[pve-devel] [PATCH storage v4 11/49] plugin: qemu block device: add hints option and EFI disk hint
Fiona Ebner
f.ebner at proxmox.com
Tue Jul 1 17:40:31 CEST 2025
For '-drive', qemu-server sets special cache options for EFI disk
using RBD. In preparation to seamlessly switch to the new '-blockdev'
interface, do the same here. Note that the issue from bug #3329, which
is solved by these cache options, still affects current versions.
With -blockdev, the cache options are split up. While cache.direct and
cache.no-flush can be set in the -blockdev options, cache.writeback is
a front-end property and was intentionally removed from the -blockdev
options by QEMU commit aaa436f998 ("block: Remove cache.writeback from
blockdev-add"). It needs to be configured as the 'write-cache'
property for the ide-hd/scsi-hd/virtio-blk device.
The default is already 'writeback' and no cache mode can be set for an
EFI drive configuration in Proxmox VE currently, so there will not be
a clash.
┌─────────────┬─────────────────┬──────────────┬────────────────┐
│ │ cache.writeback │ cache.direct │ cache.no-flush │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│writeback │ on │ off │ off │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│none │ on │ on │ off │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│writethrough │ off │ off │ off │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│directsync │ off │ on │ off │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│unsafe │ on │ off │ on │
└─────────────┴─────────────────┴──────────────┴────────────────┘
Table from 'man kvm'.
Alternatively, the option could only be set once when allocating the
RBD volume. However, then we would need to detect all cases were a
volume could potentially be used as an EFI disk later. Having a custom
disk type would help a lot there. The approach here was chosen as it
is catch-all and should not be too costly either.
Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
---
Changes in v4:
* More compact POD.
src/PVE/Storage.pm | 4 ++--
src/PVE/Storage/ISCSIDirectPlugin.pm | 2 +-
src/PVE/Storage/Plugin.pm | 23 +++++++++++++++++++++--
src/PVE/Storage/RBDPlugin.pm | 20 +++++++++++++++++++-
src/PVE/Storage/ZFSPlugin.pm | 2 +-
src/PVE/Storage/ZFSPoolPlugin.pm | 2 +-
6 files changed, 45 insertions(+), 8 deletions(-)
diff --git a/src/PVE/Storage.pm b/src/PVE/Storage.pm
index ec8b753..4920bd6 100755
--- a/src/PVE/Storage.pm
+++ b/src/PVE/Storage.pm
@@ -721,7 +721,7 @@ sub abs_filesystem_path {
# see the documentation for the plugin method
sub qemu_blockdev_options {
- my ($cfg, $volid) = @_;
+ my ($cfg, $volid, $options) = @_;
my ($storeid, $volname) = parse_volume_id($volid);
@@ -733,7 +733,7 @@ sub qemu_blockdev_options {
die "cannot use volume of type '$vtype' as a QEMU blockdevice\n"
if $vtype ne 'images' && $vtype ne 'iso' && $vtype ne 'import';
- return $plugin->qemu_blockdev_options($scfg, $storeid, $volname);
+ return $plugin->qemu_blockdev_options($scfg, $storeid, $volname, $options);
}
# used as last resort to adapt volnames when migrating
diff --git a/src/PVE/Storage/ISCSIDirectPlugin.pm b/src/PVE/Storage/ISCSIDirectPlugin.pm
index 8c6b4ab..12b894d 100644
--- a/src/PVE/Storage/ISCSIDirectPlugin.pm
+++ b/src/PVE/Storage/ISCSIDirectPlugin.pm
@@ -111,7 +111,7 @@ sub path {
}
sub qemu_blockdev_options {
- my ($class, $scfg, $storeid, $volname) = @_;
+ my ($class, $scfg, $storeid, $volname, $options) = @_;
my $lun = ($class->parse_volname($volname))[1];
diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 680bb6b..4ec4a0d 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -1965,7 +1965,7 @@ sub rename_volume {
=head3 qemu_blockdev_options
- $blockdev = $plugin->qemu_blockdev_options($scfg, $storeid, $volname)
+ $blockdev = $plugin->qemu_blockdev_options($scfg, $storeid, $volname, $options)
Returns a hash reference with the basic options needed to open the volume via QEMU's C<-blockdev>
API. This at least requires a C<< $blockdev->{driver} >> and a reference to the image, e.g.
