Re: [pve-devel] [RFC qemu-server] clone disk: fix handling of snapshot-as-volume-chain for EFI disks

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

On November 10, 2025 6:06 pm, Friedrich Weber wrote:
> On 07/11/2025 18:19, Friedrich Weber wrote:
>> Currently, cloning an EFI disk off a snapshot-as-volume-chain snapshot
>> fails with "qemu-img: Failed to load snapshot: Can't find snapshot".
>> The reason is that the special case for EFI disks calls `qemu-img dd`
>> with the additional `-l snapname` option, which is only valid for
>> qcow2-internal snapshots. For snapshot-as-volume-chain snapshots, the
>> source volume is already the volume corresponding to the snapshot.
>> 
>> Fix this by checking whether the snapshot in question is an external
>> snapshot, and if it is, omitting the `-l` option.
>> 
>> Reported in enterprise support.
> 
> Turns out this only fixes the issue in case of snapshot-as-volume-chain
> on file-level storages, not on LVM. The reason is that
> LVMPlugin::volume_snapshot_info does not set the `ext` key to 1 [1], so
> the code doesn't recognize the external snapshot.
> 
> LVMPlugin::volume_snapshot_info should probably set `ext` (assuming that
> "ext" means "external")?
> 
>> 
>> Signed-off-by: Friedrich Weber <f.weber@proxmox.com>
>> ---
>> 
>> Notes:
>>     I noticed that two oddities about volume_snapshot_info:
>>     
>>     - it doesn't seem to work for qcow2-internal snapshots: It assumes
>>       that if `qemu img info` returns a JSON object, the snapshots are
>>       internal, and if it returns a JSON array, they are "external", but
>>       this doesn't seem correct -- I think because we pass
>>       --backing-chain, qemu-img will return an one-element JSON array even
>>       for internal snapshots. Hence, volume_snapshot_info incorrectly
>>       returns a hash ref with only a single member "current" if the
>>       snapshots are internal. But since I only found callsites guarded
>>       with snapshot-as-volume-chain, I guess this didn't hurt until now?
>>     
>>       It doesn't hurt for my patch either because it only checks for
>>       existence of $info->{$snapshot}->{ext} (which also works for
>>       qcow2-internal snapshots, because alreay $info->{$snapshot} will not
>>       exist then), but that's admittedly not nice and it's probably a good
>>       idea to fix volume_snapshot_info in addition.
>>     
>>     - but in which way should we fix it? Right now, for volume-chain
>>       snapshots, it seems like volume_snapshot_info will always have a
>>       member "current" pointing to the "actual" volume, even if the volume
>>       doesn't have any snapshots. Is this intended and do we want this
>>       also in case of qcow2-internal snapshots? From reading
>>       ZFSPoolPlugin::volume_snapshot_info, it looks like that one will
>>       return an empty hash (without "current") if the volume has no
>>       snapshots.
> 
> Looks like
> 
> - ZFSPoolPlugin::volume_snapshot_info indeed doesn't return "current"
> - but there are several callsites of {LVM,}Plugin::volume_snapshot_info
> that rely on 'current' being present for snapshot-as-volume chain [2]
> 
> So the implementations differ w.r.t. returning a "current" entry or not,
> which is IMO a bit awkward API-wise. But fixing this in either way
> (removing it for {LVM,}Plugin or adding it for ZFSPoolPlugin) would
> probably require bigger changes in either the snapshot-as-volume-chain
> or the ZFS replication code?
> 
> For fixing this particular bug, I guess we could instead call
> `volume_qemu_snapshot_method` and only add the `-l snapname` parameter
> if that returns 'qemu'. But I think this is only safe because we don't
> allow enabling/disabling snapshot-as-volume-chain for file-level
> storages on-the-fly.
> 
> What do you think?

that sounds like a good approach - changing snapshot-as-volume-chain
already breaks all sorts of things anyway, so that is not supported.

cleaning up the rest would be good as well..


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