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Re: [Xen-devel] COW in Xen

Marcel Ritter wrote:

Rendon, Curtis schrieb:

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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