Re: [pve-devel] [PATCH qemu-server 2/4] blockdev: add backing_chain support

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Wed, Jul 09, 2025 at 06:21:48PM +0200, Alexandre Derumier via pve-devel wrote:
> From: Alexandre Derumier <alexandre.derumier@groupe-cyllene.com>
> To: pve-devel@lists.proxmox.com
> Subject: [PATCH qemu-server 2/4] blockdev: add backing_chain support
> Date: Wed,  9 Jul 2025 18:21:48 +0200
> Message-Id: <20250709162202.2952597-4-alexandre.derumier@groupe-cyllene.com>
> X-Mailer: git-send-email 2.39.5
> 
> We need to define name-nodes for all backing chain images,
> to be able to live rename them with blockdev-reopen
> 
> For linked clone, we don't need to definebase image(s) chain.
> They are auto added with #block nodename.
> 
> Signed-off-by: Alexandre Derumier <alexandre.derumier@groupe-cyllene.com>
> ---
>  src/PVE/QemuServer/Blockdev.pm                | 49 +++++++++++++++++++
>  src/test/cfg2cmd/simple-backingchain.conf     | 25 ++++++++++
>  src/test/cfg2cmd/simple-backingchain.conf.cmd | 33 +++++++++++++
>  src/test/run_config2command_tests.pl          | 47 ++++++++++++++++++
>  4 files changed, 154 insertions(+)
>  create mode 100644 src/test/cfg2cmd/simple-backingchain.conf
>  create mode 100644 src/test/cfg2cmd/simple-backingchain.conf.cmd
> 
> diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm
> index 5f1fdae3..2a0513fb 100644
> --- a/src/PVE/QemuServer/Blockdev.pm
> +++ b/src/PVE/QemuServer/Blockdev.pm
> @@ -360,6 +360,46 @@ my sub generate_format_blockdev {
>      return $blockdev;
>  }
>  
> +my sub generate_backing_blockdev;
> +
> +sub generate_backing_blockdev {

FYI: You can just use `my sub` once and recurse with `__SUB__->(...)`
instead of using its name.

> +    my ($storecfg, $snapshots, $deviceid, $drive, $machine_version, $options) = @_;
> +
> +    my $snap_id = $options->{'snapshot-name'};
> +    my $snapshot = $snapshots->{$snap_id};
> +    my $parentid = $snapshot->{parent};
> +
> +    my $volid = $drive->{file};
> +
> +    my $snap_file_blockdev = generate_file_blockdev($storecfg, $drive, $machine_version, $options);
> +    $snap_file_blockdev->{filename} = $snapshot->{file};
> +
> +    my $snap_fmt_blockdev =
> +        generate_format_blockdev($storecfg, $drive, $snap_file_blockdev, $options);
> +
> +    if ($parentid) {
> +        my $options = { 'snapshot-name' => $parentid };
> +        $snap_fmt_blockdev->{backing} = generate_backing_blockdev(
> +            $storecfg, $snapshots, $deviceid, $drive, $machine_version, $options,
> +        );
> +    }
> +    return $snap_fmt_blockdev;
> +}
> +
> +my sub generate_backing_chain_blockdev {
> +    my ($storecfg, $deviceid, $drive, $machine_version) = @_;
> +
> +    my $volid = $drive->{file};
> +
> +    my $snapshots = PVE::Storage::volume_snapshot_info($storecfg, $volid);
> +    my $parentid = $snapshots->{'current'}->{parent};
> +    return undef if !$parentid;
> +    my $options = { 'snapshot-name' => $parentid };
> +    return generate_backing_blockdev(
> +        $storecfg, $snapshots, $deviceid, $drive, $machine_version, $options,
> +    );
> +}
> +
>  sub generate_drive_blockdev {
>      my ($storecfg, $drive, $machine_version, $options) = @_;
>  
> @@ -371,6 +411,15 @@ sub generate_drive_blockdev {
>      my $child = generate_file_blockdev($storecfg, $drive, $machine_version, $options);
>      if (!is_nbd($drive)) {
>          $child = generate_format_blockdev($storecfg, $drive, $child, $options);
> +
> +        my $support_qemu_snapshots =
> +            PVE::Storage::volume_support_qemu_snapshot($storecfg, $drive->{file});
> +        if ($support_qemu_snapshots && $support_qemu_snapshots eq 'external') {
> +            my $backing_chain = generate_backing_chain_blockdev(
> +                $storecfg, "drive-$drive_id", $drive, $machine_version,
> +            );
> +            $child->{backing} = $backing_chain if $backing_chain;
> +        }
>      }
>  
