[pve-devel] [PATCH qemu-server] print drive commandline: prohibit using snapshot-as-volume-chain qcow2 images

[pve-devel] [PATCH qemu-server] print drive commandline: prohibit using snapshot-as-volume-chain qcow2 images

[Fiona Ebner]

Require that snapshot-as-volume-chain qcow2 images are always used in
combination with '-blockdev', rather than '-drive'. With '-drive', the
'discard-no-unref' option is not set and the fragmentation can lead to
the same issue that for '-blockdev', was solved by commit a3a9a2ab
("fix #6543: use qcow2 'discard-no-unref' option when using
snapshot-as-volume-chain").

While it would be possible to set the flag for '-drive' too, the
snapshot-as-volume-chain feature already only works with machine type
>= 10.0, see commit 6b2b45fd ("snapshot create/delete: die early for
snapshot-as-volume-chain for pre-10.0 machine version") and it's only
tested for those. Avoid accidents and other unknown issues by being
strict and prohibiting usage without '-blockdev'.

Reported-by: Friedrich Weber <f.weber@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---
 src/PVE/QemuServer.pm | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/PVE/QemuServer.pm b/src/PVE/QemuServer.pm
index e30b27cb..cfc54568 100644
--- a/src/PVE/QemuServer.pm
+++ b/src/PVE/QemuServer.pm
@@ -1335,6 +1335,13 @@ sub print_drive_commandline_full {
     my ($path, $format) =
         PVE::QemuServer::Drive::get_path_and_format($storecfg, $drive, $live_restore_name);
 
+    if ($scfg && $scfg->{'snapshot-as-volume-chain'} && $format && $format eq 'qcow2') {
+        # the print_drive_commandline_full() function is only used if machine version is < 10.0
+        die "storage for '$drive->{file}' is configured for snapshots as a volume chain - this"
+            . " requires QEMU machine version >= 10.0. See"
+            . " https://pve.proxmox.com/wiki/QEMU_Machine_Version_Upgrade\n";
+    }
+
     my $is_rbd = $path =~ m/^rbd:/;
 
     my $opts = '';
-- 
2.47.2



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[pve-devel] applied: [PATCH qemu-server] print drive commandline: prohibit using snapshot-as-volume-chain qcow2 images

[Fabian Grünbichler]

On Mon, 11 Aug 2025 15:51:33 +0200, Fiona Ebner wrote:
> Require that snapshot-as-volume-chain qcow2 images are always used in
> combination with '-blockdev', rather than '-drive'. With '-drive', the
> 'discard-no-unref' option is not set and the fragmentation can lead to
> the same issue that for '-blockdev', was solved by commit a3a9a2ab
> ("fix #6543: use qcow2 'discard-no-unref' option when using
> snapshot-as-volume-chain").
> 
> [...]

Applied, thanks!

[1/1] print drive commandline: prohibit using snapshot-as-volume-chain qcow2 images
      (no commit info)

Best regards,
-- 
Fabian Grünbichler <f.gruenbichler@proxmox.com>


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