large overhead with blktap2/vhd

large overhead with blktap2/vhd

[Bastian de Groot]

Hi everybody,

I am searching for a copy-on-write solution for Xen and tested blktap2 with 
the vhd backend.

My Test: 

I've created a master-image with "dd" and installed WindowsXP on the image. 
Then I used vhd-util to create a copy-on-write-image for the master-image. 

After booting the VM and logging in the, the cow-image had a size of 260 MB. 
That's a little bit weird, because I didn't really changend any data.

Afterwards I downloaded eclipse in the VM, to see how big the overhead is. The 
eclipse zip-file is 190.64 MB big. But the cow-image had a size of 1468.29 MB 
after downloading eclipse. That's an overhead of 670 %.

Then I unpacked eclipse. So that the actual change in the VM had 425.37 MB. At 
that moment the cow-image was 1979.79 MB big. The result is 365 % Overhead, 
that's better then the result before, but still not very good.

My questions are:
Have I done anything wrong at measuring the overhead?
Is there a bug in blktap2/vhd?
Or is this a usual result?

Here the configuration:
The host is a Xen 4.0 (I compiled it myself) on an 2.6.32-xen-amd64 debian 
kernel  (also self compiled to include some mudules statically). The guest is 
a WindowsXP 32 bit SP2.

I would appreciate your help on that very much.
Bastian

Re: large overhead with blktap2/vhd

[Christian Samsel]

Am Mittwoch, 23. Juni 2010, 17:26:05 schrieb Bastian de Groot:
> Hi everybody,
> 
> I am searching for a copy-on-write solution for Xen and tested blktap2 with
> the vhd backend.
> 
> My Test:
> 
> I've created a master-image with "dd" and installed WindowsXP on the image.
> Then I used vhd-util to create a copy-on-write-image for the master-image.
> 
> After booting the VM and logging in the, the cow-image had a size of 260
> MB. That's a little bit weird, because I didn't really changend any data.
Just one word: swap.

Christian

Re: large overhead with blktap2/vhd

[Bastian de Groot]

Am Donnerstag 24 Juni 2010 10:41:30 schrieben Sie:
> Am Mittwoch, 23. Juni 2010, 17:26:05 schrieb Bastian de Groot:
> > Hi everybody,
> >
> > I am searching for a copy-on-write solution for Xen and tested blktap2
> > with the vhd backend.
> >
> > My Test:
> >
> > I've created a master-image with "dd" and installed WindowsXP on the
> > image. Then I used vhd-util to create a copy-on-write-image for the
> > master-image.
> >
> > After booting the VM and logging in the, the cow-image had a size of 260
> > MB. That's a little bit weird, because I didn't really changend any data.
>
> Just one word: swap.
>
> Christian

Good guess! I've tested it with deactivated swap in WindowsXP, but it didn't 
have any effect. The results were the same.

Bastian