>      if ($options->{'zero-initialized'}) {
> diff --git a/src/test/cfg2cmd/simple-backingchain.conf b/src/test/cfg2cmd/simple-backingchain.conf
> new file mode 100644
> index 00000000..2c0b0f2c
> --- /dev/null
> +++ b/src/test/cfg2cmd/simple-backingchain.conf
> @@ -0,0 +1,25 @@
> +# TEST: Simple test for external snapshot backing chain
> +name: simple
> +parent: snap3
> +scsi0: localsnapext:8006/vm-8006-disk-0.qcow2,size=1G
> +scsi1: lvm-store:vm-8006-disk-0.qcow2,size=1G
> +
> +[snap1]
> +name: simple
> +scsi0: localsnapext:8006/vm-8006-disk-0.qcow2,size=1G
> +scsi1: lvm-store:vm-8006-disk-0.qcow2,size=1G
> +snaptime: 1748933042
> +
> +[snap2]
> +parent: snap1
> +name: simple
> +scsi0: localsnapext:8006/vm-8006-disk-0.qcow2,size=1G
> +scsi1: lvm-store:vm-8006-disk-0.qcow2,size=1G
> +snaptime: 1748933043
> +
> +[snap3]
> +parent: snap2
> +name: simple
> +scsi0: localsnapext:8006/vm-8006-disk-0.qcow2,size=1G
> +scsi1: lvm-store:vm-8006-disk-0.qcow2,size=1G
> +snaptime: 1748933044
> diff --git a/src/test/cfg2cmd/simple-backingchain.conf.cmd b/src/test/cfg2cmd/simple-backingchain.conf.cmd
> new file mode 100644
> index 00000000..40c957f5
> --- /dev/null
> +++ b/src/test/cfg2cmd/simple-backingchain.conf.cmd
> @@ -0,0 +1,33 @@
> +/usr/bin/kvm \
> +  -id 8006 \
> +  -name 'simple,debug-threads=on' \
> +  -no-shutdown \
> +  -chardev 'socket,id=qmp,path=/var/run/qemu-server/8006.qmp,server=on,wait=off' \
> +  -mon 'chardev=qmp,mode=control' \
> +  -chardev 'socket,id=qmp-event,path=/var/run/qmeventd.sock,reconnect-ms=5000' \
> +  -mon 'chardev=qmp-event,mode=control' \
> +  -pidfile /var/run/qemu-server/8006.pid \
> +  -daemonize \
> +  -smp '1,sockets=1,cores=1,maxcpus=1' \
> +  -nodefaults \
> +  -boot 'menu=on,strict=on,reboot-timeout=1000,splash=/usr/share/qemu-server/bootsplash.jpg' \
> +  -vnc 'unix:/var/run/qemu-server/8006.vnc,password=on' \
> +  -cpu kvm64,enforce,+kvm_pv_eoi,+kvm_pv_unhalt,+lahf_lm,+sep \
> +  -m 512 \
> +  -object '{"id":"throttle-drive-scsi0","limits":{},"qom-type":"throttle-group"}' \
> +  -object '{"id":"throttle-drive-scsi1","limits":{},"qom-type":"throttle-group"}' \
> +  -global 'PIIX4_PM.disable_s3=1' \
> +  -global 'PIIX4_PM.disable_s4=1' \
> +  -device 'pci-bridge,id=pci.1,chassis_nr=1,bus=pci.0,addr=0x1e' \
> +  -device 'pci-bridge,id=pci.2,chassis_nr=2,bus=pci.0,addr=0x1f' \
> +  -device 'piix3-usb-uhci,id=uhci,bus=pci.0,addr=0x1.0x2' \
> +  -device 'usb-tablet,id=tablet,bus=uhci.0,port=1' \
> +  -device 'VGA,id=vga,bus=pci.0,addr=0x2' \
> +  -device 'virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3,free-page-reporting=on' \
> +  -iscsi 'initiator-name=iqn.1993-08.org.debian:01:aabbccddeeff' \
> +  -device 'lsi,id=scsihw0,bus=pci.0,addr=0x5' \
> +  -blockdev '{"driver":"throttle","file":{"backing":{"backing":{"cache":{"direct":true,"no-flush":false},"driver":"qcow2","file":{"aio":"io_uring","cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","driver":"file","filename":"/var/lib/vzsnapext/images/8006/snap1-vm-8006-disk-0.qcow2","node-name":"ea91a385a49a008a4735c0aec5c6749","read-only":false},"node-name":"fa91a385a49a008a4735c0aec5c6749","read-only":false},"cache":{"direct":true,"no-flush":false},"driver":"qcow2","file":{"aio":"io_uring","cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","driver":"file","filename":"/var/lib/vzsnapext/images/8006/snap2-vm-8006-disk-0.qcow2","node-name":"ec0289317073959d450248d8cd7a480","read-only":false},"node-name":"fc0289317073959d450248d8cd7a480","read-only":false},"cache":{"direct":true,"no-flush":false},"driver":"qcow2","file":{"aio":"io_uring","cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","driv
 er":"file","filename":"/var/lib/vzsnapext/images/8006/vm-8006-disk-0.qcow2","node-name":"e74f4959037afb46eddc7313c43dfdd","read-only":false},"node-name":"f74f4959037afb46eddc7313c43dfdd","read-only":false},"node-name":"drive-scsi0","throttle-group":"throttle-drive-scsi0"}' \
