vm: keep qcow2 overlays buffered so a shared base stays in the page cache

hypervisor/cloudhypervisor/args.go

@@ -178,10 +178,18 @@ func serialConsoleFor(directBoot bool, consoleSock string) (serial, console *chR
 	return &chRuntimeFile{Mode: "Socket", Socket: consoleSock}, &chRuntimeFile{Mode: "Off"}
 }
 
+func qcow2Overlay(sc *types.StorageConfig) bool {
+	return !sc.RO && filepath.Ext(sc.Path) == ".qcow2"
+}
+
 func effectiveDirectIO(sc *types.StorageConfig, noDirectIO bool) bool {
 	if sc.DirectIO != nil {
 		return *sc.DirectIO
 	}
+	// CH applies this flag to the backing file too, so O_DIRECT would stop one page-cache copy of the shared base serving every VM.
+	if qcow2Overlay(sc) {
+		return false
+	}
 	return !sc.RO && !noDirectIO
 }
 
@@ -201,7 +209,7 @@ func storageConfigToDisk(storageConfig *types.StorageConfig, cpuCount, diskQueue
 	switch {
 	case filepath.Ext(storageConfig.Path) == ".qcow2":
 		d.ImageType = "Qcow2"
-		d.BackingFiles = !storageConfig.RO
+		d.BackingFiles = qcow2Overlay(storageConfig)
 	case storageConfig.RO:
 		d.ImageType = "Raw"
 	default:

hypervisor/cloudhypervisor/args_test.go

@@ -30,3 +30,39 @@ func TestMemoryCLIArg(t *testing.T) {
 		})
 	}
 }
+
+func TestEffectiveDirectIO(t *testing.T) {
+	tests := []struct {
+		name       string
+		sc         types.StorageConfig
+		noDirectIO bool
+		want       bool
+	}{
+		{name: "raw cow", sc: types.StorageConfig{Path: "/v/cow.raw", Role: types.StorageRoleCOW}, want: true},
+		{name: "raw cow with no-direct-io", sc: types.StorageConfig{Path: "/v/cow.raw", Role: types.StorageRoleCOW}, noDirectIO: true},
+		{name: "readonly layer", sc: types.StorageConfig{Path: "/v/base.raw", RO: true, Role: types.StorageRoleLayer}},
+		{name: "qcow2 overlay stays buffered", sc: types.StorageConfig{Path: "/v/overlay.qcow2", Role: types.StorageRoleCOW}},
+		{name: "readonly qcow2 has no backing chain", sc: types.StorageConfig{Path: "/v/base.qcow2", RO: true, Role: types.StorageRoleLayer}},
+		{name: "explicit override wins", sc: types.StorageConfig{Path: "/v/data.raw", Role: types.StorageRoleData, DirectIO: ptr(false)}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := effectiveDirectIO(&tt.sc, tt.noDirectIO); got != tt.want {
+				t.Errorf("got %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestQcow2OverlayDiskArgs(t *testing.T) {
+	sc := &types.StorageConfig{Path: "/v/overlay.qcow2", Role: types.StorageRoleCOW}
+	got := diskToCLIArg(storageConfigToDisk(sc, 1, 0, false, nil))
+	if strings.Contains(got, "direct=on") {
+		t.Errorf("qcow2 overlay must stay buffered so the shared base keeps one page-cache copy: %s", got)
+	}
+	if !strings.Contains(got, "backing_files=on") {
+		t.Errorf("qcow2 overlay must keep backing_files=on: %s", got)
+	}
+}
+
+func ptr[T any](v T) *T { return &v }

meta/sqlite/store.go

@@ -81,15 +81,6 @@ func OpenForRecovery(dbPath string, namespaces ...Namespace) (*Store, error) {
 	return openStore(dbPath, namespaces)
 }
 
-// RefuseManifest fails when a conversion manifest sits beside dbPath, meaning an offline conversion is unfinished.
-func RefuseManifest(dbPath string) error {
-	manifest := filepath.Join(filepath.Dir(dbPath), ManifestName)
-	if utils.FileExists(manifest) {
-		return fmt.Errorf("%s exists: a conversion is in flight, run `cocoon meta convert` to finish it", manifest)
-	}
-	return nil
-}
-
 func openStore(dbPath string, namespaces []Namespace) (*Store, error) {
 	// The driver creates a file on first touch; Open never creates — that is Init's job (§6) — and §4 refuses network filesystems before WAL work.
 	if !utils.FileExists(dbPath) {
@@ -381,6 +372,15 @@ func (h *txHandle) checkRead(ns string) error {
 	return nil
 }
 
+// RefuseManifest fails when a conversion manifest sits beside dbPath, meaning an offline conversion is unfinished.
+func RefuseManifest(dbPath string) error {
+	manifest := filepath.Join(filepath.Dir(dbPath), ManifestName)
+	if utils.FileExists(manifest) {
+		return fmt.Errorf("%s exists: a conversion is in flight, run `cocoon meta convert` to finish it", manifest)
+	}
+	return nil
+}
+
 func tableName(ns, table string) string {
 	return quoteIdent(ns + "__" + table)
 }