Re: COW in Xen

[Julian Chesterfield <julian@xensource.com>]

Marcel Ritter wrote:

>Rendon, Curtis schrieb:
>  
>
>>Hello,
>>
>>I had posted this question in Xen-user mailing list three 
>>days ago and I did not
>>get a single answer. On a second thought, maybe xen-devel is more
>>appropriate so
>>I am trying again in the hope that I will receive an answer:
>>
>>Does anybody have an experience (good or bad) with using
>>COW (Copy-On-Write) storage in Xen ?
>>  
>>
>>I've been using qcow images generated by qemu-img for Linux and
>>MSWIndows HVM guests, and I've been getting good results. The image
>>files are much smaller that the equivalently sized raw images.
>>
>>I simply create them (via gemi-img create -f qcom 10G) and use the
>>disk=['file:...'] line in my guest configuration files.
>>  
>>    
>>
>If I got Mark right, it's not the image type qcow itself, he's
>interested in,
>but the COW (Copy-on-write) feature. Some time ago I tried to get
>COW working with qcow images, without success. So I guess the name
>is missleading, and (in Xen) there's no COW based on qcow.
>  
>
Not true. CoW functionality is supported in the current blktap driver. 
If you create a qcow disk on the host using qcow-create with a backing 
filename argument (can be either a raw image or block device) the driver 
opens the backing file for reading-only and directs all writes to the 
qcow child image.

- Julian