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

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(a)redhat.com&gt; 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(a)redhat.com&gt; 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.