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

[Alexandre Derumier via pve-devel <pve-devel@lists.proxmox.com>]

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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>

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 {
+    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","driver":"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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