Re: [RFC storage, qemu-server] offload full/live clone to storage backend

[Fiona Ebner <f.ebner@proxmox.com>]

Adding on top of Fabian's feedback:

Am 07.07.26 um 3:48 PM schrieb Fabian Grünbichler:
> On June 30, 2026 9:20 pm, Ciro Iriarte wrote:
>> -------------------------------
>> - Add an optional plugin method, copy_image($scfg, $storeid,
>>   $src_volname, $vmid, $dst_format, $opts), that allocates and copies
>>   on the backend atomically and returns the new volname. Base class
>>   returns undef (unsupported), so existing plugins are unaffected --
>>   the driver is the sole source of the capability.

I think the base class should die, because if it doesn't advertise the
feature, the method should never be reachable anyways.

>> Online offload runs only when the freeze succeeds, giving an
>> FS-consistent copy; if there is no agent or the freeze fails, fall back
>> to drive-mirror. This needs no separate switch -- the same copy-offload
>> option governs it, and the freeze requirement keeps it from ever
>> producing a crash-consistent copy silently.
> 
> I think the question of how to incorporate freeze for cloning running
> VMs is a bit orthogonal? atm we don't freeze, so there is no cross-disk
> consistency when cloning a running VM, other than that provided by the
> mirror blockjob.

No, we do freeze when cloning a running VM:
https://git.proxmox.com/?p=qemu-server.git;a=blob;f=src/PVE/QemuServer/BlockJob.pm;h=921f046cd9ad917a1e365bd152cb250d656beec4;hb=HEAD#l168

I'm in favor of the bitmap-mirroring approach Fabian suggested, so in
the end, it would just use the existing BlockJob.pm mirror code for
completion.

>>
>> Out of scope
>> ------------
>> - Cross-storage and format-converting clones (inherently
>>   host-mediated).
> 
> agreed. cross-storage could potentially be a follow-up, and we might
> want to account for it in the copy_image signature/parameters?

I think we'd need an abstraction similar to
volume_{export,import}{,formats} so that plugins don't deal with
internal information from other plugins?