[pve-devel] [PATCH pve-storage 7/9] qcow2: add external snapshot 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 pve-storage 7/9] qcow2: add external snapshot support
Date: Tue,  3 Jun 2025 09:55:50 +0200
Message-ID: <20250603075558.627850-16-alexandre.derumier@groupe-cyllene.com>

add a snapext option to enable the feature

When a snapshot is taken, the current volume is renamed to snap volname
and a current image is created with the snap volume as backing file

Signed-off-by: Alexandre Derumier <alexandre.derumier@groupe-cyllene.com>
---
 src/PVE/Storage.pm           |   1 -
 src/PVE/Storage/Common.pm    |   3 +-
 src/PVE/Storage/DirPlugin.pm |   1 +
 src/PVE/Storage/Plugin.pm    | 241 +++++++++++++++++++++++++++++++++--
 4 files changed, 230 insertions(+), 16 deletions(-)

diff --git a/src/PVE/Storage.pm b/src/PVE/Storage.pm
index 973161f..2a2005b 100755
--- a/src/PVE/Storage.pm
+++ b/src/PVE/Storage.pm
@@ -378,7 +378,6 @@ sub volume_snapshot_rollback {
     }
 }
 
-# FIXME PVE 8.x remove $running parameter (needs APIAGE reset)
 sub volume_snapshot_delete {
     my ($cfg, $volid, $snap, $running) = @_;
 
diff --git a/src/PVE/Storage/Common.pm b/src/PVE/Storage/Common.pm
index 11b5b94..7e87a54 100644
--- a/src/PVE/Storage/Common.pm
+++ b/src/PVE/Storage/Common.pm
@@ -217,10 +217,11 @@ Returns a json with qemu image C<$filename> informations with format <$file_form
 =cut
 
 sub qemu_img_info {
-    my ($filename, $file_format, $timeout) = @_;
+    my ($filename, $file_format, $timeout, $follow_backing_files) = @_;
 
     my $cmd = ['/usr/bin/qemu-img', 'info', '--output=json', $filename];
     push $cmd->@*, '-f', $file_format if $file_format;
+    push $cmd->@*, '--backing-chain' if $follow_backing_files;
 
     my $json = '';
     my $err_output = '';
diff --git a/src/PVE/Storage/DirPlugin.pm b/src/PVE/Storage/DirPlugin.pm
index 734309f..54d8d74 100644
--- a/src/PVE/Storage/DirPlugin.pm
+++ b/src/PVE/Storage/DirPlugin.pm
@@ -83,6 +83,7 @@ sub options {
 	is_mountpoint => { optional => 1 },
 	bwlimit => { optional => 1 },
 	preallocation => { optional => 1 },
+	snapext => { optional => 1 },
    };
 }
 
diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 00eebd3..fd50b9e 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -202,6 +202,11 @@ my $defaultData = {
 	    maximum => 65535,
 	    optional => 1,
 	},
+        'snapext' => {
+	    type => 'boolean',
+	    description => 'enable external snapshot.',
+	    optional => 1,
+        },
     },
 };
 
@@ -696,6 +701,8 @@ sub filesystem_path {
     my ($vtype, $name, $vmid, undef, undef, $isBase, $format) =
 	$class->parse_volname($volname);
 
+    $name = $class->get_snap_name($volname, $snapname) if $scfg->{snapext} && $snapname;
+
     # Note: qcow2/qed has internal snapshot, so path is always
     # the same (with or without snapshot => same file).
     die "can't snapshot this image format\n"
@@ -896,6 +903,26 @@ sub alloc_image {
     return "$vmid/$name";
 }
 
