[pve-devel] [PATCH storage/qemu-server v2 0/5] avoid absolute qcow2 references

[pve-devel] [PATCH storage/qemu-server v2 0/5] avoid absolute qcow2 references

[Fabian Grünbichler]

we don't want qcow2 files to reference their backing chains via
absolute paths, as that makes renaming the base dir or VG of the storage
impossible. in most places, qemu already allows simply passing a
filename as backing-file reference, which will be interpreted as a
reference relative to the backed image.

I haven't found any further code paths that trigger absolute references,
but I might have missed some. the full backing chain should show
relative backing-file members when queried via

qemu-img info --output json --format qcow2 --backing-chain /path/to/main/image.qcow2

such, as:

        "full-backing-filename": "/var/lib/extsnap/images/210/snap-test2-vm-210-disk-0.qcow2",
        "backing-filename": "snap-test2-vm-210-disk-0.qcow2",

note that full-backing-filename will always contain the resolved,
absolute path and that is okay. we could warn about both members
containing full paths in `volume_snapshot_info`.

for existing "broken" images, an "unsafe" rebase with

qemu-img rebase -u -f qcow2 -F qcow2 -b <relative backing file path> <absolute backed filed path>

should just rewrite the qcow2 header to replace the backing file
reference - this should of course not be run while the image is being
written by other process or QEMU.

v2: incorporated Fiona's feedback, thanks!

pve-storage:

Fabian Grünbichler (4):
  plugin: fix typo in rebase log message
  lvm plugin: fix typo in rebase log message
  plugin: use relative path for qcow2 rebase command
  lvm plugin: use relative path for qcow2 rebase command

 src/PVE/Storage/LVMPlugin.pm | 6 +++---
 src/PVE/Storage/Plugin.pm    | 6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

qemu-server:

Fabian Grünbichler (1):
  blockdev-stream/-commit: make backing file relative

 src/PVE/QemuServer/Blockdev.pm | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

-- 
2.39.5



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[pve-devel] [PATCH storage v2 1/5] plugin: fix typo in rebase log message

[Fabian Grünbichler]

by directly printing the to-be-executed command, instead of copying it which is
error-prone.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
---

Notes:
    v2: join command instead of fixing manually copied message

 src/PVE/Storage/Plugin.pm | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 2bd05bd..de41bfe 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -1437,7 +1437,6 @@ sub volume_snapshot_delete {
             my $parentpath = $snapshots->{$parentsnap}->{file};
             print
                 "$volname: deleting snapshot '$snap' by rebasing '$childsnap' on top of '$parentsnap'\n";
-            print "running 'qemu-img rebase -b $parentpath -F qcow -f qcow2 $childpath'\n";
             $cmd = [
                 '/usr/bin/qemu-img',
                 'rebase',
@@ -1449,6 +1448,7 @@ sub volume_snapshot_delete {
                 'qcow2',
                 $childpath,
             ];
+            print "running '", join(' ', $cmd->@*), "'\n";
             eval { run_command($cmd) };
             if ($@) {
                 #in case of abort, the state of the snap is still clean, just a little bit bigger
-- 
2.39.5



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Re: [pve-devel] [PATCH storage v2 1/5] plugin: fix typo in rebase log message

[Fiona Ebner]

Am 29.07.25 um 1:53 PM schrieb Fabian Grünbichler:
> by directly printing the to-be-executed command, instead of copying it which is
> error-prone.
> 
> Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
> Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
> 
> Notes:
>     v2: join command instead of fixing manually copied message
> 
>  src/PVE/Storage/Plugin.pm | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
> index 2bd05bd..de41bfe 100644
> --- a/src/PVE/Storage/Plugin.pm
> +++ b/src/PVE/Storage/Plugin.pm
> @@ -1437,7 +1437,6 @@ sub volume_snapshot_delete {
>              my $parentpath = $snapshots->{$parentsnap}->{file};
>              print
>                  "$volname: deleting snapshot '$snap' by rebasing '$childsnap' on top of '$parentsnap'\n";
> -            print "running 'qemu-img rebase -b $parentpath -F qcow -f qcow2 $childpath'\n";
>              $cmd = [
>                  '/usr/bin/qemu-img',
>                  'rebase',
> @@ -1449,6 +1448,7 @@ sub volume_snapshot_delete {
>                  'qcow2',
>                  $childpath,
>              ];
> +            print "running '", join(' ', $cmd->@*), "'\n";

Style nit: we usually do concatenation with dot, was using commas for
passing a list to print intentional? Same for the next patch.

