Re: [Libguestfs] More parallelism in VDDK driver

Correct.  Using -W doesn't make sense without -m (if you only have one worker, you might as well proceed linearly than trying to randomize access, but even when you have multiple threads, there are cases where linear operations are still useful, such as 'nbdkit streaming'.  But -m is definitely the knob that controls how many outstanding I/O requests qemu-img is willing to use; and once you are using -m, using -W makes life easier for those coroutines to stay active.  The default -m1 says that at most one request is outstanding, so parallelism in the server is not utilized.  With higher -m, qemu-img issues up to that many requests without waiting for server answers, but all on the same NBD connection. Ideally, you'll get the maximum behavior as 'qemu-img -m' and 'nbdkit --threads' choose the same values; if either side has fewer in-flight operations permitted than the other, then that side has the potential to become a bottleneck.  Right now, nbdkit defaults to 16 threads (that is, up to 16 in-flight operations) for any PARALLEL plugin.