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

[Akihiko Odaki]

2021年3月9日(火) 0:37 Akihiko Odaki <akihiko.odaki@gmail.com>:
>
> 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".
>
> Regards,
> Akihiko Odaki

Let me suggest a variant of Stefan's approach:

Modify include/hw/block/block.h:DEFINE_BLOCK_PROPERTIES_BASE() to
support a new value called "host". The default values for block size
properties may be "host" or not, but they should be consistent. If
they are "host" by default, add global properties which sets
discard_granularity and opt_io_size to the old default to
hw_compat_5_2 in hw/core/machine.c. Otherwise, add global properties
which sets logical_block_size and physical_block_size to "host".

Does it sound good? I'd also like to know others opinions for the
default value ("host" or something else). I prefer "host" as the
default a little because those who need live migration should be
careful enough to set proper configurations for each device. We may
also assist users who need live migration by adding a property which
defaults all block size properties to something else "host".

Regards,
Akihiko Odaki