[pve-devel] [PATCH qemu-server v2] clone disk: fix handling of snapshot-as-volume-chain for EFI disks

[Friedrich Weber]

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 the snapshot method of the volume in question,
and only adding the `-l` option if the volume uses qcow2-internal
snapshots.

Reported in enterprise support.

Signed-off-by: Friedrich Weber <f.weber at proxmox.com>
---

Notes:
    Reproducer (STORAGE having snapshot-as-volume-chain enabled):
    
    qm create 100 --bios ovmf --efidisk0 STORAGE:1,format=qcow2
    qm snapshot 100 mysnap
    qm clone 100 101 --snap mysnap
    
    The rfc used volume_snapshot_info, but this seems to be somewhat buggy
    and the fix might be more involed (see rfc). I did not have time to
    look into fixing volume_snapshot_info -- since using
    volume_qemu_snapshot_method here also sounds appropriate and should
    fix the bug, I'm sending this new version here.
    
    changes since rfc:
    - check PVE::Storage::volume_qemu_snapshot_method instead of
      volume_snapshot_info
    
    rfc: https://lore.proxmox.com/pve-devel/20251107171910.129794-1-f.weber@proxmox.com/

 src/PVE/QemuServer.pm | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/PVE/QemuServer.pm b/src/PVE/QemuServer.pm
index cf195ccc..3ea3236c 100644
--- a/src/PVE/QemuServer.pm
+++ b/src/PVE/QemuServer.pm
@@ -7868,7 +7868,11 @@ sub clone_disk {
                 if ($src_format eq 'qcow2' && $snapname) {
                     die "cannot clone qcow2 EFI disk snapshot - requires QEMU >= 6.2\n"
                         if !min_version(kvm_user_version(), 6, 2);
-                    push $cmd->@*, '-l', $snapname;
+
+                    my $method =
+                        PVE::Storage::volume_qemu_snapshot_method($storecfg, $drive->{file});
+                    # in case of snapshot-as-volume-chain, $src_path points to the snapshot volume
+                    push $cmd->@*, '-l', $snapname if $method eq 'qemu';
                 }
                 push $cmd->@*, "bs=$bs", "osize=$size", "if=$src_path", "of=$dst_path";
                 run_command($cmd);
-- 
2.47.3