namenode should be able to retrieve block metadata from a datanode
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Key: HADOOP-3201
URL: https://issues.apache.org/jira/browse/HADOOP-3201
Project: Hadoop Core
Issue Type: New Feature
Components: dfs
Reporter: dhruba borthakur
The Append design (HADOOP-1700) requires that the namenode be able to contact
the datanode when a lease expires.
When a lease expires, the namenode has to fix up the size of the last block of
the file that was being written to. The namenode contacts the datanodes,
retrieves the block generation stamp and the length of the lastblock of all
known replicas, determines which replicas are good and which one are to be
deleted, stamps the winning replicas with a new generation stamp and deletes
losing replicas. Once this process is complete, the namenode can allow a new
writer to append to this file. Details of this design are in HADOOP-1700.
A few options available to us:
1. The namenode sends the request as a response to the next heartbeat RPC from
the datanode. This methodology is currently used for requesting block reports
from datanodes.
2. The datanode has an RPC server. A pool of threads in the namenode can be
used to make RPCs to the datanodes.
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