make disk driver cache attribute configurable

In some typical virter usage scenarios, the consistency of guest disks after a host crash is unimportant. Hence setting the cache to "unsafe" is acceptable. This improves disk performance.
LINBIT · Dec 1, 2021 · 4db9439 · 4db9439
1 parent 50840c1
commit 4db9439
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Showing 6 changed files with 22 additions and 7 deletions.
diff --git a/cmd/config.go b/cmd/config.go
@@ -43,6 +43,11 @@ network = "{{ get "libvirt.network" }}"
 # Default value: "{{ get "libvirt.static_dhcp" }}"
 static_dhcp = "{{ get "libvirt.static_dhcp" }}"
 
+# disk_cache is passed to libvirt as the disk driver cache attribute. See
+# https://libvirt.org/formatdomain.html#hard-drives-floppy-disks-cdroms.
+# Default value: "{{ get "libvirt.disk_cache" }}"
+disk_cache = "{{ get "libvirt.disk_cache" }}"
+
 [time]
 # ssh_ping_count is the number of times virter will try to connect to a VM's
 # ssh port after starting it.
@@ -94,6 +99,7 @@ func initConfig() {
 	viper.SetDefault("libvirt.pool", "default")
 	viper.SetDefault("libvirt.network", "default")
 	viper.SetDefault("libvirt.static_dhcp", false)
+	viper.SetDefault("libvirt.disk_cache", "")
 	viper.SetDefault("time.ssh_ping_count", 60)
 	viper.SetDefault("time.ssh_ping_period", time.Second)
 	viper.SetDefault("time.shutdown_timeout", 20*time.Second)

diff --git a/cmd/image_build.go b/cmd/image_build.go
@@ -181,6 +181,7 @@ func imageBuildCommand() *cobra.Command {
 				StaticDHCP:         viper.GetBool("libvirt.static_dhcp"),
 				ExtraSSHPublicKeys: extraAuthorizedKeys,
 				ConsolePath:        consolePath,
+				DiskCache:          viper.GetString("libvirt.disk_cache"),
 				Mounts:             mounts,
 			}
 

diff --git a/cmd/vm_run.go b/cmd/vm_run.go
@@ -215,6 +215,7 @@ func vmRunCommand() *cobra.Command {
 						ExtraSSHPublicKeys: extraAuthorizedKeys,
 						ConsolePath:        consolePath,
 						Disks:              disks,
+						DiskCache:          viper.GetString("libvirt.disk_cache"),
 						Mounts:             mounts,
 						ExtraNics:          nics,
 						GDBPort:            thisGDBPort,

diff --git a/internal/virter/libvirtxml.go b/internal/virter/libvirtxml.go
@@ -40,20 +40,22 @@ type VMDisk struct {
 	format     string
 }
 
-func vmDisksToLibvirtDisks(vmDisks []VMDisk) ([]lx.DomainDisk, error) {
+func vmDisksToLibvirtDisks(vmDisks []VMDisk, diskCache string) ([]lx.DomainDisk, error) {
 	devCounts := map[string]*driveletter.DriveLetter{}
 
 	var result []lx.DomainDisk
 	for _, d := range vmDisks {
 		driver := map[VMDiskDevice]lx.DomainDiskDriver{
 			VMDiskDeviceDisk: lx.DomainDiskDriver{
 				Name:    "qemu",
+				Cache:   diskCache,
 				Discard: "unmap",
 				Type:    d.format,
 			},
 			VMDiskDeviceCDROM: lx.DomainDiskDriver{
-				Name: "qemu",
-				Type: d.format,
+				Name:  "qemu",
+				Cache: diskCache,
+				Type:  d.format,
 			},
 		}[d.device]
 
@@ -104,7 +106,7 @@ func (v *Virter) vmXML(poolName string, vm VMConfig, mac string, meta *VMMeta) (
 	}
 
 	log.Debugf("input are these vmdisks: %+v", vmDisks)
-	disks, err := vmDisksToLibvirtDisks(vmDisks)
+	disks, err := vmDisksToLibvirtDisks(vmDisks, vm.DiskCache)
 	if err != nil {
 		return "", fmt.Errorf("failed to build libvirt disks: %w", err)
 	}

diff --git a/internal/virter/libvirtxml_test.go b/internal/virter/libvirtxml_test.go
@@ -12,6 +12,7 @@ func TestVmDisksToLibvirtDisks(t *testing.T) {
 	cases := []struct {
 		descr       string
 		input       []VMDisk
+		cache       string
 		expect      []lx.DomainDisk
 		expectError bool
 	}{
@@ -21,11 +22,13 @@ func TestVmDisksToLibvirtDisks(t *testing.T) {
 				VMDisk{device: VMDiskDeviceDisk, poolName: "pool", volumeName: "vol1", bus: "virtio", format: "qcow2"},
 				VMDisk{device: VMDiskDeviceCDROM, poolName: "pool", volumeName: "vol2", bus: "ide", format: "raw"},
 			},
+			cache: "unsafe",
 			expect: []lx.DomainDisk{
 				lx.DomainDisk{
 					Device: "disk",
 					Driver: &lx.DomainDiskDriver{
 						Name:    "qemu",
+						Cache:   "unsafe",
 						Discard: "unmap",
 						Type:    "qcow2",
 					},
@@ -43,8 +46,9 @@ func TestVmDisksToLibvirtDisks(t *testing.T) {
 				lx.DomainDisk{
 					Device: "cdrom",
 					Driver: &lx.DomainDiskDriver{
-						Name: "qemu",
-						Type: "raw",
+						Name:  "qemu",
+						Cache: "unsafe",
+						Type:  "raw",
 					},
 					Source: &lx.DomainDiskSource{
 						Volume: &lx.DomainDiskSourceVolume{
@@ -68,7 +72,7 @@ func TestVmDisksToLibvirtDisks(t *testing.T) {
 	}
 
 	for _, c := range cases {
-		actual, err := vmDisksToLibvirtDisks(c.input)
+		actual, err := vmDisksToLibvirtDisks(c.input, c.cache)
 		if !c.expectError && err != nil {
 			t.Errorf("on input '%s':", c.input)
 			t.Fatalf("unexpected error: %+v", err)

diff --git a/internal/virter/virter.go b/internal/virter/virter.go
@@ -142,6 +142,7 @@ type VMConfig struct {
 	ExtraSSHPublicKeys []string
 	ConsolePath        string
 	Disks              []Disk
+	DiskCache          string
 	ExtraNics          []NIC
 	Mounts             []Mount
 	GDBPort            uint