* [PATCH qemu-server 1/3] partially fix #1989: disk: add qcow2 cache options
2026-06-05 15:35 [PATCH docs/manager/qemu-server 0/3] partially fix #1989: add QEMU qcow2 cache options Erik Fastermann
@ 2026-06-05 15:35 ` Erik Fastermann
2026-06-05 15:35 ` [PATCH pve-manager 2/3] partially fix #1989: ui: qemu: disk: add qcow2 cache size config Erik Fastermann
2026-06-05 15:35 ` [PATCH pve-docs 3/3] partially fix #1989: qm: document " Erik Fastermann
2 siblings, 0 replies; 4+ messages in thread
From: Erik Fastermann @ 2026-06-05 15:35 UTC (permalink / raw)
To: pve-devel; +Cc: Erik Fastermann
Add multiple options to configure the qcow2 L2/refcount cache. This
can provide significant performance gains in some cases.
For a detailed explanation of the options see the QEMU docs [0].
Additionally the cache size can be configured based on the size of the
disk image automatically.
Both blockdev and the older drive commandline are supported, which makes
the feature accessible to all QEMU machine versions supported by PVE.
Options which only apply to disk creation (cluster_size, refcount_bits)
are not considered in this patch.
[0] https://gitlab.com/qemu-project/qemu/-/blob/master/docs/qcow2-cache.txt
Signed-off-by: Erik Fastermann <e.fastermann@proxmox.com>
---
src/PVE/QemuServer.pm | 71 ++++++++++++++++++-
src/PVE/QemuServer/Blockdev.pm | 44 ++++++++++++
src/PVE/QemuServer/Drive.pm | 122 ++++++++++++++++++++++++++++++++
src/PVE/QemuServer/QemuImage.pm | 24 +++++++
4 files changed, 260 insertions(+), 1 deletion(-)
diff --git a/src/PVE/QemuServer.pm b/src/PVE/QemuServer.pm
index 55e9f520..211927db 100644
--- a/src/PVE/QemuServer.pm
+++ b/src/PVE/QemuServer.pm
@@ -78,6 +78,7 @@ use PVE::QemuServer::Drive qw(
parse_drive
print_drive
storage_allows_io_uring_default
+ qcow2_cache_size_by_disk_size
);
use PVE::QemuServer::DriveDevice qw(print_drivedevice_full scsihw_infos);
use PVE::QemuServer::Machine;
@@ -1294,6 +1295,49 @@ sub print_drive_commandline_full {
$opts .= ",auto-remove=on";
}
+ for my $qcow2_cache_option (
+ 'qcow2-cache-size',
+ 'qcow2-l2-cache-size',
+ 'qcow2-l2-cache-entry-size',
+ 'qcow2-refcount-cache-size',
+ 'qcow2-cache-clean-interval',
+ ) {
+ if (defined($drive->{$qcow2_cache_option}) && $format ne 'qcow2') {
+ log_warn("$drive_id: $qcow2_cache_option requires disk format qcow2");
+ }
+ }
+
+ if ($format eq 'qcow2') {
+ if (
+ defined($drive->{'qcow2-cache-size-based-on-disk'})
+ && $drive->{'qcow2-cache-size-based-on-disk'}
+ ) {
+ my $cache_size = qcow2_cache_size_by_disk_size($storecfg, $drive);
+ $opts .= ",cache-size=" . int($cache_size);
+ }
+
+ if (defined($drive->{'qcow2-cache-size'})) {
+ $opts .= ",cache-size=" . int($drive->{'qcow2-cache-size'}) * 1024 * 1024;
+ }
+
+ if (defined($drive->{'qcow2-l2-cache-size'})) {
+ $opts .= ",l2-cache-size=" . int($drive->{'qcow2-l2-cache-size'}) * 1024 * 1024;
+ }
+
+ if (defined($drive->{'qcow2-l2-cache-entry-size'})) {
+ $opts .= ",l2-cache-entry-size=" . int($drive->{'qcow2-l2-cache-entry-size'});
+ }
+
+ if (defined($drive->{'qcow2-refcount-cache-size'})) {
+ $opts .=
+ ",refcount-cache-size=" . int($drive->{'qcow2-refcount-cache-size'}) * 1024 * 1024;
+ }
+
+ if (defined($drive->{'qcow2-cache-clean-interval'})) {
+ $opts .= ",cache-clean-interval=" . int($drive->{'qcow2-cache-clean-interval'});
