diff --git a/cmd/heavy-load/.gitignore b/cmd/heavy-load/.gitignore
new file mode 100644
index 0000000..c44904f
--- /dev/null
+++ b/cmd/heavy-load/.gitignore
@@ -0,0 +1,3 @@
+/main
+/_watcher
+/_storage
diff --git a/cmd/heavy-load/Makefile b/cmd/heavy-load/Makefile
new file mode 100644
index 0000000..0cf94c6
--- /dev/null
+++ b/cmd/heavy-load/Makefile
@@ -0,0 +1,13 @@
+all: _watcher _storage main
+
+_%:
+	mkdir -p $@
+
+main:
+	go build -o $@
+
+clean:
+	rm -rf main
+
+realclean: clean
+	rm -rf _watcher _storage
diff --git a/cmd/heavy-load/main.go b/cmd/heavy-load/main.go
new file mode 100644
index 0000000..4420cda
--- /dev/null
+++ b/cmd/heavy-load/main.go
@@ -0,0 +1,159 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"flag"
+	"io"
+	"log"
+	"os"
+	"sync"
+	"time"
+
+	"github.com/yowcow/goromdb/handler/simplehandler"
+	"github.com/yowcow/goromdb/loader"
+	"github.com/yowcow/goromdb/storage/boltstorage"
+	"github.com/yowcow/goromdb/watcher"
+)
+
+var (
+	concurrency int
+	duration    int
+	help        bool
+	logger      *log.Logger
+
+	bucket      = "goromdb"
+	watcherFile = "_watcher/data.db"
+	storagePath = "_storage"
+
+	sourceDataFile = "../../data/store/sample-boltdb.db"
+	sourceMD5File  = "../../data/store/sample-boltdb.db.md5"
+)
+
+func init() {
+	flag.IntVar(&concurrency, "c", 1, "concurrency")
+	flag.IntVar(&duration, "d", 1, "duration in seconds")
+	flag.BoolVar(&help, "h", false, "show help")
+	flag.Parse()
+
+	if help {
+		flag.Usage()
+		os.Exit(0)
+	}
+}
+
+func init() {
+	logger = log.New(os.Stdout, "", log.Ldate|log.Ltime)
+}
+
+func init() {
+	if _, err := os.Stat(sourceDataFile); os.IsNotExist(err) {
+		logger.Println(sourceDataFile, "is not found")
+		os.Exit(1)
+	}
+	if _, err := os.Stat(sourceMD5File); os.IsNotExist(err) {
+		logger.Println(sourceMD5File, "is not found")
+		os.Exit(2)
+	}
+}
+
+func main() {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(duration)*time.Second+3*time.Second)
+	defer cancel()
+
+	// create watcher
+	wcr := watcher.New(watcherFile, 1, logger)
+	filein := wcr.Start(ctx)
+
+	// create storage
+	stg := boltstorage.New(bucket)
+
+	// create loader
+	ldr, err := loader.New(storagePath, "data.db")
+	if err != nil {
+		panic(err)
+	}
+
+	// create handler
+	hdr := simplehandler.New(stg, logger)
+
+	var wg sync.WaitGroup
+
+	// start goromdb handler
+	wg.Add(1)
+	go func(w *sync.WaitGroup) {
+		defer w.Done()
+		done := hdr.Start(filein, ldr)
+		<-done
+	}(&wg)
+
+	// start infinite file loading
+	wg.Add(1)
+	go func(w *sync.WaitGroup) {
+		defer w.Done()
+		tc := time.NewTicker(500 * time.Millisecond)
+		for {
+			// copy data.db
+			if _, err := os.Stat(watcherFile); os.IsNotExist(err) {
+				if r, err := os.Open(sourceDataFile); err == nil {
+					if w, err := os.OpenFile(watcherFile+".tmp", os.O_WRONLY|os.O_CREATE, 0644); err == nil {
+						io.Copy(w, r)
+						w.Close()
+					}
+					r.Close()
+				}
+				os.Rename(watcherFile+".tmp", watcherFile)
+			}
+			// copy data.db.md5
+			if _, err := os.Stat(watcherFile + ".md5"); os.IsNotExist(err) {
+				if r, err := os.Open(sourceMD5File); err == nil {
+					if w, err := os.OpenFile(watcherFile+".md5.tmp", os.O_WRONLY|os.O_CREATE, 0644); err == nil {
+						io.Copy(w, r)
+						w.Close()
+					}
+					r.Close()
+				}
+				os.Rename(watcherFile+".md5.tmp", watcherFile+".md5")
+			}
+
+			select {
+			case <-tc.C:
+			case <-ctx.Done():
+				tc.Stop()
+				return
+			}
+		}
+	}(&wg)
+
+	time.Sleep(3 * time.Second) // wait 3 secs
+
+	// start infinite `Get` calls
+	f := func(id int, w *sync.WaitGroup, c context.Context, l *log.Logger) {
+		defer w.Done()
+		for {
+			_, err := hdr.Get([]byte("hoge"))
+			if err != nil {
+				l.Println("worker", id, "got error:", err)
+			}
+
+			select {
+			case <-c.Done():
+				return
+			default:
+			}
+		}
+	}
+
+	logbuf := new(bytes.Buffer)
+	l := log.New(logbuf, "", log.Ldate|log.Ltime)
+	wg.Add(concurrency)
+	for i := 0; i < concurrency; i++ {
+		go f(i+1, &wg, ctx, l)
+	}
+
+	// wait for everybody
+	wg.Wait()
+
+	logger.Println("===== errors during `Get()` calls =====")
+	io.WriteString(os.Stdout, logbuf.String())
+}
diff --git a/storage/bdbstorage/storage.go b/storage/bdbstorage/storage.go
index 5bb74a9..59eb3ba 100644
--- a/storage/bdbstorage/storage.go
+++ b/storage/bdbstorage/storage.go
@@ -15,12 +15,12 @@ var (
 // Storage represents a BDB storage
 type Storage struct {
 	db  *atomic.Value
-	mux *sync.Mutex
+	mux *sync.RWMutex
 }
 
