[HDFS-826] Allow a mechanism for an application to detect that datanode(s) have died in the write pipeline

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.20-append
• Fix Version/s: 0.20-append, 0.20.205.0, 0.21.0
• Component/s: hdfs-client
• Hadoop Flags: Reviewed
• Tags: hbase
• Assignee: Dhruba Borthakur
• Reporter: Dhruba Borthakur
• Created: 10/Dec/09 20:55
• Updated: 02/May/13 02:29
• Resolved: 15/Mar/10 07:24

Description

HDFS does not replicate the last block of the file that is being currently written to by an application. Every datanode death in the write pipeline decreases the reliability of the last block of the currently-being-written block. This situation can be improved if the application can be notified of a datanode death in the write pipeline. Then, the application can decide what is the right course of action to be taken on this event.

In our use-case, the application can close the file on the first datanode death, and start writing to a newly created file. This ensures that the reliability guarantee of a block is close to 3 at all time.

One idea is to make DFSOutoutStream. write() throw an exception if the number of datanodes in the write pipeline fall below minimum.replication.factor that is set on the client (this is backward compatible).

Issue Links

• blocks: HBASE-2234 Roll Hlog if any datanode in the write pipeline dies
• is blocked by: HADOOP-6450 Enhance FSDataOutputStream to allow retrieving the current number of replicas of current block
• is depended upon by: HDFS-142 In 0.20, move blocks being written into a blocksBeingWritten directory