+ }
+ }
+
# my $file_param = $live_restore_name ? "file.file.filename" : "file";
my $file_param = "file";
if ($live_restore_name) {
@@ -5217,6 +5266,26 @@ sub vmconfig_update_disk {
|| safe_string_ne($drive->{ssd}, $old_drive->{ssd})
|| safe_string_ne($drive->{vendor}, $old_drive->{vendor})
|| safe_string_ne($drive->{ro}, $old_drive->{ro})
+ || safe_string_ne(
+ $drive->{'qcow2-cache-size'},
+ $old_drive->{'qcow2-cache-size'},
+ )
+ || safe_num_ne(
+ $drive->{'qcow2-l2-cache-size'},
+ $old_drive->{'qcow2-l2-cache-size'},
+ )
+ || safe_num_ne(
+ $drive->{'qcow2-l2-cache-entry-size'},
+ $old_drive->{'qcow2-l2-cache-entry-size'},
+ )
+ || safe_num_ne(
+ $drive->{'qcow2-refcount-cache-size'},
+ $old_drive->{'qcow2-refcount-cache-size'},
+ )
+ || safe_num_ne(
+ $drive->{'qcow2-cache-clean-interval'},
+ $old_drive->{'qcow2-cache-clean-interval'},
+ )
) {
die "skip\n";
}
diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm
index 101c747c..0dd5067d 100644
--- a/src/PVE/QemuServer/Blockdev.pm
+++ b/src/PVE/QemuServer/Blockdev.pm
@@ -9,6 +9,7 @@ use File::stat;
use JSON;
use PVE::JSONSchema qw(json_bool);
+use PVE::RESTEnvironment qw(log_warn);
use PVE::Storage;
use PVE::QemuServer::Drive qw(drive_is_cdrom);
@@ -403,6 +404,49 @@ sub generate_format_blockdev {
$blockdev->{'discard-no-unref'} = JSON::true if $format eq 'qcow2';
}
+ for my $qcow2_cache_option (
+ 'qcow2-cache-size',
+ 'qcow2-l2-cache-size',
+ 'qcow2-l2-cache-entry-size',
+ 'qcow2-refcount-cache-size',
+ 'qcow2-cache-clean-interval',
+ ) {
+ if (defined($drive->{$qcow2_cache_option}) && $format ne 'qcow2') {
+ log_warn("$drive_id: $qcow2_cache_option requires disk format qcow2");
+ }
+ }
+
+ if ($format eq 'qcow2') {
+ if (
+ defined($drive->{'qcow2-cache-size-based-on-disk'})
+ && $drive->{'qcow2-cache-size-based-on-disk'}
+ ) {
+ my $cache_size = qcow2_cache_size_by_disk_size($storecfg, $drive);
+ $blockdev->{'cache-size'} = int($cache_size);
+ }
+
+ if (defined($drive->{'qcow2-cache-size'})) {
+ $blockdev->{'cache-size'} = int($drive->{'qcow2-cache-size'}) * 1024 * 1024;
+ }
+
+ if (defined($drive->{'qcow2-l2-cache-size'})) {
+ $blockdev->{'l2-cache-size'} = int($drive->{'qcow2-l2-cache-size'}) * 1024 * 1024;
+ }
+
+ if (defined($drive->{'qcow2-l2-cache-entry-size'})) {
+ $blockdev->{'l2-cache-entry-size'} = int($drive->{'qcow2-l2-cache-entry-size'});
+ }
+
+ if (defined($drive->{'qcow2-refcount-cache-size'})) {
+ $blockdev->{'refcount-cache-size'} =
+ int($drive->{'qcow2-refcount-cache-size'}) * 1024 * 1024;
+ }
+
+ if (defined($drive->{'qcow2-cache-clean-interval'})) {
+ $blockdev->{'cache-clean-interval'} = int($drive->{'qcow2-cache-clean-interval'});
+ }
+ }
+
return $blockdev;
}
diff --git a/src/PVE/QemuServer/Drive.pm b/src/PVE/QemuServer/Drive.pm
index b80b7dbb..e123947e 100644
--- a/src/PVE/QemuServer/Drive.pm
+++ b/src/PVE/QemuServer/Drive.pm
@@ -7,6 +7,7 @@ use Storable qw(dclone);
use IO::File;
use List::Util qw(first);
+use POSIX;
use PVE::RESTEnvironment qw(log_warn);
use PVE::Storage;
@@ -26,6 +27,7 @@ our @EXPORT_OK = qw(
parse_drive
print_drive
storage_allows_io_uring_default
+ qcow2_cache_size_by_disk_size
);
my $DROPPED_PROPERTIES = ['cyls', 'heads', 'secs', 'trans'];
@@ -256,6 +258,46 @@ my %drivedesc_base = (
optional => 1,
default => 0,
},
+ 'qcow2-cache-size' => {
