large overhead with blktap2/vhd

[Bastian de Groot <degroot@univention.de>]

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