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

[Akihiko Odaki]

2021年3月9日(火) 19:26 Daniel P. Berrangé <berrange@redhat.com>:
>
> On Tue, Mar 09, 2021 at 10:37:18AM +0100, Kevin Wolf wrote:
> > Am 09.03.2021 um 05:52 hat Akihiko Odaki geschrieben:
> > > 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
> >
> > I'm not sure if it's a good idea, but maybe we could make it so that the
> > default is "host" only as long as you didn't specify -nodefaults? Then
> > libvirt would automatically keep the old behaviour (because it always
> > sets -nodefaults) and manual invocations would usually get the new one.
> >
> > Of course, when I start with "I'm not sure if it's a good idea", it's
> > usually not, but I wanted to share the thought anyway...
>
> Can you elaborate on what the actual live migration problem is, and
> its impact ?  This patch is touching the block backends, so I'm
> wondering how backend data ends up having an impact on the migration
> stream which is all frontend device data ?  I'm especially concerned
> by the mention that some block backends already have this problem,
> and wondering if it already impacts libvirt ?
>
> Using -nodefaults is good practice, but I'm still uncomfortable saying
> that its use is a requirement if you want migration to work, as that
> feels like a change in semantics for non-libvirt users (who can be
> mgmt apps, nor merely human interactive users).
>
> Regards,
> Daniel
> --
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>

A problem is that block drivers else "file" already provide
logical_block_sizes and physical_block_sizes, which depends on their
backends, via bdrv_probe_blocksizes. It is apparently tolerated
because those block drivers have physical backends, unlike "file"
block driver which has files as its backends.

This patch has two important changes:
1. It adds new members, discard_granularity and opt_io_size, to the
value bdrv_probe_blocksizes.
2. It implements bdrv_probe_blocksizes which returns
discard_granularity and opt_io_size for "file" block driver.

2 is unacceptable here because it is a change for "file" block driver.
Now we are discussing how it can be fixed. A possible solution is to
make the feature to infer the default value from the backend opt-in.
Stefan's suggestion adds a non-default value, "host" to opt-in the
feature. However, it breaks the current behaviors of other block
drivers which already decides logical_block_sizes and
physical_block_sizes from their backends. My variant fixes the
compatibility issue by adding a global property to hw_compat_5_2.

Regards,
Akihiko Odaki