[Libguestfs] [PATCH libnbd] copy: Set POSIX_FADV_SEQUENTIAL and POSIX_FADV_DONTNEED

[Richard W.M. Jones]

I'm trying to emulate what we do in nbdkit-file-plugin to avoid the
page cache getting trashed when copying to/from local files:
https://gitlab.com/nbdkit/nbdkit/-/commit/aa5a2183a6d16afd919174b5ba8222b2bccf4039

This patch works well when reading from a local file as you can see
from the results in the commit message.  However when writing to a
local file it has absolutely no effect, and I've no idea why.  (Even
checked with strace and it's making the correct fadvise64 system
calls).

For the reading case it has a slight performance impact which could be
problematic.  Should we only enable it with a flag?  I would prefer to
fix it so it's always beneficial, but it's not too easy to do that
without adding more threads and synchronization.

Rich.