> +  -device 'scsi-hd,bus=scsihw0.0,scsi-id=0,drive=drive-scsi0,id=scsi0,write-cache=on' \
> +  -blockdev '{"driver":"throttle","file":{"backing":{"backing":{"cache":{"direct":true,"no-flush":false},"driver":"qcow2","file":{"aio":"native","cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","driver":"host_device","filename":"/dev/veegee/snap1-vm-8006-disk-0.qcow2","node-name":"e25f58d3e6e11f2065ad41253988915","read-only":false},"node-name":"f25f58d3e6e11f2065ad41253988915","read-only":false},"cache":{"direct":true,"no-flush":false},"driver":"qcow2","file":{"aio":"native","cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","driver":"host_device","filename":"/dev/veegee/snap2-vm-8006-disk-0.qcow2","node-name":"e9415bb5e484c1e25d25063b01686fe","read-only":false},"node-name":"f9415bb5e484c1e25d25063b01686fe","read-only":false},"cache":{"direct":true,"no-flush":false},"driver":"qcow2","file":{"aio":"native","cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","driver":"host_device","filename":"
 /dev/veegee/vm-8006-disk-0.qcow2","node-name":"e87358a470ca311f94d5cc61d1eb428","read-only":false},"node-name":"f87358a470ca311f94d5cc61d1eb428","read-only":false},"node-name":"drive-scsi1","throttle-group":"throttle-drive-scsi1"}' \
> +  -device 'scsi-hd,bus=scsihw0.0,scsi-id=1,drive=drive-scsi1,id=scsi1,write-cache=on' \
> +  -machine 'type=pc+pve0'
> diff --git a/src/test/run_config2command_tests.pl b/src/test/run_config2command_tests.pl
> index 1262a0df..61302f6b 100755
> --- a/src/test/run_config2command_tests.pl
> +++ b/src/test/run_config2command_tests.pl
> @@ -21,6 +21,7 @@ use PVE::QemuServer::Helpers;
>  use PVE::QemuServer::Monitor;
>  use PVE::QemuServer::QMPHelpers;
>  use PVE::QemuServer::CPUConfig;
> +use PVE::Storage;
>  
>  my $base_env = {
>      storage_config => {
> @@ -34,6 +35,15 @@ my $base_env = {
>                  type => 'dir',
>                  shared => 0,
>              },
> +            localsnapext => {
> +                content => {
> +                    images => 1,
> +                },
> +                path => '/var/lib/vzsnapext',
> +                type => 'dir',
> +                shared => 0,
> +                snapext => 1,
> +            },
>              noimages => {
>                  content => {
>                      iso => 1,
> @@ -264,6 +274,43 @@ $storage_module->mock(
>      deactivate_volumes => sub {
>          return;
>      },
> +    volume_snapshot_info => sub {
> +        my ($cfg, $volid) = @_;
> +
> +        my ($storeid, $volname) = PVE::Storage::parse_volume_id($volid);
> +
> +        my $snapshots = {};
> +        if ($storeid eq 'localsnapext') {
> +            $snapshots = {
> +                current => {
> +                    file => 'var/lib/vzsnapext/images/8006/vm-8006-disk-0.qcow2',
> +                    parent => 'snap2',
> +                },
> +                snap2 => {
> +                    file => '/var/lib/vzsnapext/images/8006/snap2-vm-8006-disk-0.qcow2',
> +                    parent => 'snap1',
> +                },
> +                snap1 => {
> +                    file => '/var/lib/vzsnapext/images/8006/snap1-vm-8006-disk-0.qcow2',
> +                },
> +            };
> +        } elsif ($storeid eq 'lvm-store') {
> +            $snapshots = {
> +                current => {
> +                    file => '/dev/veegee/vm-8006-disk-0.qcow2',
> +                    parent => 'snap2',
> +                },
> +                snap2 => {
> +                    file => '/dev/veegee/snap2-vm-8006-disk-0.qcow2',
> +                    parent => 'snap1',
> +                },
> +                snap1 => {
> +                    file => '/dev/veegee/snap1-vm-8006-disk-0.qcow2',
> +                },
> +            };
> +        }
> +        return $snapshots;
> +    },
>  );
>  
>  my $file_stat_module = Test::MockModule->new("File::stat");
> -- 
> 2.39.5


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