Re: [RFC] dm-userspace

[Dan Smith <danms@us.ibm.com>]

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EVH> It seems like this would be really useful for prototyping and
EVH> debugging new device mapper modules (like the dm-cache ideas I
EVH> posted about a few weeks back).

Yes, I think that is a major benefit to going with the dm-userspace
idea over the cow-specific dm-cow.

EVH> 1) how would you handle permissions?  IIRC FUSE allows normal
EVH> users to bind their own FUSE userspace file systems, would
EVH> something similar happen for dm-userspace or would even binding a
EVH> userspace device-mapper module require root?

Hmm, I think that due to the way device-mapper works, this would be
difficult.  Without the ability to create a pseudo-device with a
dm-userspace target, I think you'd be out of luck.

EVH> 2) How close would the userspace API be to the kernel
EVH> device-mapper API?  It'd be nice to have something close so that
EVH> userspace code could easily be migrated into the kernel (for
EVH> performance reasons) as appropriate.

Well, currently I pass basically the same information to userspace.
You get the location of the access and whether it was a read or a
write.  The userspace module passes back a destination location,
device, and whether or not to copy the area from a source location
(which gives you an interface to kcopyd).

I think that we could easily add a layer to be able to run simple
device-mapper modules in userspace with it, similar to how nfsim
works, which may be very useful to people trying to write new
device-mapper targets.  What are people's thoughts on this?

Something I should mention here: to simplify things and reduce
communication, the current module blocks contiguous regions of the
disk together so that you can talk about whole chunks at a time,
instead of each individual bio request, which may be of varying size
and location.  Block sizes can be no smaller than 512 bytes.  I think
that most device-mapper work will be dealing with fixed blocks of some
size, so this shouldn't be a problem.

EVH> 3) When do you think you'll be able to post a patch for RFC?

I'm currently just cleaning some things up at the moment.  I would be
glad to post a patch and a sample userspace app if people would be
willing to take a look at it.

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

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