Xen devel - Aug 2006 - [PATCH 0 of 6] dm-userspace xen integration patches

Here are our latest dm-userspace-xen integration patches.  A few things have
changed since our last patch:  synchronous metadata flushing and block-dmu
script updates to avoid xend races, a makefile change to ensure we check for
autotools, and a change to the dscow creation tool to allow for large dscow
files.

I''d like to point out that having dm-userspace integrated into xen
allows a
user to quickly and easily specify a base device and where they''d like
to store
the changes.  All they have to do is change one line in their xm config file,
and they''re good to go.

Our main focus here is really block devices, such as LVMS, nbd, iSCSI.  We do
support file-backed images as an experimental feature and encourage using it
with care since there are some potential issues with the current loop driver.

Comments are much appreciated.

--
Thanks,
Ryan Grimm
IBM Linux Technology Center

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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.

-- 
Dan Smith
IBM Linux Technology Center
Open Hypervisor Team
email: danms@us.ibm.com


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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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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@us.ibm.com


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