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Re: [dm-devel] dm-userspace (no in-kernel cache version)

FT> I'm not sure how this works. Have you explained the details of
FT> this feature in the earlier thread?

For example, in our CoW case:

A write is made to a previously unremapped block.  We queue that block
and let userspace make a mapping.  Then, we allow the write to happen,
but intercept the completion (endio) and notify userspace that it has
completed.  Then, userspace flushes its metadata to disk and notifies
the kernel to allow the endio to complete.  Without this, a crash
could result in an inconsistent CoW device.  (Note this only needs to
happen on the _initial_ write to a block)

FT> I see. I don't think that you need to implement it now (simple
FT> code is always better for mainline inclusion), however, it would
FT> be nice to have dmu_event structure for this.

Well, it's crucial for our CoW work, so unless Alasdair would prefer
me to leave it out in the initial upstream push, I would like to
include it.  It was already implemented and working in my original

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

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