>              eval { run_command($cmd) };
>              if ($@) {
>                  #in case of abort, the state of the snap is still clean, just a little bit bigger



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Re: [pve-devel] [PATCH storage v2 1/5] plugin: fix typo in rebase log message

[Fabian Grünbichler]

On July 29, 2025 2:04 pm, Fiona Ebner wrote:
> Am 29.07.25 um 1:53 PM schrieb Fabian Grünbichler:
>> by directly printing the to-be-executed command, instead of copying it which is
>> error-prone.
>> 
>> Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
>> Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
>> ---
>> 
>> Notes:
>>     v2: join command instead of fixing manually copied message
>> 
>>  src/PVE/Storage/Plugin.pm | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
>> index 2bd05bd..de41bfe 100644
>> --- a/src/PVE/Storage/Plugin.pm
>> +++ b/src/PVE/Storage/Plugin.pm
>> @@ -1437,7 +1437,6 @@ sub volume_snapshot_delete {
>>              my $parentpath = $snapshots->{$parentsnap}->{file};
>>              print
>>                  "$volname: deleting snapshot '$snap' by rebasing '$childsnap' on top of '$parentsnap'\n";
>> -            print "running 'qemu-img rebase -b $parentpath -F qcow -f qcow2 $childpath'\n";
>>              $cmd = [
>>                  '/usr/bin/qemu-img',
>>                  'rebase',
>> @@ -1449,6 +1448,7 @@ sub volume_snapshot_delete {
>>                  'qcow2',
>>                  $childpath,
>>              ];
>> +            print "running '", join(' ', $cmd->@*), "'\n";
> 
> Style nit: we usually do concatenation with dot, was using commas for
> passing a list to print intentional? Same for the next patch.

force of habit, no real reason - feel free to fixup on applying :)

>>              eval { run_command($cmd) };
>>              if ($@) {
>>                  #in case of abort, the state of the snap is still clean, just a little bit bigger
> 
> 


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[pve-devel] [PATCH storage v2 2/5] lvm plugin: fix typo in rebase log message

[Fabian Grünbichler]

this was copied over from Plugin.pm

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
---
 src/PVE/Storage/LVMPlugin.pm | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/PVE/Storage/LVMPlugin.pm b/src/PVE/Storage/LVMPlugin.pm
index c1f5474..5a84e82 100644
--- a/src/PVE/Storage/LVMPlugin.pm
+++ b/src/PVE/Storage/LVMPlugin.pm
@@ -1120,7 +1120,6 @@ sub volume_snapshot_delete {
         my $parentpath = $snapshots->{$parentsnap}->{file};
         print
             "$volname: deleting snapshot '$snap' by rebasing '$childsnap' on top of '$parentsnap'\n";
-        print "running 'qemu-img rebase -b $parentpath -F qcow -f qcow2 $childpath'\n";
         $cmd = [
             '/usr/bin/qemu-img',
             'rebase',
@@ -1132,6 +1131,7 @@ sub volume_snapshot_delete {
             'qcow2',
             $childpath,
         ];
+        print "running '", join(' ', $cmd->@*), "'\n";
         eval { run_command($cmd) };
         if ($@) {
             #in case of abort, the state of the snap is still clean, just a little bit bigger
-- 
2.39.5



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[pve-devel] [PATCH storage v2 3/5] plugin: use relative path for qcow2 rebase command

[Fabian Grünbichler]

otherwise the resulting qcow2 file will contain an absolute path, which makes
changing the backing path of the directory storage impossible.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
---
 src/PVE/Storage/Plugin.pm | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index de41bfe..f54a38c 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -1434,14 +1434,14 @@ sub volume_snapshot_delete {
 
         } else {
             #we rebase the child image on the parent as new backing image
-            my $parentpath = $snapshots->{$parentsnap}->{file};
             print
                 "$volname: deleting snapshot '$snap' by rebasing '$childsnap' on top of '$parentsnap'\n";
+            my $rel_parent_path = get_snap_name($class, $volname, $parentsnap);
             $cmd = [
                 '/usr/bin/qemu-img',
                 'rebase',
                 '-b',
-                $parentpath,
+                $rel_parent_path,
                 '-F',
                 'qcow2',
                 '-f',
-- 
2.39.5



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[pve-devel] [PATCH storage v2 4/5] lvm plugin: use relative path for qcow2 rebase command

[Fabian Grünbichler]

otherwise the resulting qcow2 file will contain an absolute path, which makes
renaming the backing VG of the storage impossible.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
---

Notes:
    v2: drop unused variable

 src/PVE/Storage/LVMPlugin.pm | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/PVE/Storage/LVMPlugin.pm b/src/PVE/Storage/LVMPlugin.pm
index 5a84e82..46c8d8e 100644
--- a/src/PVE/Storage/LVMPlugin.pm
+++ b/src/PVE/Storage/LVMPlugin.pm
@@ -1117,14 +1117,14 @@ sub volume_snapshot_delete {
 
