Re: [pve-devel] [PATCH v1 pve-storage 6/8] pluginbase: document image operation methods

["Fabian Grünbichler" <f.gruenbichler@proxmox.com>]

On April 2, 2025 6:32 pm, Max Carrara wrote:
> On Mon Mar 31, 2025 at 5:12 PM CEST, Fabian Grünbichler wrote:
>> On March 26, 2025 3:20 pm, Max Carrara wrote:
>> > Add documentation for the following methods:
>> > - list_images
>> > - create_base
>> > - clone_image
>> > - alloc_image
>> > - free_image
>> > 
>> > Signed-off-by: Max Carrara <m.carrara@proxmox.com>
>> > Co-authored-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
>> > ---
>> >  src/PVE/Storage/PluginBase.pm | 111 ++++++++++++++++++++++++++++++++++
>> >  1 file changed, 111 insertions(+)
>> > 
>> > diff --git a/src/PVE/Storage/PluginBase.pm b/src/PVE/Storage/PluginBase.pm
>> > index b3ce684..37b1471 100644
>> > --- a/src/PVE/Storage/PluginBase.pm
>> > +++ b/src/PVE/Storage/PluginBase.pm
>> > @@ -721,26 +721,137 @@ sub on_delete_hook {
>> >  
>> >  =cut
>> >  
>> > +=head3 $plugin->list_images($storeid, \%scfg [, $vmid, \@vollist, \%cache])
>> > +
>> > +B<REQUIRED:> Must be implemented in every storage plugin.
>> > +
>> > +Returns a listref of all disk images of a storage. If the storage does not
>> > +support storing disk images, returns an empty listref.
>> > +
>> > +Optionally, if C<\@vollist> is provided, return only disks whose volume ID is
>> > +within C<\@vollist>. Note that this usually has higher precedence than
>> > +C<$vmid>.
> 
> Upfront note: Unless I otherwise comment something, I agree with you.
> Just sparing myself from writing and you from reading too many ACKs :P
> 
>>
>> what does usually mean? what does $vmid do?
> 
> Sorry, this got lost when tossing out some redundant paragraphs here.
> 
> The "usually" can be dropped; C<$vmid> is the guest's ID (mentioned in
> the DESCRIPTION in patch 01) and unless C<\@vollist> is provided, the
> images that the given C<$vmid> owns will be returned.

in case that wasn't obvious - I know what $vmid is (and also what it
does) - it should be explained to readers that don't (yet).

So I think this should say something like:

If C<$vmid> is set, only images owned by that VMID must be included in
the return value.

If C<\@vollist> is set and not empty, the return value must be filtered
using the contained volids.

If both C<$vmid> and C<\@vollist> are set, only C<\@vollist> should be
honored.

and about that last part - do we actually have callers that set both? if
not, should we instead recommend treating it as an error and document it
as such?

