[jira] [Created] (HDFS-13739) Option to disable Rack Local Write Preference to avoid Major Storage Imbalance across DataNodes caused by uneven spread of Datanodes across Racks

["Hari Sekhon (JIRA)" <ji...@apache.org>]

Hari Sekhon created HDFS-13739:
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             Summary: Option to disable Rack Local Write Preference to avoid Major Storage Imbalance across DataNodes caused by uneven spread of Datanodes across Racks
                 Key: HDFS-13739
                 URL: https://issues.apache.org/jira/browse/HDFS-13739
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: balancer & mover, block placement, datanode, fs, hdfs, hdfs-client, namenode, nn, performance
    Affects Versions: 2.7.3
         Environment: Hortonworks HDP 2.6
            Reporter: Hari Sekhon


Current HDFS write pattern of "local node, rack local node, other rack node" is good for most purposes but when there is an uneven layout of datanodes across racks it can cause major storage imbalance across nodes with some nodes filling up and others being half empty.

I have observed this on a cluster where half the nodes were 85% full and the other half were only 50% full.

Rack layouts like the following illustrate this - the nodes in the same rack will only choose to send half their block replicas to each other, so they will fill up first, while other nodes will receive far fewer replica blocks:
{code:java}
NumNodes - Rack 
2 - rack 1
2 - rack 2
1 - rack 3
1 - rack 4 
1 - rack 5
1 - rack 6{code}
In this case if I reduce the number of replicas to 2 then I get an almost perfect spread of blocks across all datanodes because HDFS has no choice but to maintain the only 2nd replica on a different rack. If I increase the replicas back to 3 it goes back to 85% on half the nodes and 50% on the other half, because the extra replicas choose to replicate only to rack local nodes.


 Why not just run the HDFS balancer to fix it you might say? This is a heavily loaded HBase cluster - aside from destroying HBase's data locality and performance by moving blocks out from underneath RegionServers - as soon as an HBase major compaction occurs (at least weekly), all blocks will get re-written by HBase and the HDFS client will again write to local node, rack local node, other rack node and resulting in the same storage imbalance again. Hence this cannot be solved by running HDFS balancer on HBase clusters - or for any application sitting on top of HDFS that has any HDFS block churn.



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