@@ -1992,12 +1992,31 @@ Arguments:
=item C<$volume>: The volume name.
+=item C<$options>: A hash reference with additional options.
+
+=over
+
+=item C<< $options->{hints} >>: A hash reference with hints indicating what the volume will be used
+for. This can be safely ignored if no concrete issues are known with your plugin. For certain use
+cases, setting additional (plugin-specific) options might be very beneficial however. An example is
+setting the correct cache options for an EFI disk on RBD. The list of hints might get expanded in
+the future.
+
+=over
+
+=item C<< $options->{hints}->{'efi-disk'} >>: (optional) If set, the volume will be used as the EFI
+disk of a VM, containing its OMVF variables.
+
+=back
+
+=back
+
=back
=cut
sub qemu_blockdev_options {
- my ($class, $scfg, $storeid, $volname) = @_;
+ my ($class, $scfg, $storeid, $volname, $options) = @_;
my $blockdev = {};
diff --git a/src/PVE/Storage/RBDPlugin.pm b/src/PVE/Storage/RBDPlugin.pm
index 6e70a19..c40c845 100644
--- a/src/PVE/Storage/RBDPlugin.pm
+++ b/src/PVE/Storage/RBDPlugin.pm
@@ -374,6 +374,16 @@ my sub rbd_volume_exists {
return 0;
}
+# Needs to be public, so qemu-server can mock it for cfg2cmd.
+sub rbd_volume_config_set {
+ my ($scfg, $storeid, $volname, $key, $value) = @_;
+
+ my $cmd = $rbd_cmd->($scfg, $storeid, 'config', 'image', 'set', $volname, $key, $value);
+ run_rbd_command($cmd, errmsg => "rbd config image set $volname $key $value error");
+
+ return;
+}
+
# Configuration
sub type {
@@ -514,7 +524,7 @@ sub path {
}
sub qemu_blockdev_options {
- my ($class, $scfg, $storeid, $volname) = @_;
+ my ($class, $scfg, $storeid, $volname, $options) = @_;
my $cmd_option = PVE::CephConfig::ceph_connect_option($scfg, $storeid);
my ($name) = ($class->parse_volname($volname))[1];
@@ -547,6 +557,14 @@ sub qemu_blockdev_options {
$blockdev->{user} = "$cmd_option->{userid}" if $cmd_option->{keyring};
+ # SPI flash does lots of read-modify-write OPs, without writeback this gets really slow #3329
+ if ($options->{hints}->{'efi-disk'}) {
+ # Querying the value would just cost more and the 'rbd image config get' command will just
+ # fail if the config has not been set yet, so it's not even straight-forward to do so.
+ # Simply set the value (possibly again).
+ rbd_volume_config_set($scfg, $storeid, $name, 'rbd_cache_policy', 'writeback');
+ }
+
return $blockdev;
}
diff --git a/src/PVE/Storage/ZFSPlugin.pm b/src/PVE/Storage/ZFSPlugin.pm
index c03fcca..0f64898 100644
--- a/src/PVE/Storage/ZFSPlugin.pm
+++ b/src/PVE/Storage/ZFSPlugin.pm
@@ -248,7 +248,7 @@ sub path {
}
sub qemu_blockdev_options {
- my ($class, $scfg, $storeid, $volname) = @_;
+ my ($class, $scfg, $storeid, $volname, $options) = @_;
my $name = ($class->parse_volname($volname))[1];
my $guid = $class->zfs_get_lu_name($scfg, $name);
diff --git a/src/PVE/Storage/ZFSPoolPlugin.pm b/src/PVE/Storage/ZFSPoolPlugin.pm
index 86c52ea..e775dae 100644
--- a/src/PVE/Storage/ZFSPoolPlugin.pm
+++ b/src/PVE/Storage/ZFSPoolPlugin.pm
@@ -163,7 +163,7 @@ sub path {
}
sub qemu_blockdev_options {
- my ($class, $scfg, $storeid, $volname) = @_;
+ my ($class, $scfg, $storeid, $volname, $options) = @_;
my $format = ($class->parse_volname($volname))[6];
--
2.47.2
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