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Re: [dm-devel] [RFC] dm-userspace

just curious, will the speed be a problem here? considering each time it
needs to contact user space for mapping a piece of data. and the size
unit is per sector in dm?

do u have any benchmark results about overhead?


On Wed, 2006-04-26 at 15:45 -0700, Dan Smith wrote:
> Xen needs to be able to directly access disk formats such as QEMU's
> qcow, VMware's vmdk, and possibly others.  Most of these formats are
> based on copy-on-write ideas, and thus have a base image and a bunch
> of modified blocks stored elsewhere.  Presenting this to a virtual
> machine transparently as a normal block device would be ideal.  The
> solution I propose is to use device-mapper for redirecting block
> accesses to the appropriate locations within either the base image or
> the COW space, with the following constraints:
> 1. The block-allocation algorithm and formatting scheme should not be
>    in the kernel.  This gives the most flexibility and puts the
>    complexity in userspace.
> 2. Actual data flow should happen only in the kernel, and userspace
>    should be able to control it without the blocks being passed back
>    and forth.
> So, I developed a generic device-mapper target called dm-userspace
> which allows a userspace application to control the block mapping in a
> mostly generic way.  With the functionality it provides, I was able to
> write a userspace daemon that handles the mapping of blocks such that
> a qcow file could be presented as a single block device, mounted and
> accessed as if it were a normal disk.  If/when VMware releases their
> vmdk spec under the GPL, adding support for it would be relatively
> simple.  This would give us a unified block device to export to the
> virtual machine, that would be backed by a complex format such as vmdk
> or qcow.
> In addition to providing support for the above scenario, dm-userspace
> could be used for other things as well.  It's possible that new
> device-mapper targets could be developed in userspace using a special
> application that used dm-userspace to simulate the kernel
> environment.  Additionally, filesystem debuggers may be able to use
> dm-userspace to provide interactive control and logging of disk
> writes. 
> A patch against to add dm-userspace to the kernel is
> available here:
>   http://static.danplanet.com/dm-userspace/dmu-
> After you have a patched kernel, you can build the (very tiny) helper
> library and example program, available here:
>   http://static.danplanet.com/dm-userspace/libdmu-0.1.tar.gz
> Comments would be appreciated :)
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