>> > +
>> > +C<die>s in case of errors.
>> > +
>> > +This method may reuse L<< cached information via C<\%cache>|/"CACHING EXPENSIVE OPERATIONS" >>.
>> > +
>> > +The returned listref has the following structure:
>> > +
>> > +    [
>> > +	{
>> > +	    ctime => "1689163322", # creation time as unix timestamp
>> > +	    format => "raw",
>> > +	    size => 8589934592, # in bytes!
>> > +	    vmid => 101,
>> > +	    volid => "local-lvm:base-101-disk-0", # volume ID, storage-specific
>> > +	},
>> > +	# [...]
>> > +    ]
>> > +
>> > +=cut
>> > +
>> >  sub list_images {
>> >      my ($class, $storeid, $scfg, $vmid, $vollist, $cache) = @_;
>> >      croak "implement me in sub-class\n";
>> >  }
>> >  
>> > +=head3 $plugin->create_base($storeid, \%scfg, $volname)
>> > +
>> > +B<OPTIONAL:> May be implemented in a storage plugin.
>> > +
>> > +Creates a base volume from an existing volume, allowing the volume to be
>>
>> this is misleading - it doesn't create a base volume from an existing
>> volume, it *converts* an existing volume into a base volume!
> 
> Oh dang, thanks for pointing this out! Sorry for the oversight.
> 
>>
>> > +L<< cloned|/"$plugin->clone_image(...)" >>. This cloned volume (usually
>> > +a disk image) may then be used as a base for the purpose of creating linked
>>
>> this is wrong? there is no cloned volume yet? and all of this only
>> applies to images and not other volumes?
>>
>> > +clones. See L<C<PVE::Storage::LvmThinPlugin>> and
>> > +L<C<PVE::Storage::ZFSPoolPlugin>> for example implementations.
>> > +
>> > +On completion, returns the name of the new base volume (the new C<$volname>).
>> > +
>> > +This method is called in the context of C<L<< cluster_lock_storage()|/"cluster_lock_storage(...)" >>>,
>> > +i.e. when the storage is B<locked>.
>>
>> Shouldn't this say that it *should* be called in a locked context?
>>
>> > +
>> > +=cut
>> > +
>> >  sub create_base {
>> >      my ($class, $storeid, $scfg, $volname) = @_;
>> >      croak "implement me in sub-class\n";
>> >  }
>> >  
>> > +=head3 $plugin->clone_image($scfg, $storeid, $volname, $vmid [, $snap])
>> > +
>> > +=head3 $plugin->clone_image(...)
>> > +
>> > +B<REQUIRED:> Must be implemented in every storage plugin.
>>
>> not really? there's a feature guard for it, so only storage plugins that
>> support it should ever get it called anyway..
>>
>> what does $vmid mean?
> 
> See above; the guest's ID. Since it was unclear, the parameter should
> probably be explained here (and also above) instead of (just) in the
> DESCRIPTION section.

I know what a vmid is ;) the question is what does this parameter *mean*
for this particular method?

C<$vmid> contains the owner of the new volume, which must be encoded in
the returned volid.

or something like that?

>> > +
>> > +Clones a disk image or a snapshot of an image, returning the name of the new
>> > +image (the new C<$volname>). Note that I<cloning> here means to create a linked
>> > +clone and not duplicating an image. See L<C<PVE::Storage::LvmThinPlugin>> and
>> > +L<C<PVE::Storage::ZFSPoolPlugin>> for example implementations.
>>
>> then why not start with
>>
>> "Creates a linked clone of an existing disk image or snapshot of an
>> image"
>>
>> ? maybe also include that unless the linked clone is 100% independent
>> from the base (only true for LVM thin atm?), the new volid should encode
>> the base->clone relation in the volname?
>>
>> > +
>> > +C<die>s in case of an error of if the underlying storage doesn't support
>>
>> s/of/or/
>>
>> > +cloning images.
>> > +
>> > +This method is called in the context of C<L<< cluster_lock_storage()|/"cluster_lock_storage(...)" >>>,
>> > +i.e. when the storage is B<locked>.
>>
>> same as above
>>
>> > +
>> > +=cut
>> > +
>> >  sub clone_image {
>> >      my ($class, $scfg, $storeid, $volname, $vmid, $snap) = @_;
>> >      croak "implement me in sub-class\n";
>> >  }
>> >  
>> > +=head3 $plugin->alloc_image($storeid, $scfg, $vmid, $fmt, $name, $size)
>> > +
>> > +B<REQUIRED:> Must be implemented in every storage plugin.
>> > +
>> > +Allocates a disk image with the given format C<$fmt> and C<$size> in bytes,
>> > +returning the name of the new image (the new C<$volname>). See
>> > +C<L<< plugindata()|/"$plugin->plugindata()" >>> for all disk formats.
>> > +
>> > +Optionally, if given, set the name of the image to C<$name>. If C<$name> isn't
>> > +provided, the next name should be determined via C<L<< find_free_diskname()|/"$plugin->find_free_diskname(...)" >>>.
>>
>> a suitable name should be automatically generated, unless we really want
>> to stabilize find_free_diskname as part of the API..
>>
>> what does $vmid mean?

here as well, similar to clone_image above


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