[pve-devel] [PATCH qemu-server 16/22] print drive device: explicitly set write-cache starting with machine version 10.0

[Fiona Ebner]

With the 'blockdev' command line option, 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.

Table from 'man kvm':

┌─────────────┬─────────────────┬──────────────┬────────────────┐
│             │ 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             │
└─────────────┴─────────────────┴──────────────┴────────────────┘

Suggested-by: Alexandre Derumier <alexandre.derumier at groupe-cyllene.com>
Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
---
 PVE/QemuServer.pm | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
index 82304096..f9e1b3f9 100644
--- a/PVE/QemuServer.pm
+++ b/PVE/QemuServer.pm
@@ -1317,6 +1317,8 @@ sub print_drivedevice_full {
     my $device = '';
     my $maxdev = 0;
 
+    my $machine_version = extract_version($machine_type, kvm_user_version());
+
     my $drive_id = PVE::QemuServer::Drive::get_drive_id($drive);
     if ($drive->{interface} eq 'virtio') {
 	my $pciaddr = print_pci_addr("$drive_id", $bridges, $arch, $machine_type);
@@ -1327,7 +1329,6 @@ sub print_drivedevice_full {
 	my ($maxdev, $controller, $controller_prefix) = scsihw_infos($conf, $drive);
 	my $unit = $drive->{index} % $maxdev;
 
-	my $machine_version = extract_version($machine_type, kvm_user_version());
 	my $device_type = PVE::QemuServer::Drive::get_scsi_device_type(
 	    $drive, $storecfg, $machine_version);
 
@@ -1403,6 +1404,15 @@ sub print_drivedevice_full {
 	$device .= ",serial=$serial";
     }
 
+    if (min_version($machine_version, 10, 0)) {
+	if (!drive_is_cdrom($drive)) {
+	    my $write_cache = 'on';
+	    if (my $cache = $drive->{cache}) {
+		$write_cache = 'off' if $cache eq 'writethrough' || $cache eq 'directsync';
+	    }
+	    $device .= ",write-cache=$write_cache";
+	}
+    }
 
     return $device;
 }
-- 
2.39.5