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@xxxxxxxxxx
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