[HADOOP-4483] getBlockArray in DatanodeDescriptor does not honor passed in maxblocks value

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 0.18.1
• Fix Version/s: 0.18.2

Environment

hadoop-0.18.1 running on a cluster of 16 nodes.

• Hadoop Flags: Reviewed
• Assignee: Ahad Rana
• Reporter: Ahad Rana
• Created: 21/Oct/08 20:38
• Updated: 08/Jul/09 16:43
• Resolved: 29/Oct/08 21:57

Description

The getBlockArray method in DatanodeDescriptor.java should honor the passed in maxblocks parameter. In its current form it passed in an array sized to min(maxblocks,blocks.size()) into the Collections.toArray method. As the javadoc for Collections.toArray indicates, the toArray method may discard the passed in array (and allocate a new array) if the number of elements returned by the iterator exceeds the size of the passed in array. As a result, the flawed implementation of this method would return all the invalid blocks for a data node in one go, and thus trigger the NameNode to send a DNA_INVALIDATE command to the DataNode with an excessively large number of blocks. This INVALIDATE command, in turn, could potentially take a very long time to process at the DataNode, and since DatanodeCommand(s) are processed in between heartbeats at the DataNode, this would trigger the NameNode to consider the DataNode to be offline / unresponsive (due to a lack of heartbeats).

In our use-case at CommonCrawl.org, we regularly do large scale hdfs file deletions after certain stages of our map-reduce pipeline. These deletes would make certain DataNode(s) unresponsive, and thus impact the cluster's capability to properly balance file-system reads / writes across the whole available cluster. This problem only surfaced once we migrated from our 16.2 deployment to the current 18.1 release.