diff --git a/bolt_openbsd.go b/bolt_openbsd.go
index bf47aa1a6..5bbf80bfe 100644
--- a/bolt_openbsd.go
+++ b/bolt_openbsd.go
@@ -9,7 +9,7 @@ func msync(db *DB) error {
 }
 
 func fdatasync(db *DB) error {
-	if db.data != nil {
+	if db.data != nil && !db.mmapFallback {
 		return msync(db)
 	}
 	return db.file.Sync()
diff --git a/bolt_plan9.go b/bolt_plan9.go
new file mode 100644
index 000000000..a97e505e3
--- /dev/null
+++ b/bolt_plan9.go
@@ -0,0 +1,25 @@
+//go:build plan9
+
+package bbolt
+
+import "time"
+
+func fdatasync(db *DB) error {
+	return db.file.Sync()
+}
+
+func flock(_ *DB, _ bool, _ time.Duration) error {
+	return nil
+}
+
+func funlock(_ *DB) error {
+	return nil
+}
+
+func mmap(db *DB, sz int) error {
+	return mmapFallback(db, sz)
+}
+
+func munmap(db *DB) error {
+	return munmapFallback(db)
+}
diff --git a/db.go b/db.go
index 2ab120394..ec0e33155 100644
--- a/db.go
+++ b/db.go
@@ -121,10 +121,9 @@ type DB struct {
 	path     string
 	openFile func(string, int, os.FileMode) (*os.File, error)
 	file     *os.File
-	// `dataref` isn't used at all on Windows, and the golangci-lint
-	// always fails on Windows platform.
+	// dataref keeps the mapped byte slice on Unix, or the heap mirror used by mmap fallback.
 	//nolint
-	dataref  []byte // mmap'ed readonly, write throws SEGV
+	dataref  []byte
 	data     *[common.MaxMapSize]byte
 	datasz   int
 	meta0    *common.Meta
@@ -154,6 +153,8 @@ type DB struct {
 	// Read only mode.
 	// When true, Update() and Begin(true) return ErrDatabaseReadOnly immediately.
 	readOnly bool
+
+	mmapFallback bool
 }
 
 // Path returns the path to currently open database file.
@@ -514,8 +515,16 @@ func (db *DB) mmap(minsz int) (err error) {
 	// gofail: var mapError string
 	// return errors.New(mapError)
 	if err = mmap(db, size); err != nil {
-		lg.Errorf("[GOOS: %s, GOARCH: %s] mmap failed, size: %d, error: %v", runtime.GOOS, runtime.GOARCH, size, err)
-		return err
+		if !isMmapUnsupported(err) {
+			lg.Errorf("[GOOS: %s, GOARCH: %s] mmap failed, size: %d, error: %v", runtime.GOOS, runtime.GOARCH, size, err)
+			return err
+		}
+		mmapErr := err
+		lg.Warningf("[GOOS: %s, GOARCH: %s] mmap unsupported, size: %d, falling back to heap mirror: %v", runtime.GOOS, runtime.GOARCH, size, mmapErr)
+		if err = mmapFallback(db, size); err != nil {
+			lg.Errorf("[GOOS: %s, GOARCH: %s] mmap fallback failed, size: %d, mmap error: %v, fallback error: %v", runtime.GOOS, runtime.GOARCH, size, mmapErr, err)
+			return fmt.Errorf("mmap unsupported: %v; fallback error: %w", mmapErr, err)
+		}
 	}
 
 	// Perform unmmap on any error to reset all data fields:
@@ -556,6 +565,7 @@ func (db *DB) invalidate() {
 	db.dataref = nil
 	db.data = nil
 	db.datasz = 0
+	db.mmapFallback = false
 
 	db.meta0 = nil
 	db.meta1 = nil
@@ -567,7 +577,13 @@ func (db *DB) munmap() error {
 
