From a3a9a2ab190a6e8beee79b25c033f76aaadc97b5 Mon Sep 17 00:00:00 2001 From: Fiona Ebner Date: Thu, 24 Jul 2025 13:16:50 +0200 Subject: [PATCH] fix #6543: use qcow2 'discard-no-unref' option when using snapshot-as-volume-chain Without the 'discard-no-unref', a qcow2 file can grow beyond what 'qemu-img measure' reports, because of fragmentation. This can lead to IO errors with qcow2 on top of LVM storages, where the containing LV is allocated with that size. Guard enabling the option with having 'snapshot-as-volume-chain' in the storage configuration for now. Enabling it always should be evaluated a bit more and tested on different storages. It is a runtime-only option just affecting how referencing clusters is handled during discard in qcow2 and nothing else, so it is also fine for existing images and migration streams. While 'snapshot-as-volume-chain' is not the perfect proxy, as that's not only for LVM, it's an experimental feature that covers the LVM case and it seems like a nice fit to try out the new option on file-based storages too. Suggested-by: Alexandre Derumier Signed-off-by: Fiona Ebner --- src/PVE/QemuServer/Blockdev.pm | 7 ++++++ src/PVE/QemuServer/QemuImage.pm | 20 +++++++++++++++++ src/test/cfg2cmd/simple-backingchain.conf.cmd | 2 +- src/test/run_qemu_img_convert_tests.pl | 22 ++++++++++++++----- 4 files changed, 44 insertions(+), 7 deletions(-) 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 {