+my sub alloc_backed_image {
+    my ($class, $storeid, $scfg, $volname, $backing_snap) = @_;
+
+    my $path = $class->path($scfg, $volname, $storeid);
+    my $backing_path = $class->path($scfg, $volname, $storeid, $backing_snap);
+
+    eval { PVE::Storage::Common::qemu_img_create_qcow2_backed($scfg, $path, $backing_path, 'qcow2') };
+    if ($@) {
+	unlink $path;
+	die "$@";
+    }
+}
+
+my sub free_snap_image {
+    my ($class, $storeid, $scfg, $volname, $snap) = @_;
+
+    my $path = $class->path($scfg, $volname, $storeid, $snap);
+    unlink($path) || die "unlink '$path' failed - $!\n";
+}
+
 sub free_image {
     my ($class, $storeid, $scfg, $volname, $isBase, $format) = @_;
 
@@ -918,6 +945,17 @@ sub free_image {
 	    return undef;
 	}
 
+	#delete external snapshots
+	if($scfg->{snapext}) {
+	    my $snapshots =  $class->volume_snapshot_info($scfg, $storeid, $volname);
+	    for my $snapid (sort { $snapshots->{$b}->{order} <=> $snapshots->{$a}->{order} } keys %$snapshots) {
+		my $snap = $snapshots->{$snapid};
+		next if $snapid eq 'current';
+		next if !$snap->{ext};
+		free_snap_image($class, $storeid, $scfg, $volname, $snapid);
+	    }
+	}
+
 	unlink($path) || die "unlink '$path' failed - $!\n";
     }
 
@@ -1122,11 +1160,33 @@ sub volume_snapshot {
 
     die "can't snapshot this image format\n" if $volname !~ m/\.(qcow2|qed)$/;
 
-    my $path = $class->filesystem_path($scfg, $volname);
+    if($scfg->{snapext}) {
 
-    my $cmd = ['/usr/bin/qemu-img', 'snapshot','-c', $snap, $path];
+	my $vmid = ($class->parse_volname($volname))[2];
 
-    run_command($cmd);
+	#if running, the old current has been renamed with blockdev-reopen by qemu
+	if (!$running) {
+	    #rename current volume to snap volume
+	    $class->rename_volume($scfg, $storeid, $volname, $vmid, undef, 'current', $snap);
+	}
+
+	eval { alloc_backed_image($class, $storeid, $scfg, $volname, $snap) };
+	if ($@) {
+	    warn "$@ \n";
+	    #if running, the revert is done by qemu with blockdev-reopen
+	    if (!$running) {
+		eval { $class->rename_volume($scfg, $storeid, $volname, $vmid, undef, $snap, 'current') };
+		warn $@ if $@;
+	    }
+	    die "can't allocate new volume $volname with $snap backing image\n";
+	}
+
+    } else {
+
+	my $path = $class->filesystem_path($scfg, $volname);
+	my $cmd = ['/usr/bin/qemu-img', 'snapshot','-c', $snap, $path];
+	run_command($cmd);
+    }
 
     return undef;
 }
@@ -1137,6 +1197,21 @@ sub volume_snapshot {
 sub volume_rollback_is_possible {
     my ($class, $scfg, $storeid, $volname, $snap, $blockers) = @_;
 
+    if ($scfg->{snapext}) {
+	#technically, we could manage multibranch, we it need lot more work for snapshot delete
+	#we need to implemente block-stream from deleted snapshot to all others child branchs
+	#when online, we need to do a transaction for multiple disk when delete the last snapshot
+	#and need to merge in current running file
+
+	my $snappath = $class->path($scfg, $volname, $storeid, $snap);
+	my $snapshots = $class->volume_snapshot_info($scfg, $storeid, $volname);
+	my $parentsnap = $snapshots->{current}->{parent};
+
+	return 1 if $parentsnap eq $snap;
+
+	die "can't rollback, '$snap' is not most recent snapshot on '$volname'\n";
+    }
+
     return 1;
 }
 
@@ -1145,11 +1220,22 @@ sub volume_snapshot_rollback {
 
     die "can't rollback snapshot this image format\n" if $volname !~ m/\.(qcow2|qed)$/;
 
