不把被释放的块合并起来，而是让他们保持当前大小以期望不久会有一个相同大小的请求。
Rather than coalescing freed chunks, leave them at their current sizes in hopes that another request for the same size will come along soon.
你的大脑总是试着去这样做，但是不久它就会愿意存在于当前。
The mind is always trying to do, and it won't be long before it volunteers to stay as the presence.
在当前实现中，checkpoint只发生在Namenode启动时，在不久的将来我们将实现支持周期性的checkpoint。
In the current implementation, a checkpoint only occurs when the NameNode starts up. Work is in progress to support periodic checkpointing in the near future.
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