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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