-    my $path = $class->filesystem_path($scfg, $volname);
-
-    my $cmd = ['/usr/bin/qemu-img', 'snapshot','-a', $snap, $path];
+    if ($scfg->{snapext}) {
+	#simply delete the current snapshot and recreate it
+	eval { free_snap_image($class, $storeid, $scfg, $volname, 'current') };
+	if ($@) {
+	    die "can't delete old volume $volname: $@\n";
+	}
 
-    run_command($cmd);
+	eval { alloc_backed_image($class, $storeid, $scfg, $volname, $snap) };
+	if ($@) {
+	    die "can't allocate new volume $volname: $@\n";
+	}
+    } else {
+	my $path = $class->filesystem_path($scfg, $volname);
+	my $cmd = ['/usr/bin/qemu-img', 'snapshot','-a', $snap, $path];
+	run_command($cmd);
+    }
 
     return undef;
 }
@@ -1159,15 +1245,71 @@ sub volume_snapshot_delete {
 
     die "can't delete snapshot for this image format\n" if $volname !~ m/\.(qcow2|qed)$/;
 
-    return 1 if $running;
+    my $cmd = "";
 
-    my $path = $class->filesystem_path($scfg, $volname);
+    if ($scfg->{snapext}) {
 
-    $class->deactivate_volume($storeid, $scfg, $volname, $snap, {});
+	#qemu has already live commit|stream the snapshot, therefore we only have to drop the image itself
+	if ($running) {
+	    eval { free_snap_image($class, $storeid, $scfg, $volname, $snap) };
+	    if ($@) {
+		die "can't delete snapshot $snap of volume $volname: $@\n";
+	    }
+	    return;
+	}
+
+	my $snapshots = $class->volume_snapshot_info($scfg, $storeid, $volname);
+	my $snappath = $snapshots->{$snap}->{file};
+	my $snap_volname = $snapshots->{$snap}->{volname};
+	die "volume $snappath is missing" if !-e $snappath;
+
+	my $parentsnap = $snapshots->{$snap}->{parent};
+	my $childsnap = $snapshots->{$snap}->{child};
+	my $childpath = $snapshots->{$childsnap}->{file};
+
+	#if first snapshot,as it should be bigger,  we merge child, and rename the snapshot to child
+	if(!$parentsnap) {
+	    print "$volname: deleting snapshot '$snap' by commiting snapshot '$childsnap'\n";
+	    print "running 'qemu-img commit $childpath'\n";
+	    $cmd = ['/usr/bin/qemu-img', 'commit', $childpath];
+	    eval { run_command($cmd) };
+	    if ($@) {
+		warn "The state of $snap is now invalid. Don't try to clone or rollback it. You can only try to delete it again later\n";
+		die "error commiting $childsnap to $snap; $@\n";
+	    }
+
+	    print"rename $snappath to $childpath\n";
+	    rename($snappath, $childpath) ||
+		die "rename '$snappath' to '$childpath' failed - $!\n";
+
+	} 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";
+	    print "running 'qemu-img rebase -b $parentpath -F qcow -f qcow2 $childpath'\n";
+	    $cmd = ['/usr/bin/qemu-img', 'rebase', '-b', $parentpath, '-F', 'qcow2', '-f', 'qcow2', $childpath];
+	    eval { run_command($cmd) };
+	    if ($@) {
+		#in case of abort, the state of the snap is still clean, just a little bit bigger
+		die "error rebase $childsnap from $parentsnap; $@\n";
+	    }
+	    #delete the old snapshot file (not part of the backing chain anymore)
+	    eval { free_snap_image($class, $storeid, $scfg, $volname, $snap) };
+	    if ($@) {
+		die "error delete old snapshot volume $snap_volname: $@\n";
+	    }
+	}
+
+    } else {
 
-    my $cmd = ['/usr/bin/qemu-img', 'snapshot','-d', $snap, $path];
+	return 1 if $running;
 
-    run_command($cmd);
+	my $path = $class->filesystem_path($scfg, $volname);
+	$class->deactivate_volume($storeid, $scfg, $volname, $snap, {});
+
+	$cmd = ['/usr/bin/qemu-img', 'snapshot','-d', $snap, $path];
+	run_command($cmd);
+    }
 
     return undef;
 }
@@ -1441,7 +1583,52 @@ sub status {
 sub volume_snapshot_info {
     my ($class, $scfg, $storeid, $volname) = @_;
 
