[Libguestfs] [PATCH nbdkit 0/5 ] vddk: Implement parallel thread model

[Richard W.M. Jones]

NOTE: This is not complete, I'm just posting it for early review.  In
particular I need to test and benchmark this a lot more to make sure
it really is a performance improvement.

At the moment nbdkit-vddk-plugin suffers because it works
synchronously.  We found in testing that copying multiple VMs in
parallel is much faster than copying a single VM, the obvious reason
being that each single nbdkit spends a lot of time waiting on round
trips to the VMware server.

Since VDDK 6.0 (but only implemented in VDDK 6.7+) the library has
offered a form of asynchronous read and write operation.  We can use
that to implement the parallel thread model, hopefully allowing
multiple outstanding read/write commands to overlap in flight.

One thing of note is that multi-conn is probably not possible without
a lot more work.  The reason is that VDDK locks the backing disk when
you open a handle, so multiple connections (ie. handles) cannot be
opened.  I am also not clear if VDDK/VMware offers the correct data
integrity guarantees to allow us to implement multi-conn, and since
the whole lot of closed source there's no easy way to find out.

Rich.