 // New creates and returns a storage
 func New() *Storage {
-	return &Storage{new(atomic.Value), new(sync.Mutex)}
+	return &Storage{new(atomic.Value), new(sync.RWMutex)}
 }
 
 // Load loads a new db handle into storage, and closes old db handle if exists
@@ -54,15 +54,18 @@ func openBDB(file string) (*bdb.BerkeleyDB, error) {
 
 // Get finds a given key in db, and returns its value
 func (s *Storage) Get(key []byte) ([]byte, error) {
-	s.mux.Lock()
-	defer s.mux.Unlock()
+	s.mux.RLock()
+	defer s.mux.RUnlock()
+
 	db := s.getDB()
 	if db == nil {
 		return nil, storage.InternalError("couldn't load db")
 	}
+
 	v, err := db.Get(bdb.NoTxn, key, 0)
 	if err != nil {
 		return nil, storage.KeyNotFoundError(key)
 	}
+
 	return v, nil
 }
diff --git a/storage/boltstorage/storage.go b/storage/boltstorage/storage.go
index 696fdd2..672d23f 100644
--- a/storage/boltstorage/storage.go
+++ b/storage/boltstorage/storage.go
@@ -56,14 +56,14 @@ func openDB(file string) (*bolt.DB, error) {
 
 // Get finds a given key in db, and returns its value
 func (s *Storage) Get(key []byte) ([]byte, error) {
+	s.mux.RLock()
+	defer s.mux.RUnlock()
+
 	db := s.getDB()
 	if db == nil {
 		return nil, storage.InternalError("couldn't load db")
 	}
 
-	s.mux.RLock()
-	defer s.mux.RUnlock()
-
 	return getFromBucket(db, s.bucket, key)
 }