feat: firecracker best path

internal/config/config.go

@@ -20,6 +20,7 @@ type Config struct {
 	MachineDisksDir       string
 	SnapshotsDir          string
 	RuntimeDir            string
+	DiskCloneMode         DiskCloneMode
 	SocketPath            string
 	HTTPAddr              string
 	EgressInterface       string
@@ -28,6 +29,16 @@ type Config struct {
 	GuestLoginCAPublicKey string
 }
 
+// DiskCloneMode controls how the daemon materializes writable machine disks.
+type DiskCloneMode string
+
+const (
+	// DiskCloneModeReflink requires an O(1) copy-on-write clone and never falls back to a full copy.
+	DiskCloneModeReflink DiskCloneMode = "reflink"
+	// DiskCloneModeCopy performs a full sparse copy. Use only for local development or emergency fallback.
+	DiskCloneModeCopy DiskCloneMode = "copy"
+)
+
 // Load loads and validates the firecracker-host daemon configuration from the environment.
 func Load() (Config, error) {
 	rootDir := filepath.Clean(strings.TrimSpace(os.Getenv("FIRECRACKER_HOST_ROOT_DIR")))
@@ -39,6 +50,7 @@ func Load() (Config, error) {
 		MachineDisksDir:       filepath.Join(rootDir, "machine-disks"),
 		SnapshotsDir:          filepath.Join(rootDir, "snapshots"),
 		RuntimeDir:            filepath.Join(rootDir, "runtime"),
+		DiskCloneMode:         loadDiskCloneMode(os.Getenv("FIRECRACKER_HOST_DISK_CLONE_MODE")),
 		SocketPath:            filepath.Join(rootDir, defaultSocketName),
 		HTTPAddr:              strings.TrimSpace(os.Getenv("FIRECRACKER_HOST_HTTP_ADDR")),
 		EgressInterface:       strings.TrimSpace(os.Getenv("FIRECRACKER_HOST_EGRESS_INTERFACE")),
@@ -81,6 +93,9 @@ func (c Config) Validate() error {
 	if strings.TrimSpace(c.RuntimeDir) == "" {
 		return fmt.Errorf("runtime dir is required")
 	}
+	if err := c.DiskCloneMode.Validate(); err != nil {
+		return err
+	}
 	if strings.TrimSpace(c.SocketPath) == "" {
 		return fmt.Errorf("socket path is required")
 	}
@@ -99,3 +114,21 @@ func (c Config) FirecrackerRuntimeConfig() firecracker.RuntimeConfig {
 		JailerBinaryPath:      c.JailerBinaryPath,
 	}
 }
+
+func loadDiskCloneMode(raw string) DiskCloneMode {
+	value := strings.TrimSpace(raw)
+	if value == "" {
+		return DiskCloneModeReflink
+	}
+	return DiskCloneMode(value)
+}
+
+// Validate reports whether the clone mode is safe to use.
+func (m DiskCloneMode) Validate() error {
+	switch m {
+	case DiskCloneModeReflink, DiskCloneModeCopy:
+		return nil
+	default:
+		return fmt.Errorf("FIRECRACKER_HOST_DISK_CLONE_MODE must be %q or %q", DiskCloneModeReflink, DiskCloneModeCopy)
+	}
+}

internal/config/config_test.go

@@ -0,0 +1,40 @@
+package config
+
+import "testing"
+
+func TestLoadDiskCloneModeDefaultsToReflink(t *testing.T) {
+	t.Parallel()
+
+	if got := loadDiskCloneMode(""); got != DiskCloneModeReflink {
+		t.Fatalf("disk clone mode = %q, want %q", got, DiskCloneModeReflink)
+	}
+}
+
+func TestDiskCloneModeValidate(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		mode    DiskCloneMode
+		wantErr bool
+	}{
+		{name: "reflink", mode: DiskCloneModeReflink},
+		{name: "copy", mode: DiskCloneModeCopy},
+		{name: "empty", mode: "", wantErr: true},
+		{name: "unknown", mode: "auto", wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := tt.mode.Validate()
+			if tt.wantErr && err == nil {
+				t.Fatal("Validate() error = nil, want error")
+			}
+			if !tt.wantErr && err != nil {
+				t.Fatalf("Validate() error = %v, want nil", err)
+			}
+		})
+	}
+}