-    die "volume_snapshot_info is not implemented for $class";
+    my $path = $class->filesystem_path($scfg, $volname);
+    my ($vtype, $name, $vmid, $basename, $basevmid, $isBase, $format) = $class->parse_volname($volname);
+
+    my $json = PVE::Storage::Common::qemu_img_info($path, undef, 10, 1);
+    die "failed to query file information with qemu-img\n" if !$json;
+    my $json_decode = eval { decode_json($json) };
+    if ($@) {
+	die "Can't decode qemu snapshot list. Invalid JSON: $@\n";
+    }
+    my $info = {};
+    my $order = 0;
+    if (ref($json_decode) eq 'HASH') {
+	#internal snapshots is a hashref
+	my $snapshots = $json_decode->{snapshots};
+	for my $snap (@$snapshots) {
+	    my $snapname = $snap->{name};
+	    $info->{$snapname}->{order} = $snap->{id};
+	    $info->{$snapname}->{timestamp} = $snap->{'date-sec'};
+
+	}
+    } elsif (ref($json_decode) eq 'ARRAY') {
+	#no snapshot or external  snapshots is an arrayref
+	my $snapshots = $json_decode;
+	for my $snap (@$snapshots) {
+	    my $snapfile = $snap->{filename};
+	    my $snapname = parse_snapname($snapfile);
+	    $snapname = 'current' if !$snapname;
+	    my $snapvolname = $class->get_snap_volname($volname, $snapname);
+
+	    $info->{$snapname}->{order} = $order;
+	    $info->{$snapname}->{file}= $snapfile;
+	    $info->{$snapname}->{volname} = "$snapvolname";
+	    $info->{$snapname}->{volid} = "$storeid:$snapvolname";
+	    $info->{$snapname}->{ext} = 1;
+
+	    my $parentfile = $snap->{'backing-filename'};
+	    if ($parentfile) {
+		my $parentname = parse_snapname($parentfile);
+		$info->{$snapname}->{parent} = $parentname;
+		$info->{$parentname}->{child} = $snapname;
+	    }
+	    $order++;
+	}
+    }
+
+    return $info;
 }
 
 sub activate_storage {
@@ -1896,7 +2083,7 @@ sub qemu_blockdev_options {
        # the snapshot alone.
        my $format = ($class->parse_volname($volname))[6];
        die "cannot attach only the snapshot of a '$format' image\n"
-           if $options->{'snapshot-name'} && ($format eq 'qcow2' || $format eq 'qed');
+           if $options->{'snapshot-name'} && !$scfg->{snapext} && ($format eq 'qcow2' || $format eq 'qed');
 
        # The 'file' driver only works for regular files. The check below is taken from
        # block/file-posix.c:hdev_probe_device() in QEMU. Do not bother with detecting 'host_cdrom'
@@ -1935,4 +2122,30 @@ sub config_aware_base_mkdir {
     }
 }
 
+sub get_snap_name {
+    my ($class, $volname, $snapname) = @_;
+
+    my ($vtype, $name, $vmid, $basename, $basevmid, $isBase, $format) = $class->parse_volname($volname);
+    $name = !$snapname || $snapname eq 'current' ? $name : "snap-$snapname-$name";
+    return $name;
+}
+
+sub get_snap_volname {
+    my ($class, $volname, $snapname) = @_;
+
+    my $vmid = ($class->parse_volname($volname))[2];
+    my $name = $class->get_snap_name($volname, $snapname);
+    return "$vmid/$name";
+}
+
+sub parse_snapname {
+    my ($name) = @_;
+
+    my $basename = basename($name);
+    if ($basename =~ m/^snap-(.*)-vm(.*)$/) {
+	return $1;
+    }
+    return undef;
+}
+
 1;
-- 
2.39.5



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