diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm
index 9d84117e..6e105571 100644
--- a/src/PVE/QemuServer/Blockdev.pm
+++ b/src/PVE/QemuServer/Blockdev.pm
@@ -373,6 +373,13 @@ my sub generate_format_blockdev {
         $blockdev->{size} = int($options->{size});
     }
 
+    # see bug #6543: without this option, fragmentation can lead to the qcow2 file growing larger
+    # than what qemu-img measure reports, which is problematic for qcow2-on-top-of-LVM
+    # TODO test and consider enabling this in general
+    if ($scfg && $scfg->{'snapshot-as-volume-chain'}) {
+        $blockdev->{'discard-no-unref'} = JSON::true if $format eq 'qcow2';
+    }
+
     return $blockdev;
 }
 
diff --git a/src/PVE/QemuServer/QemuImage.pm b/src/PVE/QemuServer/QemuImage.pm
index 026c24e9..804cbc8e 100644
--- a/src/PVE/QemuServer/QemuImage.pm
+++ b/src/PVE/QemuServer/QemuImage.pm
@@ -3,6 +3,9 @@ package PVE::QemuServer::QemuImage;
 use strict;
 use warnings;
 
+use Fcntl qw(S_ISBLK);
+use File::stat;
+
 use PVE::Format qw(render_bytes);
 use PVE::Storage;
 use PVE::Tools;
@@ -27,6 +30,18 @@ sub convert_iscsi_path {
     die "cannot convert iscsi path '$path', unknown format\n";
 }
 
+my sub qcow2_target_image_opts {
+    my ($path, @qcow2_opts) = @_;
+
+    my $st = File::stat::stat($path) or die "stat for '$path' failed - $!\n";
+
+    my $driver = S_ISBLK($st->mode) ? 'host_device' : 'file';
+
+    my $qcow2_opts_str = ',' . join(',', @qcow2_opts);
+
+    return "driver=qcow2$qcow2_opts_str,file.driver=$driver,file.filename=$path";
+}
+
 # The possible options are:
 # bwlimit - The bandwidth limit in KiB/s.
 # is-zero-initialized - If the destination image is zero-initialized.
@@ -71,6 +86,8 @@ sub convert {
     my $dst_format = checked_volume_format($storecfg, $dst_volid);
     my $dst_path = PVE::Storage::path($storecfg, $dst_volid);
     my $dst_is_iscsi = ($dst_path =~ m|^iscsi://|);
+    my $dst_needs_discard_no_unref =
+        $dst_scfg->{'snapshot-as-volume-chain'} && $dst_format eq 'qcow2';
     my $support_qemu_snapshots = PVE::Storage::volume_qemu_snapshot_method($storecfg, $src_volid);
 
     my $cmd = [];
@@ -94,6 +111,9 @@ sub convert {
     if ($dst_is_iscsi) {
         push @$cmd, '--target-image-opts';
         $dst_path = convert_iscsi_path($dst_path);
+    } elsif ($dst_needs_discard_no_unref) {
+        push @$cmd, '--target-image-opts';
+        $dst_path = qcow2_target_image_opts($dst_path, 'discard-no-unref=true');
     } else {
         push @$cmd, '-O', $dst_format;
     }
diff --git a/src/test/cfg2cmd/simple-backingchain.conf.cmd b/src/test/cfg2cmd/simple-backingchain.conf.cmd
index cae2ad4b..4ac24b93 100644
--- a/src/test/cfg2cmd/simple-backingchain.conf.cmd
+++ b/src/test/cfg2cmd/simple-backingchain.conf.cmd
@@ -26,7 +26,7 @@
   -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 '{"detect-zeroes":"on","discard":"ignore","driver":"throttle","file":{"backing":{"backing":{"cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","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},"detect-zeroes":"on","discard":"ignore","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},"detect-zeroes":"on","discard":"ignore","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","read-only":false,"throttle-group":"throttle-drive-scsi0"}' \
+  -blockdev '{"detect-zeroes":"on","discard":"ignore","driver":"throttle","file":{"backing":{"backing":{"cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","discard-no-unref":true,"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},"detect-zeroes":"on","discard":"ignore","discard-no-unref":true,"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},"detect-zeroes":"on","discard":"ignore","discard-no-unref":true,"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","read-only":false,"throttle-group":"throttle-drive-scsi0"}' \
   -device 'scsi-hd,bus=scsihw0.0,scsi-id=0,drive=drive-scsi0,id=scsi0,device_id=drive-scsi0,write-cache=on' \
   -blockdev '{"detect-zeroes":"on","discard":"ignore","driver":"throttle","file":{"backing":{"backing":{"cache":{"direct":true,"no-flush":false},"detect-zeroes":"on","discard":"ignore","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},"detect-zeroes":"on","discard":"ignore","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},"detect-zeroes":"on","discard":"ignore","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","read-only":false,"throttle-group":"throttle-drive-scsi1"}' \
   -device 'scsi-hd,bus=scsihw0.0,scsi-id=1,drive=drive-scsi1,id=scsi1,device_id=drive-scsi1,write-cache=on' \
diff --git a/src/test/run_qemu_img_convert_tests.pl b/src/test/run_qemu_img_convert_tests.pl
index 64c98327..3c8f09f0 100755
--- a/src/test/run_qemu_img_convert_tests.pl
+++ b/src/test/run_qemu_img_convert_tests.pl
@@ -542,10 +542,10 @@ my $tests = [
             "-n",
             "-f",
             "raw",
-            "-O",
-            "qcow2",
+            "--target-image-opts",
             "/var/lib/vz/images/$vmid/vm-$vmid-disk-0.raw",
-            "/var/lib/vzsnapext/images/$vmid/vm-$vmid-disk-0.qcow2",
+            "driver=qcow2,discard-no-unref=true,file.driver=file,"
+                . "file.filename=/var/lib/vzsnapext/images/$vmid/vm-$vmid-disk-0.qcow2",
         ],
     },
     {
@@ -560,16 +560,26 @@ my $tests = [
             "-n",
             "-f",
             "raw",
-            "-O",
-            "qcow2",
+            "--target-image-opts",
             "/var/lib/vz/images/$vmid/vm-$vmid-disk-0.raw",
-            "/dev/pve/vm-$vmid-disk-0.qcow2",
+            "driver=qcow2,discard-no-unref=true,file.driver=host_device,"
+                . "file.filename=/dev/pve/vm-$vmid-disk-0.qcow2",
         ],
     },
 ];
 
 my $command;
 
+my $file_stat_module = Test::MockModule->new("File::stat");
+$file_stat_module->mock(
+    stat => sub {
+        my ($path) = @_;
+        my $st = $file_stat_module->original('stat')->('./run_qemu_img_convert_tests.pl');
+        $st->[2] = 25008 if $path =~ m!/dev/!; # block device
+        return $st;
+    },
+);
+
 my $storage_module = Test::MockModule->new("PVE::Storage");
 $storage_module->mock(
     config => sub {