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

[Julian Chesterfield <jac90@xxxxxxxxx>]

Dan,

On 25 Aug 2006, at 23:04, Dan Smith wrote:

> RG> Our main focus here is really block devices, such as LVMS, nbd,
> RG> iSCSI.  We do support file-backed images as an experimental
> RG> feature and encourage using it with care since there are some
> RG> potential issues with the current loop driver.
>
> I think that in the past dm-userspace has been compared to blktap as a
> competing method for providing cow support for image files.  While we
> do support files with the existing loop driver, we are currently
> most concerned with providing cow support against native block
> devices.
Just to clarify, the Blktap device driver supports both block devices 
and image files. In order to create a block device backed CoW image use 
the qcow-create command:
qcow-create 0 <qcow filename> <block device>

when you insert the qcow filename into the config file:

disk = [ 'tap:qcow:<qcow filename>, sda1, w' ]

it will open both the backing device handle and the qcow image. 
Similarly you can open a raw block device without CoW support by 
setting the disk parameter field to 'tap:aio:<block device> etc...'
We've been working on some improvements to the CoW architecture within 
the userspace blktap tools that we will push out to -unstable soon.
- Julian


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