Re: [Xen-devel] Re: [PATCH 0 of 6] dm-userspace xen integration patches

[Dan Smith <danms@xxxxxxxxxx>]

JC> Just to clarify, the Blktap device driver supports both block
JC> devices and image files.

Sorry, I didn't mean to imply that it only supported files.

JC> In order to create a block device backed CoW image use the
JC> qcow-create command:

JC> qcow-create 0 <qcow filename> <block device>

So, this is still putting the CoW data in a file, but with the base
image data in a block device, right?

JC> Similarly you can open a raw block device without CoW support

Right, but we are interested in CoW between two block devices.  That's
why we have created a special CoW format (qcow wastes a lot of space
to do sparse allocation, which is pointless and slow on a block
device).  We looking to support things like using two iSCSI block
devices to do CoW (i.e. with no files involved).  We are also
interested in doing things that have nothing to do with cow, using
dm-userspace as an enabling component.

-- 
Dan Smith
IBM Linux Technology Center
Open Hypervisor Team
email: danms@xxxxxxxxxx

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