parameters look like, specifically, what if type?
> ide, scsi, virtio?
>   

This is probably the change to cache=writethrough.  I bet if you set 
cache=writeback then you'll see this go away.

qcow needs some love.

Regards,

Anthony Liguori

Re: qcow2 slowdown in kvm-78,79

[Thomas Mueller]

On Wed, 19 Nov 2008 12:26:41 -0600, Anthony Liguori wrote:

> Ryan Harper wrote:
>> * Henrik Holst <henrik@witsbits.com> [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?
>>
>> what's your -drive parameters look like, specifically, what if type?
>> ide, scsi, virtio?
>>   
>>   
> This is probably the change to cache=writethrough.  I bet if you set
> cache=writeback then you'll see this go away.

this was it. cache=writeback and the throughput goes from 10mb/s to 80/mb/
s with if=scsi

thanks.

- Thomas

Re: qcow2 slowdown in kvm-78,79

[Anthony Liguori]

Thomas Mueller wrote:
> On Wed, 19 Nov 2008 12:26:41 -0600, Anthony Liguori wrote:
>   
>>
>> This is probably the change to cache=writethrough.  I bet if you set
>> cache=writeback then you'll see this go away.
>>     
>
> this was it. cache=writeback and the throughput goes from 10mb/s to 80/mb/
> s with if=scsi
>   

Yeah, we'll most likely switch back to cache=writeback for the default 
for qcow2 images.  qcow2 needs some lovin'.

Regards,

Anthony Liguori

> thanks.
>
> - Thomas
>
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>   

Re: qcow2 slowdown in kvm-78,79

[walt]

Anthony Liguori wrote:
> Ryan Harper wrote:
>> * Henrik Holst <henrik@witsbits.com> [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?
>>
>> what's your -drive parameters look like, specifically, what if type?
>> ide, scsi, virtio?
>
> This is probably the change to cache=writethrough. I bet if you set
> cache=writeback then you'll see this go away.

Would this affect 'qemu-img commit'?  It took over an hour to commit
a fresh install of Vista on a qcow2 virtual disk. (The base image
expanded from 3GB to 11GB, which is a lot of committing, though.)

Re: qcow2 slowdown in kvm-78,79

[Anthony Liguori]

>
> Would this affect 'qemu-img commit'?  It took over an hour to commit
> a fresh install of Vista on a qcow2 virtual disk. (The base image
> expanded from 3GB to 11GB, which is a lot of committing, though.)

Yes.

Regards,

Anthony Liguori

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> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
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Re: qcow2 slowdown in kvm-78,79

[Henrik Holst]

ons 2008-11-19 klockan 12:26 -0600 skrev Anthony Liguori:
> Ryan Harper wrote:
> > * Henrik Holst <henrik@witsbits.com> [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

Re: qcow2 slowdown in kvm-78,79

[Henrik Holst]

> > 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.

ignore that remark, I retested with cache=writeback and everything is OK
again. I must have misspelled somehow and qemu didn't quit on me or
there was something else going on at the time of my tests.

Thanks!

/Henrik Holst
https://witsbits.com