[pve-devel] [PATCH qemu-server] fix #6713: snapshot volume chain: fix snapshot after disk move with zeroinit

[Aaron Lauterer]

Tested by moving a disk from RBD to a directory storage with snapshot 
chains enabled. Verified without the patch that the error in the bug 
still happens.
Then tested again after a cold boot of the VM with the patch. Rollback 
also worked.

Tested-By: Aaron Lauterer <a.lauterer at proxmox.com>

On  2025-09-19  14:09, Fiona Ebner wrote:
> After mirror, the node below throttle might not be the format node,
> but can also be a zeroinit node. In particular, this is the node that
> needs to be used by when replacing the blockdev for volume-chain
> snapshots for the 'current' snapshot. Look up the actually inserted
> node below throttle, rather than assuming that it's the format node.
> 
> Also removes the $src_file_blockdev_name variable that has been unused
> since commit e7cf7c00 ("blockdev: delete/replace: re-use detach()
> helper").
> 
> Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
> ---
>   src/PVE/QemuServer/Blockdev.pm | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm
> index d4062148..8e6749e8 100644
> --- a/src/PVE/QemuServer/Blockdev.pm
> +++ b/src/PVE/QemuServer/Blockdev.pm
> @@ -924,9 +924,15 @@ sub blockdev_replace {
>       my $volid = $drive->{file};
>       my $drive_id = PVE::QemuServer::Drive::get_drive_id($drive);
>   
> -    my $src_name_options = $src_snap eq 'current' ? {} : { 'snapshot-name' => $src_snap };
> -    my $src_file_blockdev_name = get_node_name('file', $drive_id, $volid, $src_name_options);
> -    my $src_fmt_blockdev_name = get_node_name('fmt', $drive_id, $volid, $src_name_options);
> +    my $src_name_options = {};
> +    my $src_blockdev_name;
> +    if ($src_snap eq 'current') {
> +        # there might be other nodes on top like zeroinit, look up the current node below throttle
> +        $src_blockdev_name = get_node_name_below_throttle($vmid, $deviceid, 1);
> +    } else {
> +        $src_name_options = { 'snapshot-name' => $src_snap };
> +        $src_blockdev_name = get_node_name('fmt', $drive_id, $volid, $src_name_options);
> +    }
>   
>       my $target_file_blockdev = generate_file_blockdev(
>           $storecfg,
> @@ -989,7 +995,7 @@ sub blockdev_replace {
>       }
>   
>       # delete old file|fmt nodes
> -    eval { detach($vmid, $src_fmt_blockdev_name); };
> +    eval { detach($vmid, $src_blockdev_name); };
>       warn "detaching block node for $src_snap failed - $@" if $@;
>   }
>