Re: [PATCH] nbd/server: Advertise MULTI_CONN for shared writable exports

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Aug 27, 2021 at 10:09:16AM -0500, Eric Blake wrote:
> +# Parallel client connections are easier with libnbd than qemu-io:
> +if ! (nbdsh --version) >/dev/null 2>&1; then

I'm curious why the parentheses are needed here?

> +export nbd_unix_socket
> +nbdsh -c '
> +import os
> +sock = os.getenv("nbd_unix_socket")
> +h = []
> +
> +for i in range(3):
> +  h.append(nbd.NBD())
> +  h[i].connect_unix(sock)
> +  assert h[i].can_multi_conn()
> +
> +buf1 = h[0].pread(1024 * 1024, 0)
> +if buf1 != b"\x01" * 1024 * 1024:
> +  print("Unexpected initial read")
> +buf2 = b"\x03" * 1024 * 1024
> +h[1].pwrite(buf2, 0)                   #1
> +h[2].flush()
> +buf1 = h[0].pread(1024 * 1024, 0)
> +if buf1 == buf2:
> +  print("Flush appears to be consistent across connections")

The test is ... simple.

Would it be a better test if the statement at line #1 above was
h.aio_pwrite instead, so the operation is only started?  This would
depend on some assumptions inside libnbd: That the h[1] socket is
immediately writable and that libnbd's statement will write before
returning, which are likely to be true, but perhaps you could do
something with parsing libnbd debug statements to check that the state
machine has started the write.

Another idea would be to make the write at line #1 be very large, so
that perhaps the qemu block layer would split it up.  After the flush
you would read the beginning and end bytes of the written part to
approximate a check that the whole write has been flushed so parts of
it are not hanging around in the cache of the first connection.

Anyway the patch as written seems fine to me so:

Reviewed-by: Richard W.M. Jones <rjones@redhat.com>

Rich.

-- 
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