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

[Richard W.M. Jones]

On Sun, Jan 30, 2022 at 10:42:01PM +0200, Nir Soffer wrote:
> On Sun, Jan 30, 2022 at 8:29 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> >
> > On Sun, Jan 30, 2022 at 06:18:03PM +0200, Nir Soffer wrote:
> > > 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.
> >
> > Got it.
> >
> > Anyhow let's discuss in more detail if I ever get a patch series that
> > works!  We probably need to make the various blocksize(s) into a
> > configuration option.
> 
> I think we can avoid more configuration and use something like:
> 
>     min_extent_size = max(src.min_block_size, dst.min_block_size) //
> or preferred size
>
>     request_size = ROUND_UP(request_size, min_extent_size)
> 
>     sparse = ROUND_UP(sparse, dst.min_block_size)

I think this is worth considering too.  It's a lot simpler than what
I'm thinking about.

Rich.

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