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Re: [Xen-devel] COW in Xen

  • To: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
  • From: "Mark Ryden" <markryde@xxxxxxxxx>
  • Date: Tue, 6 Mar 2007 13:23:42 +0200
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Rendon, Curtis" <Curtis.Rendon@xxxxxxx>, Marcel Ritter <Marcel.Ritter@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Tue, 06 Mar 2007 03:22:49 -0800
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First , thanks , Marcel

If I got Mark right, it's not the image type qcow itself, he's
interested in,
but the COW (Copy-on-write) feature.

 You did indeed got me right; I am not talking specifically about
some format like qcow  itself but my question is general. As I said
before, it seems that there are many
Copy-on-Write solutions; at least according to the Xen wiki page on COW
For example, Xen blktap, DmUserspace ,UnionFS, BenRin's Copy-on-Write
md device,Translucency, parallax, CowNFS , and LVM snapshots (and yet
I don't know yet whether MarkW XenFS should support cow).

I am not an expert on CoE. As I understand, there are two main
categories for Copy On Write solutions: file based solutions and block
based solutions.

It's obvious to me that there are some pros and cons for each method.
It seems to me that no doubt that when using a solution which should
patch the kernel
this can be thought of as a drawback by many. Also when the solution
is tightly coupled to a filesystem (like unionFS or CoWNFS) this maybe
can seem a drawback. (conmparing to block device solutions). So if
somebody can give a short analyze of pros and cons of the different
CoW techniques available for Xen
and the roadmap ahead this can make things better understood.

On 3/6/07, Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx> wrote:
> > 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.
> I'd really like to get a simple file based COW - is this planned for
> future Xen releases? Maybe someone can comment?

AFAIK it works in xen-unstable, so will be in 3.0.5.


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