 	// gofail: var unmapError string
 	// return errors.New(unmapError)
-	if err := munmap(db); err != nil {
+	var err error
+	if db.mmapFallback {
+		err = munmapFallback(db)
+	} else {
+		err = munmap(db)
+	}
+	if err != nil {
 		db.Logger().Errorf("[GOOS: %s, GOARCH: %s] munmap failed, db.datasz: %d, error: %v", runtime.GOOS, runtime.GOARCH, db.datasz, err)
 		return fmt.Errorf("unmap error: %w", err)
 	}
@@ -575,6 +591,16 @@ func (db *DB) munmap() error {
 	return nil
 }
 
+func (db *DB) writeAt(b []byte, off int64) (int, error) {
+	n, err := db.ops.writeAt(b, off)
+	if n > 0 && db.mmapFallback {
+		db.mmaplock.Lock()
+		db.copyToMmapFallback(b[:n], off)
+		db.mmaplock.Unlock()
+	}
+	return n, err
+}
+
 // mmapSize determines the appropriate size for the mmap given the current size
 // of the database. The minimum size is 32KB and doubles until it reaches 1GB.
 // Returns an error if the new mmap size is greater than the max allowed.
@@ -676,7 +702,7 @@ func (db *DB) init() error {
 	p.SetCount(0)
 
 	// Write the buffer to our data file.
-	if _, err := db.ops.writeAt(buf, 0); err != nil {
+	if _, err := db.writeAt(buf, 0); err != nil {
 		db.Logger().Errorf("writeAt failed: %w", err)
 		return err
 	}
diff --git a/db_whitebox_test.go b/db_whitebox_test.go
index 8b517fc0a..f4bb9f974 100644
--- a/db_whitebox_test.go
+++ b/db_whitebox_test.go
@@ -55,6 +55,35 @@ func TestOpenWithPreLoadFreelist(t *testing.T) {
 	}
 }
 
+func TestMmapFallbackReadWrite(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "db")
+	db, err := Open(path, 0600, nil)
+	require.NoError(t, err)
+	defer db.Close()
+
+	sz := db.datasz
+	require.NoError(t, db.munmap())
+	require.NoError(t, mmapFallback(db, sz))
+	db.meta0 = db.page(0).Meta()
+	db.meta1 = db.page(1).Meta()
+
+	require.True(t, db.mmapFallback)
+	require.NoError(t, db.Update(func(tx *Tx) error {
+		bucket, err := tx.CreateBucket([]byte("widgets"))
+		if err != nil {
+			return err
+		}
+		return bucket.Put([]byte("key"), []byte("value"))
+	}))
+
+	require.NoError(t, db.View(func(tx *Tx) error {
+		bucket := tx.Bucket([]byte("widgets"))
+		require.NotNil(t, bucket)
+		require.Equal(t, []byte("value"), bucket.Get([]byte("key")))
+		return nil
+	}))
+}
+
 func TestMethodPage(t *testing.T) {
 	testCases := []struct {
 		name            string
diff --git a/mlock_plan9.go b/mlock_plan9.go
new file mode 100644
index 000000000..2b2eb730e
--- /dev/null
+++ b/mlock_plan9.go
@@ -0,0 +1,15 @@
+//go:build plan9
+
+package bbolt
+
+import "errors"
+
+// mlock locks memory of db file
+func mlock(_ *DB, _ int) error {
+	return errors.New("mlock is not supported on plan9")
+}
+
+// munlock unlocks memory of db file
+func munlock(_ *DB, _ int) error {
+	return errors.New("munlock is not supported on plan9")
+}
diff --git a/mlock_unix.go b/mlock_unix.go
index 9a0fd332c..4e208427e 100644
--- a/mlock_unix.go
+++ b/mlock_unix.go
@@ -1,4 +1,4 @@
-//go:build !windows
+//go:build !windows && !plan9
 