+ type => 'integer',
+ minimum => 1,
+ maximum => 10 * 1024,
+ description => 'Cache size for qcow2 disks in MiB',
+ optional => 1,
+ },
+ 'qcow2-cache-size-based-on-disk' => {
+ type => 'boolean',
+ description =>
+ 'Automatically pick a qcow2 cache size based on the configured disk size',
+ optional => 1,
+ },
+ 'qcow2-l2-cache-size' => {
+ type => 'integer',
+ minimum => 1,
+ maximum => 10 * 1024,
+ description => 'L2 cache size for qcow2 disks in MiB',
+ optional => 1,
+ },
+ 'qcow2-l2-cache-entry-size' => {
+ type => 'integer',
+ minimum => 512,
+ maximum => 2 * 1024 * 1024,
+ description => 'L2 cache entry size for qcow2 disks in bytes',
+ optional => 1,
+ },
+ 'qcow2-refcount-cache-size' => {
+ type => 'integer',
+ minimum => 1,
+ maximum => 10 * 1024,
+ description => 'Refcount cache size for qcow2 disks in MiB',
+ optional => 1,
+ },
+ 'qcow2-cache-clean-interval' => {
+ type => 'integer',
+ minimum => 0,
+ description => 'Cache clean interval for qcow2 disks in seconds',
+ optional => 1,
+ },
);
my %iothread_fmt = (
@@ -838,6 +880,52 @@ sub parse_drive {
}
}
+ my $cache_option_count =
+ defined($res->{'qcow2-cache-size'}) +
+ defined($res->{'qcow2-l2-cache-size'}) +
+ defined($res->{'qcow2-refcount-cache-size'});
+
+ if ($cache_option_count > 2) {
+ warn "at most two of qcow2-cache-size, qcow2-l2-cache-size, qcow2-refcount-cache-size"
+ . " can be set simultaneously\n";
+ ++$error;
+ }
+
+ if (
+ defined($res->{'qcow2-l2-cache-entry-size'})
+ && !is_power_of_two($res->{'qcow2-l2-cache-entry-size'})
+ ) {
+ warn "qcow2-l2-cache-entry-size must be a power of two\n";
+ ++$error;
+ }
+
+ if (
+ defined($res->{'qcow2-cache-size-based-on-disk'})
+ && $res->{'qcow2-cache-size-based-on-disk'}
+ ) {
+ if (
+ defined($res->{'qcow2-cache-size'})
+ || defined($res->{'qcow2-l2-cache-size'})
+ || defined($res->{'qcow2-refcount-cache-size'})
+ ) {
+ warn "qcow2-cache-size-based-on-disk is not compatible with cache-size,"
+ . " l2-cache-size or refcount-cache-size being set\n";
+ ++$error;
+ }
+ }
+
+ if (defined($res->{'qcow2-cache-size'})) {
+ if (($res->{'qcow2-l2-cache-size'} // 0) >= $res->{'qcow2-cache-size'}) {
+ warn "qcow2-l2-cache-size is larger than or equal to qcow2-cache-size\n";
+ ++$error;
+ }
+
+ if (($res->{'qcow2-refcount-cache-size'} // 0) >= $res->{'qcow2-cache-size'}) {
+ warn "qcow2-refcount-cache-size is larger than or equal to qcow2-cache-size\n";
+ ++$error;
+ }
+ }
+
return if $error;
return if $res->{mbps_rd} && $res->{mbps};
@@ -857,6 +945,11 @@ sub parse_drive {
return $res;
}
+sub is_power_of_two {
+ my ($n) = @_;
+ return $n > 0 && (($n & ($n - 1)) == 0);
+}
+
sub print_drive {
my ($drive, $with_alloc) = @_;
my $skip = ['index', 'interface'];
@@ -1179,4 +1272,33 @@ sub drive_qmp_peer {
return drive_uses_qsd_fuse($storecfg, $drive) ? qsd_qmp_peer($vmid) : vm_qmp_peer($vmid);
}
+sub qcow2_cache_size_by_disk_size {
+ # The calculation is adapted from here:
+ # https://gitlab.com/qemu-project/qemu/-/blob/master/docs/qcow2-cache.txt#L97-98
+ # As this combines the refcount and L2 cache sizes, QEMU is free to
+ # choose a different value for each of them, which means the L2 cache
+ # is not necessarily 4 times bigger than the refcount cache with the
+ # default cluster_size, refcount_bits and no extended L2 entries.
