XL: GetObject caching implemented for XL. (#2017)

The object cache implementation is XL cache, which defaults
to 8GB worth of read cache. Currently GetObject() transparently
writes to this cache upon first client read and then subsequently
serves reads from the same cache.

Currently expiration is not implemented.

xl-v1-object.go

@@ -27,6 +27,7 @@ import (
 	"time"
 
 	"github.com/minio/minio/pkg/mimedb"
+	"github.com/minio/minio/pkg/objcache"
 )
 
 /// Object Operations
@@ -92,6 +93,45 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64, length i
 		eInfos = append(eInfos, metaArr[index].Erasure)
 	}
 
+	// Save the writer.
+	mw := writer
+
+	// Object cache enabled block.
+	if xl.objCacheEnabled {
+		// Validate if we have previous cache.
+		cachedBuffer, err := xl.objCache.Open(path.Join(bucket, object))
+		if err == nil { // Cache hit.
+			// Advance the buffer to offset as if it was read.
+			if _, err = cachedBuffer.Seek(startOffset, 0); err != nil { // Seek to the offset.
+				return err
+			}
+			// Write the requested length.
+			if _, err = io.CopyN(writer, cachedBuffer, length); err != nil {
+				return err
+			}
+			return nil
+		} // Cache miss.
+
+		// For unknown error, return and error out.
+		if err != objcache.ErrKeyNotFoundInCache {
+			return err
+		} // Cache has not been found, fill the cache.
+
+		// Proceed to set the cache.
+		var newBuffer io.Writer
+		// Cache is only set if whole object is being read.
+		if startOffset == 0 && length == xlMeta.Stat.Size {
+			// Create a new entry in memory of length.
+			newBuffer, err = xl.objCache.Create(path.Join(bucket, object), length)
+			if err != nil {
+				// Perhaps cache is full, returns here.
+				return err
+			}
+			// Create a multi writer to write to both memory and client response.
+			mw = io.MultiWriter(newBuffer, writer)
+		}
+	}
+
 	totalBytesRead := int64(0)
 	// Read from all parts.
 	for ; partIndex <= lastPartIndex; partIndex++ {
@@ -109,7 +149,7 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64, length i
 		}
 
 		// Start reading the part name.
-		n, err := erasureReadFile(writer, onlineDisks, bucket, pathJoin(object, partName), partName, eInfos, partOffset, readSize, partSize)
+		n, err := erasureReadFile(mw, onlineDisks, bucket, pathJoin(object, partName), partName, eInfos, partOffset, readSize, partSize)
 		if err != nil {
 			return err
 		}