[pve-devel] [PATCH v3 storage 1/9] plugin: add method to get qemu blockdevice options for volume

[Fiona Ebner]

Am 01.07.25 um 11:28 schrieb Thomas Lamprecht:
> Am 26.06.25 um 16:40 schrieb Fiona Ebner:
>> +=item C<$scfg>
>> +
>> +The hash reference with the storage configuration.
>> +
>> +=item C<$storeid>
>> +
>> +The storage ID.
>> +
>> +=item C<$volume>
>> +
>> +The volume name.
> 
> bit much POD noise for my taste, couldn't the item's be at least placed
> on a single line? E.g.:
> 
> =item C<$volume>: The volume name.
> =item ...

Sure, fine by me, but blank lines between two =item elements seem to be
necessary.

>> +
>> +=back
>> +
>> +=cut
>> +
>> +sub qemu_blockdev_options {
>> +    my ($class, $scfg, $storeid, $volname) = @_;
> 
> might make sense to pass some versioning information here, as otherwise this
> is a bit to tight coupling for my taste and might cause long term maintenance
> headache.
> 
> It might be potentially enough to have the QEMU version and machine version
> passed here, as then one can generate block dev options that can be understood
> by qemu(-server) and are also compatible with the target machine version.

It would seem pretty strange to me that an option for accessing an image
would depend on the machine version. We already rely on having an image
accessed via different drivers/options to be fully equivalent for
migration (or we couldn't combine block mirror and migration). OTOH, the
machine version is the natural guard and there is less potential for a
plugin to break live migration by accidentally setting or changing an
option it shouldn't have. And if a plugin depends on a specific binary
version for a feature, it can just also guard with the machine version
corresponding to that binary version. So I'd just pass along the machine
version and not the binary version.

> And are we sure that we want to allow passing arbitrary options here?
> Could we at least generate some simple schema automatically from the QAPI or
> the like? Could be also a specially prepared JSON file just for this purpose
> shipped by our qemu package, or tracked separately so that we can track the
> version in which a option first appeared or became obsolete.

Sorry, I don't understand what you mean here.

Verify the block device options in the return value that the plugin
implementation has given us? I don't think verifying would give us much,
as QEMU will already complain the same way.

Plugins should just use the most basic options to access/open the image.
Other options will be set by qemu-server. The POD mentions this, but
maybe it should be emphasized more?

>> +
>> +    my $blockdev = {};
>> +
>> +    my ($path) = $class->filesystem_path($scfg, $volname);
>> +
>> +    if ($path =~ m|^/|) {
>> +        # The 'file' driver only works for regular files. The check below is taken from
>> +        # block/file-posix.c:hdev_probe_device() in QEMU. Do not bother with detecting 'host_cdrom'
>> +        # devices here, those are not managed by the storage layer.
>> +        my $st = File::stat::stat($path) or die "stat for '$path' failed - $!\n";
>> +        my $driver = (S_ISCHR($st->mode) || S_ISBLK($st->mode)) ? 'host_device' : 'file';
>> +        $blockdev = { driver => $driver, filename => $path };
>> +    } else {
>> +        die "storage plugin doesn't implement qemu_blockdev_options() method\n";
>> +    }
> 
> Should we rather default to an empty set of extra options? At least for external
> plugins that would be the safer choice for upgrading, might not always work but
> as is users can only loose FWICT?

What extra options do you mean? The default implementation here only
sets driver and filename which is the most minimal possible.