+
+ my ($storecfg, $drive) = @_;
+
+ my $img_info = PVE::QemuServer::QemuImage::info($storecfg, $drive->{file});
+ my $cluster_size = $img_info->{'cluster-size'} // 65536;
+ my $refcount_bits = $img_info->{'format-specific'}->{data}->{'refcount-bits'} // 16;
+ my $has_extended_l2 = $img_info->{'format-specific'}->{data}->{'extended-l2'} // 0;
+
+ my $l2_multiplier = $has_extended_l2 ? 16 : 8;
+ my $l2_cache_size = ($drive->{size} * $l2_multiplier) / $cluster_size;
+
+ $l2_cache_size = 2 * $cluster_size if $l2_cache_size < (2 * $cluster_size);
+ $l2_cache_size = ceil($l2_cache_size / $cluster_size) * $cluster_size;
+
+ my $refcount_cache_size = ($drive->{size} * $refcount_bits) / (8 * $cluster_size);
+
+ $refcount_cache_size = 4 * $cluster_size if $refcount_cache_size < (4 * $cluster_size);
+ $refcount_cache_size = ceil($refcount_cache_size / $cluster_size) * $cluster_size;
+
+ return $l2_cache_size + $refcount_cache_size;
+}
+
1;
diff --git a/src/PVE/QemuServer/QemuImage.pm b/src/PVE/QemuServer/QemuImage.pm
index 71be3abb..9c9db4e8 100644
--- a/src/PVE/QemuServer/QemuImage.pm
+++ b/src/PVE/QemuServer/QemuImage.pm
@@ -15,6 +15,30 @@ use PVE::QemuServer::Blockdev;
use PVE::QemuServer::Drive qw(checked_volume_format);
use PVE::QemuServer::Helpers;
+sub info {
+ my ($storecfg, $volid) = @_;
+
+ my $disk_path = PVE::Storage::abs_filesystem_path($storecfg, $volid, 1);
+ my $cmd = ['/usr/bin/qemu-img', 'info', '--output=json', $disk_path];
+
+ my $res = eval {
+ my $output = '';
+ PVE::Tools::run_command(
+ $cmd,
+ outfunc => sub {
+ my $line = shift;
+ $output .= $line;
+ },
+ );
+ decode_json($output);
+ };
+
+ my $err = $@;
+ die "qemu-img info failed: $err" if $err;
+
+ return $res;
+}
+
sub convert_iscsi_path {
my ($path) = @_;
--
2.47.3
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH pve-manager 2/3] partially fix #1989: ui: qemu: disk: add qcow2 cache size config
2026-06-05 15:35 [PATCH docs/manager/qemu-server 0/3] partially fix #1989: add QEMU qcow2 cache options Erik Fastermann
2026-06-05 15:35 ` [PATCH qemu-server 1/3] partially fix #1989: disk: add " Erik Fastermann
@ 2026-06-05 15:35 ` Erik Fastermann
2026-06-05 15:35 ` [PATCH pve-docs 3/3] partially fix #1989: qm: document " Erik Fastermann
2 siblings, 0 replies; 4+ messages in thread
From: Erik Fastermann @ 2026-06-05 15:35 UTC (permalink / raw)
To: pve-devel; +Cc: Erik Fastermann
Add the option to control the size of the qcow2 cache in the vm disk
create and edit dialogs. This can provide significant performance gains
in some vm configurations.
For a detailed explanation see the QEMU docs [0]. Currently only the
cache-size is configurable in the frontend, either directly or based on
the size of the disk.
