diff --git a/lib/fs/reader_at_timing_test.go b/lib/fs/reader_at_timing_test.go
new file mode 100644
index 0000000000..e28b453953
--- /dev/null
+++ b/lib/fs/reader_at_timing_test.go
@@ -0,0 +1,56 @@
+package fs
+
+import (
+	"fmt"
+	"io/ioutil"
+	"testing"
+)
+
+func BenchmarkReaderAtMustReadAt(b *testing.B) {
+	b.Run("mmap_on", func(b *testing.B) {
+		benchmarkReaderAtMustReadAt(b, true)
+	})
+	b.Run("mmap_off", func(b *testing.B) {
+		benchmarkReaderAtMustReadAt(b, false)
+	})
+}
+
+func benchmarkReaderAtMustReadAt(b *testing.B, isMmap bool) {
+	prevDisableMmap := *disableMmap
+	*disableMmap = !isMmap
+	defer func() {
+		*disableMmap = prevDisableMmap
+	}()
+
+	path := "BenchmarkReaderAtMustReadAt"
+	const fileSize = 1024 * 1024
+	data := make([]byte, fileSize)
+	if err := ioutil.WriteFile(path, data, 0600); err != nil {
+		b.Fatalf("cannot create %q: %s", path, err)
+	}
+	defer MustRemoveAll(path)
+	r, err := OpenReaderAt(path)
+	if err != nil {
+		b.Fatalf("error in OpenReaderAt(%q): %s", path, err)
+	}
+	defer r.MustClose()
+
+	b.ResetTimer()
+	for _, bufSize := range []int{1, 10, 100, 1000, 10000} {
+		b.Run(fmt.Sprintf("%d", bufSize), func(b *testing.B) {
+			b.ReportAllocs()
+			b.SetBytes(int64(bufSize))
+			b.RunParallel(func(pb *testing.PB) {
+				buf := make([]byte, bufSize)
+				var offset int64
+				for pb.Next() {
+					if len(buf)+int(offset) > fileSize {
+						offset = 0
+					}
+					r.MustReadAt(buf, offset)
+					offset += int64(len(buf))
+				}
+			})
+		})
+	}
+}