FUSE: add chunk cache for recently accessed file chunks

weed/pb/pb_cache/chunk_cache.go

@@ -0,0 +1,32 @@
+package pb_cache
+
+import (
+	"time"
+
+	"github.com/karlseguin/ccache"
+)
+
+// a global cache for recently accessed file chunks
+type ChunkCache struct {
+	cache *ccache.Cache
+}
+
+func NewChunkCache() *ChunkCache {
+	return &ChunkCache{
+		cache: ccache.New(ccache.Configure().MaxSize(1000).ItemsToPrune(100)),
+	}
+}
+
+func (c *ChunkCache) GetChunk(fileId string) []byte {
+	item := c.cache.Get(fileId)
+	if item == nil {
+		return nil
+	}
+	data := item.Value().([]byte)
+	item.Extend(time.Hour)
+	return data
+}
+
+func (c *ChunkCache) SetChunk(fileId string, data []byte) {
+	c.cache.Set(fileId, data, time.Hour)
+}