[0] https://gitlab.com/qemu-project/qemu/-/blob/master/docs/qcow2-cache.txt
Signed-off-by: Erik Fastermann <e.fastermann@proxmox.com>
---
www/manager6/Parser.js | 2 +-
www/manager6/qemu/HDEdit.js | 67 +++++++++++++++++++++++++++++++++++++
2 files changed, 68 insertions(+), 1 deletion(-)
diff --git a/www/manager6/Parser.js b/www/manager6/Parser.js
index 36df8e9d..f8d2bdad 100644
--- a/www/manager6/Parser.js
+++ b/www/manager6/Parser.js
@@ -207,7 +207,7 @@ Ext.define('PVE.Parser', {
if (!p || p.match(/^\s*$/)) {
return undefined; // continue
}
- let match = p.match(/^([a-z_]+)=(\S+)$/);
+ let match = p.match(/^([0-9a-z-]+)=(\S+)$/);
if (!match) {
if (!p.match(/[=]/)) {
res.file = p;
diff --git a/www/manager6/qemu/HDEdit.js b/www/manager6/qemu/HDEdit.js
index 1bb2bfda..3716a0b0 100644
--- a/www/manager6/qemu/HDEdit.js
+++ b/www/manager6/qemu/HDEdit.js
@@ -18,6 +18,8 @@ Ext.define('PVE.qemu.HDInputPanel', {
isSCSI: false,
isVirtIO: false,
isSCSISingle: false,
+ isQcow2: false,
+ qcow2CacheSizeBasedOnDisk: false,
},
},
@@ -54,6 +56,22 @@ Ext.define('PVE.qemu.HDInputPanel', {
vm.set('isSCSISingle', value === 'virtio-scsi-single');
},
},
+ 'field[name=diskformat]': {
+ change: function (f, value) {
+ this.getViewModel().set('isQcow2', value === 'qcow2');
+ },
+ afterrender: function (f) {
+ this.getViewModel().set('isQcow2', f.getValue() === 'qcow2');
+ },
+ },
+ 'field[name=qcow2CacheSizeBasedOnDisk]': {
+ change: function (f, value) {
+ this.getViewModel().set('qcow2CacheSizeBasedOnDisk', value);
+ },
+ afterrender: function (f) {
+ this.getViewModel().set('qcow2CacheSizeBasedOnDisk', f.getValue());
+ },
+ },
},
init: function (view) {
@@ -88,6 +106,20 @@ Ext.define('PVE.qemu.HDInputPanel', {
me.drive.format = values.diskformat;
}
+ if (me.drive?.format === 'qcow2' || me.drive?.file?.endsWith('.qcow2')) {
+ if (values.qcow2CacheSize) {
+ me.drive['qcow2-cache-size'] = values.qcow2CacheSize;
+ } else {
+ delete me.drive['qcow2-cache-size'];
+ }
+ }
+
+ PVE.Utils.propertyStringSet(
+ me.drive,
+ values.qcow2CacheSizeBasedOnDisk,
+ 'qcow2-cache-size-based-on-disk',
+ );
+
PVE.Utils.propertyStringSet(me.drive, !values.backup, 'backup', '0');
PVE.Utils.propertyStringSet(me.drive, values.noreplicate, 'replicate', 'no');
PVE.Utils.propertyStringSet(me.drive, values.discard, 'discard', 'on');
@@ -156,6 +188,8 @@ Ext.define('PVE.qemu.HDInputPanel', {
values.iothread = PVE.Parser.parseBoolean(drive.iothread);
values.readOnly = PVE.Parser.parseBoolean(drive.ro);
values.aio = drive.aio || '__default__';
+ values.qcow2CacheSizeBasedOnDisk = drive['qcow2-cache-size-based-on-disk'];
+ values.qcow2CacheSize = drive['qcow2-cache-size'];
values.mbps_rd = drive.mbps_rd;
values.mbps_wr = drive.mbps_wr;
@@ -167,6 +201,12 @@ Ext.define('PVE.qemu.HDInputPanel', {
values.iops_wr_max = drive.iops_wr_max;
me.setValues(values);
+
+ me.getViewModel().set(
+ 'isQcow2',
+ values.diskformat === 'qcow2' || values.hdimage?.endsWith('.qcow2'),
+ );
+ this.getViewModel().set('qcow2CacheSizeBasedOnDisk', values.qcow2CacheSizeBasedOnDisk);
},
setNodename: function (nodename) {
@@ -355,6 +395,33 @@ Ext.define('PVE.qemu.HDInputPanel', {
disabled: '{!isVirtIO && !isSCSI}',
},
},
+ {
+ xtype: 'numberfield',
+ name: 'qcow2CacheSize',
+ minValue: 1,
+ fieldLabel: gettext('qcow2 cache') + ' (MiB)',
+ emptyText: gettext('Default cache size'),
+ bind: {
+ disabled: '{!isQcow2 || qcow2CacheSizeBasedOnDisk}',
+ },
+ listeners: {
+ disable: function (field) {
+ console.log(field);
+ field.setValue(null);
+ },
+ },
+ },
+ {
+ xtype: 'proxmoxcheckbox',
+ name: 'qcow2CacheSizeBasedOnDisk',
+ reference: 'qcow2CacheSizeBasedOnDisk',
+ defaultValue: 0,
+ fieldLabel: gettext('qcow2 cache based on disk'),
+ clearOnDisable: true,
+ bind: {
+ disabled: '{!isQcow2}',
+ },
+ },
);
advancedColumn2.push(
--
2.47.3
^ permalink raw reply related [flat|nested] 4+ messages in thread