refactor memory mapped file into backend storage

weed/storage/backend/memory_map/memory_map.go

@@ -21,8 +21,6 @@ type MemoryMap struct {
 	End_of_file            int64
 }
 
-var FileMemoryMap = make(map[string]*MemoryMap)
-
 func ReadMemoryMapMaxSizeMb(memoryMapMaxSizeMbString string) (uint32, error) {
 	if memoryMapMaxSizeMbString == "" {
 		return 0, nil

weed/storage/backend/memory_map/memory_map_backend.go

@@ -0,0 +1,60 @@
+package memory_map
+
+import (
+	"os"
+	"time"
+
+	"github.com/chrislusf/seaweedfs/weed/storage/backend"
+)
+
+var (
+	_ backend.DataStorageBackend = &MemoryMappedFile{}
+)
+
+type MemoryMappedFile struct {
+	mm *MemoryMap
+}
+
+func NewMemoryMappedFile(f *os.File, memoryMapSizeMB uint32) *MemoryMappedFile {
+	mmf := &MemoryMappedFile{
+		mm : new(MemoryMap),
+	}
+	mmf.mm.CreateMemoryMap(f, 1024*1024*uint64(memoryMapSizeMB))
+	return mmf
+}
+
+func (mmf *MemoryMappedFile) ReadAt(p []byte, off int64) (n int, err error) {
+	readBytes, e :=  mmf.mm.ReadMemory(uint64(off), uint64(len(p)))
+	if e != nil {
+		return 0, e
+	}
+	// TODO avoid the extra copy
+	copy(p, readBytes)
+	return len(readBytes), nil
+}
+
+func (mmf *MemoryMappedFile) WriteAt(p []byte, off int64) (n int, err error) {
+	mmf.mm.WriteMemory(uint64(off), uint64(len(p)), p)
+	return len(p), nil
+}
+
+func (mmf *MemoryMappedFile) Truncate(off int64) error {
+	return nil
+}
+
+func (mmf *MemoryMappedFile) Close() error {
+	mmf.mm.DeleteFileAndMemoryMap()
+	return nil
+}
+
+func (mmf *MemoryMappedFile) GetStat() (datSize int64, modTime time.Time, err error) {
+	stat, e := mmf.mm.File.Stat()
+	if e == nil {
+		return stat.Size(), stat.ModTime(), nil
+	}
+	return 0, time.Time{}, err
+}
+
+func (mmf *MemoryMappedFile) String() string {
+	return mmf.mm.File.Name()
+}