     } else {
         #we rebase the child image on the parent as new backing image
-        my $parentpath = $snapshots->{$parentsnap}->{file};
         print
             "$volname: deleting snapshot '$snap' by rebasing '$childsnap' on top of '$parentsnap'\n";
+        my $rel_parent_path = get_snap_name($class, $volname, $parentsnap);
         $cmd = [
             '/usr/bin/qemu-img',
             'rebase',
             '-b',
-            $parentpath,
+            $rel_parent_path,
             '-F',
             'qcow2',
             '-f',
-- 
2.39.5



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[pve-devel] [PATCH qemu-server v2 5/5] blockdev-stream/-commit: make backing file relative

[Fabian Grünbichler]

to avoid the resulting qcow2 file referencing its backing file via an absolute
path, which makes renaming the base of the storage impossible.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
---

Notes:
    v2: move logic into its own helper

 src/PVE/QemuServer/Blockdev.pm | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm
index 6e105571..08bc48dd 100644
--- a/src/PVE/QemuServer/Blockdev.pm
+++ b/src/PVE/QemuServer/Blockdev.pm
@@ -5,6 +5,7 @@ use warnings;
 
 use Digest::SHA;
 use Fcntl qw(S_ISBLK S_ISCHR);
+use File::Basename qw(basename dirname);
 use File::stat;
 use JSON;
 
@@ -885,6 +886,20 @@ sub blockdev_delete {
     PVE::Storage::volume_snapshot_delete($storecfg, $volid, $snap, 1);
 }
 
+my sub blockdev_relative_backing_file {
+    my ($backing, $backed) = @_;
+
+    my $backing_file = $backing->{filename};
+    my $backed_file = $backed->{filename};
+
+    if (dirname($backing_file) eq dirname($backed_file)) {
+        # make backing file relative if in same directory
+        return basename($backing_file);
+    }
+
+    return $backing_file;
+}
+
 sub blockdev_replace {
     my (
         $storecfg,
@@ -954,12 +969,15 @@ sub blockdev_replace {
         $parent_fmt_blockdev->{backing} = $target_fmt_blockdev->{'node-name'};
         mon_cmd($vmid, 'blockdev-reopen', options => [$parent_fmt_blockdev]);
 
+        my $backing_file =
+            blockdev_relative_backing_file($target_file_blockdev, $parent_file_blockdev);
+
         #change backing-file in qcow2 metadatas
         mon_cmd(
             $vmid, 'change-backing-file',
             device => $deviceid,
             'image-node-name' => $parent_fmt_blockdev->{'node-name'},
-            'backing-file' => $target_file_blockdev->{filename},
+            'backing-file' => $backing_file,
         );
     }
 
@@ -1069,11 +1087,13 @@ sub blockdev_stream {
         { 'snapshot-name' => $snap },
     );
 
+    my $backing_file = blockdev_relative_backing_file($parent_file_blockdev, $target_file_blockdev);
+
     my $job_id = "stream-$deviceid";
     my $jobs = {};
     my $options = { 'job-id' => $job_id, device => $target_fmt_blockdev->{'node-name'} };
     $options->{'base-node'} = $parent_fmt_blockdev->{'node-name'};
-    $options->{'backing-file'} = $parent_file_blockdev->{filename};
+    $options->{'backing-file'} = $backing_file;
 
     mon_cmd($vmid, 'block-stream', %$options);
     $jobs->{$job_id} = {};
-- 
2.39.5



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[pve-devel] applied-series: [PATCH storage/qemu-server v2 0/5] avoid absolute qcow2 references

[Fiona Ebner]

Am 29.07.25 um 1:53 PM schrieb Fabian Grünbichler:
> we don't want qcow2 files to reference their backing chains via
> absolute paths, as that makes renaming the base dir or VG of the storage
> impossible. in most places, qemu already allows simply passing a
> filename as backing-file reference, which will be interpreted as a
> reference relative to the backed image.
> 
> I haven't found any further code paths that trigger absolute references,
> but I might have missed some. the full backing chain should show
> relative backing-file members when queried via
> 
> qemu-img info --output json --format qcow2 --backing-chain /path/to/main/image.qcow2
> 
> such, as:
> 
>         "full-backing-filename": "/var/lib/extsnap/images/210/snap-test2-vm-210-disk-0.qcow2",
>         "backing-filename": "snap-test2-vm-210-disk-0.qcow2",
> 
> note that full-backing-filename will always contain the resolved,
> absolute path and that is okay. we could warn about both members
> containing full paths in `volume_snapshot_info`.
> 
> for existing "broken" images, an "unsafe" rebase with
> 
> qemu-img rebase -u -f qcow2 -F qcow2 -b <relative backing file path> <absolute backed filed path>
> 
> should just rewrite the qcow2 header to replace the backing file
> reference - this should of course not be run while the image is being
> written by other process or QEMU.

Applied series, with the discussed fix-up squashed in, thanks!


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