Re: [Libguestfs] [PATCH libnbd] copy: Implement destination preferred block size
On Sun, Jan 30, 2022 at 11:10 AM Richard W.M. Jones <rjones(a)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(a)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.