FAQ
All replicas for a block end up in same rack
--------------------------------------------

Key: HDFS-1480
URL: https://issues.apache.org/jira/browse/HDFS-1480
Project: Hadoop HDFS
Issue Type: Bug
Reporter: T Meyarivan
Priority: Minor


It appears that all replicas of a block can end up in the same rack. The likelihood of such replicas seems to be directly related to decommissioning of nodes.

Post rolling OS upgrade (decommission 3-10% of nodes, re-install etc, add them back) of a running cluster, all replicas of about 0.16% of blocks ended up in the same rack.

Hadoop Namenode UI etc doesn't seem to know about such incorrectly replicated blocks. "hadoop fsck .." does report that the blocks must be replicated on additional racks.

Looking at ReplicationTargetChooser.java, following seem suspect:

snippet-01:

"""
int maxNodesPerRack =
(totalNumOfReplicas-1)/clusterMap.getNumOfRacks()+2;

"""

snippet-02:

"""
if (counter>maxTargetPerLoc) {
logr.debug("Node "+NodeBase.getPath(node)+
" is not chosen because the rack has too many chosen nodes");
return false;
}
"""

snippet-03:

"""
default:
chooseRandom(numOfReplicas, NodeBase.ROOT, excludedNodes,
blocksize, maxNodesPerRack, results);
}
"""

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postedOct 26, '10 at 11:50p
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