diff --git a/db_mmap_test.go b/db_mmap_test.go
new file mode 100644
index 0000000..41ab462
--- /dev/null
+++ b/db_mmap_test.go
@@ -0,0 +1,10 @@
+//go:build !plan9
+// +build !plan9
+
+package pogreb
+
+import (
+	"github.com/akrylysov/pogreb/fs"
+)
+
+var testFileSystems = []fs.FileSystem{fs.Mem, fs.OSMMap, fs.OS}
diff --git a/db_rpc_test.go b/db_rpc_test.go
new file mode 100644
index 0000000..b0761f0
--- /dev/null
+++ b/db_rpc_test.go
@@ -0,0 +1,10 @@
+//go:build plan9
+// +build plan9
+
+package pogreb
+
+import (
+	"github.com/akrylysov/pogreb/fs"
+)
+
+var testFileSystems = []fs.FileSystem{fs.Mem, fs.OS}
diff --git a/db_test.go b/db_test.go
index e25b145..bd7a2d3 100644
--- a/db_test.go
+++ b/db_test.go
@@ -32,7 +32,7 @@ func TestMain(m *testing.M) {
 		SetLogger(log.New(io.Discard, "", 0))
 	}
 	// Run tests against all file systems.
-	for _, fsys := range []fs.FileSystem{fs.Mem, fs.OSMMap, fs.OS} {
+	for _, fsys := range testFileSystems {
 		var tmpDir string
 		if fsys == fs.Mem {
 			testFS = fsys
diff --git a/fs/os_mmap.go b/fs/os_mmap.go
index 3fbe40b..deabf1d 100644
--- a/fs/os_mmap.go
+++ b/fs/os_mmap.go
@@ -1,3 +1,5 @@
+//go:build !plan9
+
 package fs
 
 import (
@@ -161,3 +163,8 @@ func (f *osMMapFile) Close() error {
 	}
 	return f.File.Close()
 }
+
+// Return a default FileSystem for this platform.
+func DefaultFileSystem() FileSystem {
+	return OSMMap
+}
diff --git a/fs/os_mmap_unix.go b/fs/os_mmap_unix.go
index bb2e295..ea6fc7a 100644
--- a/fs/os_mmap_unix.go
+++ b/fs/os_mmap_unix.go
@@ -1,4 +1,5 @@
-//go:build !windows
+//go:build !(plan9 || windows)
+// +build !plan9,!windows
 
 package fs
 
diff --git a/fs/os_mmap_windows.go b/fs/os_mmap_windows.go
index af8434b..ff7ccd7 100644
--- a/fs/os_mmap_windows.go
+++ b/fs/os_mmap_windows.go
@@ -1,4 +1,5 @@
 //go:build windows
+// +build windows
 
 package fs
 
diff --git a/fs/os_plan9.go b/fs/os_plan9.go
new file mode 100644
index 0000000..8819013
--- /dev/null
+++ b/fs/os_plan9.go
@@ -0,0 +1,26 @@
+//go:build plan9
+// +build plan9
+
+package fs
+
+import (
+	"os"
+	"syscall"
+)
+
+func createLockFile(name string, perm os.FileMode) (LockFile, bool, error) {
+	acquiredExisting := false
+	if _, err := os.Stat(name); err == nil {
+		acquiredExisting = true
+	}
+	f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE, syscall.DMEXCL|perm)
+	if err != nil {
+		return nil, false, err
+	}
+	return &osLockFile{f, name}, acquiredExisting, nil
+}
+
+// Return a default FileSystem for this platform.
+func DefaultFileSystem() FileSystem {
+	return OS
+}
diff --git a/fs/os_unix.go b/fs/os_unix.go
index bcc6230..f410c3f 100644
--- a/fs/os_unix.go
+++ b/fs/os_unix.go
@@ -1,4 +1,5 @@
-//go:build !windows
+//go:build !(plan9 || windows)
+// +build !plan9,!windows
 
 package fs
 
diff --git a/fs/os_windows.go b/fs/os_windows.go
index 097a23f..c5ca330 100644
--- a/fs/os_windows.go
+++ b/fs/os_windows.go
@@ -1,4 +1,5 @@
 //go:build windows
+// +build windows
 
 package fs
 
diff --git a/options.go b/options.go
index 78e7138..cee92e8 100644
--- a/options.go
+++ b/options.go
@@ -26,7 +26,7 @@ type Options struct {
 	//
 	// Default: fs.OSMMap.
 	FileSystem fs.FileSystem
-	rootFS     fs.FileSystem
+	rootFS fs.FileSystem
 
 	maxSegmentSize             uint32
 	compactionMinSegmentSize   uint32
@@ -39,7 +39,7 @@ func (src *Options) copyWithDefaults(path string) *Options {
 		opts = *src
 	}
 	if opts.FileSystem == nil {
-		opts.FileSystem = fs.OSMMap
+		opts.FileSystem = fs.DefaultFileSystem()
 	}
 	opts.rootFS = opts.FileSystem
 	opts.FileSystem = fs.Sub(opts.FileSystem, path)