Re: [PATCH v2] block/file-posix: Optimize for macOS

[Stefan Hajnoczi]

On Tue, Mar 09, 2021 at 12:37:35AM +0900, Akihiko Odaki wrote:
> 2021年3月9日(火) 0:17 Stefan Hajnoczi <stefanha@redhat.com>:
> >
> > The live migration compatibility issue is still present. Migrating to
> > another host might not work if the block limits are different.
> >
> > Here is an idea for solving it:
> >
> > Modify include/hw/block/block.h:DEFINE_BLOCK_PROPERTIES_BASE() to
> > support a new value called "host". The default behavior remains
> > unchanged for live migration compatibility but now you can use "host" if
> > you know it's okay but don't care about migration compatibility.
> >
> > The downside to this approach is that users must explicitly say
> > something like --drive ...,opt_io_size=host. But it's still better than
> > the situation we have today where user must manually enter values for
> > their disk.
> >
> > Does this sound okay to everyone?
> >
> > Stefan
> 
> I wonder how that change affects other block drivers implementing
> bdrv_probe_blocksizes. As far as I know, the values they report are
> already used by default, which is contrary to the default not being
> "host".

The behavior should remain unchanged for existing QEMU command-lines.
The block drivers that report values by default should continue to do
so:

block/file-posix.c: only reports values for s390 DASD devices.

block/nvme.c: reports the NVMe PCI adapter's values.

block/raw-format.c: propagates its child's values unless the "offset"
                    option is incompatible with them.

These block drivers would default to logical_block_size=host and
physical_block_size=host, while other block drivers would not.

Stefan

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