off-heap usage, and divide the whole off-heap memory as many small DirectByteBuffer which are composed as a ​reservoir​. each DirectByteBuffer has a fixed size, such as 64KB. You can tune this by: a. hbase to allocate an fixed-size DirectByteBuffer. 3. If the desired size is less than 64KB, the allocation will be an SingleByteBuff backend by one fixed-size DirectByteBuffer. 4. If the desired size is greater than 64KB, then the allocation will be an MultiByteBuff composited by multiple fixed-size DirectByteBuffer (or with an extra HeapByteBuffer for the reminder based rule#2); While the memory management

BucketCache StoreFileScanner Return a DirectByteBuffer(off, len) which reference to the offheap block. Rpc Handler offheap onheap Use the DirectByteBuffer in all the read path. branch-1.5 branch-2