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[dm-devel] [RFC] dm-userspace

Hi List,

As you all know, I'm working on a userspace-controlled CoW
implementation using device-mapper.  The idea is to offer a pseudo
device through device-mapper that has CoW behavior, but where the
block allocation decisions are made from userspace.

My thoughts are that it might be best to abandon the concept of the
"dm-cow" target and instead work on a "dm-userspace" target.  The
userspace cow application I'm working on would remain mostly the
same.  Similarly, dm-userspace would look almost identical to my
dm-cow does right now, but more generic.  The target would simply
present the details of the data passed to the map() function to
userspace, which would respond with a target device and sector of
where to send the request.

A generic dm-userspace target would allow for testing of new
algorithms (RAID, CoW, etc) from a userspace application, as well as
some more interesting things involving distributed applications.  Just
like FUSE allows for some neat (although not necessarily
high-performance) tricks, dm-userspace could allow the same thing for
block devices.

Would the device-mapper maintainers be interested in accepting
something like dm-userspace upstream?

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

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