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

代码@1：先重点解释 begin、diff 两个局部变量的含义：  begin 通俗的一点讲，就是将消息写入 Commitlog 文件所持有锁的时间，精确说是将消息体 追加到内存映射文件(DirectByteBuffer)或 pageCache(FileChannel#map)该过程中开 始持有锁的时间戳，具体的代码请参考：CommitLog#putMessage。  diff 一次消息追加过程中持有锁的总时长，即往内存映射文件或 作情况下，会出现缺页中断 降低，内存加锁，污染页的回写。 第二种，DirectByteBuffer(堆外内存)+PageCache 的两层架构方式，这样子可以实 现读写消息分离，写入消息时候写到的是 DirectByteBuffer——堆外内存中,读消息走的是 PageCache(对于,DirectByteBuffer 是两步刷盘，一步是刷到 PageCache，还有一步 本文来自『中间 从这里可以看出，TransientStorePool 默认会初始化 5 个 DirectByteBuffer(对外内 存)，并提供内存锁定功能，即这部分内存不会被置换，可通过 transientStorePoolSize 参数控制。在消息写入消息时，首先从池子中获取一个 DirectByteBuffer 进行消息的追加， 那当 5 个 DirectByteBuffer 全部写满消息后，该如何处理呢？从 RocketMQ

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