 package bbolt
 
diff --git a/mmap_error_plan9.go b/mmap_error_plan9.go
new file mode 100644
index 000000000..c0103d1fd
--- /dev/null
+++ b/mmap_error_plan9.go
@@ -0,0 +1,7 @@
+//go:build plan9
+
+package bbolt
+
+func isMmapUnsupported(error) bool {
+	return false
+}
diff --git a/mmap_error_unix.go b/mmap_error_unix.go
new file mode 100644
index 000000000..0a7ef3c49
--- /dev/null
+++ b/mmap_error_unix.go
@@ -0,0 +1,17 @@
+//go:build !windows && !plan9
+
+package bbolt
+
+import (
+	"errors"
+
+	"golang.org/x/sys/unix"
+)
+
+func isMmapUnsupported(err error) bool {
+	return errors.Is(err, unix.ENOSYS) ||
+		errors.Is(err, unix.ENODEV) ||
+		errors.Is(err, unix.EOPNOTSUPP) ||
+		errors.Is(err, unix.ENOTSUP) ||
+		errors.Is(err, unix.EINVAL)
+}
diff --git a/mmap_error_windows.go b/mmap_error_windows.go
new file mode 100644
index 000000000..5884aed66
--- /dev/null
+++ b/mmap_error_windows.go
@@ -0,0 +1,12 @@
+package bbolt
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+func isMmapUnsupported(err error) bool {
+	return errors.Is(err, windows.ERROR_INVALID_FUNCTION) ||
+		errors.Is(err, windows.ERROR_NOT_SUPPORTED)
+}
diff --git a/mmap_fallback.go b/mmap_fallback.go
new file mode 100644
index 000000000..d525bd7be
--- /dev/null
+++ b/mmap_fallback.go
@@ -0,0 +1,49 @@
+package bbolt
+
+import (
+	"fmt"
+	"io"
+	"unsafe"
+
+	"github.com/sagernet/bbolt/internal/common"
+)
+
+func mmapFallback(db *DB, sz int) error {
+	b := make([]byte, sz)
+
+	info, err := db.file.Stat()
+	if err != nil {
+		return fmt.Errorf("file stat: %w", err)
+	}
+
+	readSize := int64(sz)
+	if info.Size() < readSize {
+		readSize = info.Size()
+	}
+	if readSize > 0 {
+		if _, err := db.file.ReadAt(b[:readSize], 0); err != nil && err != io.EOF {
+			return fmt.Errorf("file read: %w", err)
+		}
+	}
+
+	db.dataref = b
+	db.data = (*[common.MaxMapSize]byte)(unsafe.Pointer(&b[0]))
+	db.datasz = sz
+	db.mmapFallback = true
+	return nil
+}
+
+func munmapFallback(db *DB) error {
+	db.dataref = nil
+	db.data = nil
+	db.datasz = 0
+	db.mmapFallback = false
+	return nil
+}
+
+func (db *DB) copyToMmapFallback(b []byte, off int64) {
+	if off < 0 || off >= int64(len(db.dataref)) {
+		return
+	}
+	copy(db.dataref[off:], b)
+}
diff --git a/tx.go b/tx.go
index fa2453432..6a124fb7e 100644
--- a/tx.go
+++ b/tx.go
@@ -542,7 +542,7 @@ func (tx *Tx) write() error {
 			}
 			buf := common.UnsafeByteSlice(unsafe.Pointer(p), written, 0, int(sz))
 
-			if _, err := tx.db.ops.writeAt(buf, offset); err != nil {
+			if _, err := tx.db.writeAt(buf, offset); err != nil {
 				lg.Errorf("writeAt failed, offset: %d: %w", offset, err)
 				return err
 			}
@@ -604,7 +604,7 @@ func (tx *Tx) writeMeta() error {
 
 	// Write the meta page to file.
 	tx.db.metalock.Lock()
-	if _, err := tx.db.ops.writeAt(buf, int64(p.Id())*int64(tx.db.pageSize)); err != nil {
+	if _, err := tx.db.writeAt(buf, int64(p.Id())*int64(tx.db.pageSize)); err != nil {
 		tx.db.metalock.Unlock()
 		lg.Errorf("writeAt failed, pgid: %d, pageSize: %d, error: %v", p.Id(), tx.db.pageSize, err)
 		return err