[dm-devel] Latest dm-userspace kernel code

[Dan Smith]

FT> I think that it would be better for the kerne module to provide
FT> only generic mechanisms.

I definitely see the value in removing the copy functionality from the
kernel pieces, I just want to make sure that performance will not
suffer. 

>> In order for this to perform well, we will need to use AIO, right?
FT> I'm not sure about this. Can you elaborate?

Well, assuming that a copy operation (and an fsync() to make sure it
is on-disk) is a non-trivial operation, it would be good to allow the
copy to happen in the background while still being able to service
other requests from the kernel.

In cowd, I need to write metadata when blocks are mapped, and fsync()
them to make sure they are on disk before I allow the IO to complete.
If I do this with a normal write()/fsync() (or open the file O_SYNC),
then cowd is blocked until the write is complete.  This means that
other requests that are in the queue, which may not require a copy,
are also blocked unnecessarily.  I assume that unless copy operations
are performed asynchronously (as they are now by letting the kernel do
it), the problem will get even worse.

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