Re: [Xen-devel] COW in Xen

[Julian Chesterfield <julian@xxxxxxxxxxxxx>]

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


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