output: Don't use nbdkit-file-plugin cache=none when writing

nbdkit-file-plugin flag cache=none is meant to ensure that the file
plugin does not pollute the page cache.  However it doesn't yet work
very well, especially for writing.  As you can see here:

https://gitlab.com/nbdkit/nbdkit/-/blob/048d5b9818c88355e596824355269773e5c4f6ad/plugins/file/file.c#L594

it is currently implemented by flushing the whole file (including
parallel requests), and then evicting the data from the page cache.
This implementation is naive.  (See nbdcopy copy/file-ops.c for a much
better implementation.)

This causes quite a considerable slow down when writing to a local
file (eg -os local).  The penalty varies between machines and possibly
kernels.  On my Fedora Rawhide AMD server with 12 cores and SSDs, the
slow down is under 5%.  But on my RHEL 9 Intel NUC server with 2 cores
and a hard disk, it makes a really huge difference, nearly doubling
the total time taken.

I left a note in the code that we should fix this in nbdkit and re-add
this option or similar when it is working.

I also removed the fadvise=sequential option.  This is associated with
cache=none in the nbdkit documentation, and anyway we are not doing
sequential writes.  (This makes no measurable difference in my testing)

output/output.ml

@@ -98,8 +98,14 @@ let output_to_local_file ?(changeuid = fun f -> f ())
      let cmd = Nbdkit.add_arg cmd "file" filename in
      let cmd =
        if Nbdkit.version nbdkit_config >= (1, 22, 0) then (
-         let cmd = Nbdkit.add_arg cmd "fadvise" "sequential" in
-         let cmd = Nbdkit.add_arg cmd "cache" "none" in
+         (* nbdkit 1.28 has a very naive implementation of
+          * page cache eviction.  We need to copy the one from
+          * nbdcopy copy/file-ops.c.  Until then do not use
+          * this flag because it causes a large slow down on
+          * some machines. XXX
+          *)
+         (*let cmd = Nbdkit.add_arg cmd "fadvise" "sequential" in
+           let cmd = Nbdkit.add_arg cmd "cache" "none" in*)
          cmd
        )
        else cmd in