[Libguestfs] [PATCH libnbd] copy: Implement destination preferred block size

[Nir Soffer]

On Sun, Jan 30, 2022 at 11:10 AM Richard W.M. Jones <rjones at redhat.com> wrote:
>
> On Sun, Jan 30, 2022 at 12:45:37AM +0200, Nir Soffer wrote:
> > On Fri, Jan 28, 2022 at 10:37 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> > > +  .get_preferred_block_size = nbd_ops_get_preferred_block_size,
> >
> > Why preferred block size and not minimum block size? For example if we
> > write 256k when the minimum block size is 64k, wouldn't qemu block layer
> > handle the write properly, creating 4 compressed clusters?
>
> My theory was that if the destination prefers a particular block size,
> and we're going to all this effort anyway, we might as well use the
> preference.  For the qcow2/compress filter the two values are
> identical.
>
> > When not using a compress filter, qemu-nbd reports block size of 4k, and
> > using this value it will kill performance.
>
> Not sure I understand?

I think this was a mistake - if we use the preferred size only for
alignment, not
for limiting the size of the requests, it should be ok to use the
preferred block size.