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plugin: qemu block device: add hints option and EFI disk hint

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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@proxmox.com>
This commit is contained in:
Fiona Ebner
2025-07-02 18:27:44 +02:00
committed by Fabian Grünbichler
parent f9c390bdfd
commit 590fb76238
6 changed files with 45 additions and 8 deletions
Download Patch File Download Diff File Expand all files Collapse all files

23
src/PVE/Storage/Plugin.pm
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@ -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 = {};