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https://issues.apache.org/jira/browse/HDFS-343?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12747725#action_12747725
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Boris Shkolnik commented on HDFS-343:
-------------------------------------
I agree with Enis, that using random policy in highly heterogeneous clusters we
may end up with severely underutilized nodes.
(One extreme use-case is adding a new/restored empty node to a running
cluster).
I don't know about pluggable policy, but some modification/improvements to
existing one can help.
Using probability(based on usage) instead of direct placement should address
issue #1.
We also need to make sure that this overwrites only the "random" part of the
placing policy.
> Better Target selection for block replication
> ---------------------------------------------
>
> Key: HDFS-343
> URL: https://issues.apache.org/jira/browse/HDFS-343
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Enis Soztutar
>
> Block replication policy tends to balance the number of blocks in each
> datanode in the long run, however with heterogeneous clusters with varying
> number of disks per node, the nodes with one disk fill quickly while nodes
> with 3 disks still have 60% free disk space. This also reduces the advantage
> of using more than one disk for parallel IO, since machines with multiple
> disks are not used as much.
> The javadoc of the ReplicationTargetChooser reads as :
> The replica placement strategy is that if the writer is on a datanode, the
> 1st replica is placed on the local machine, otherwise a random datanode. The
> 2nd replica is placed on a datanode that is on a different rack. The 3rd
> replica is placed on a datanode which is on the same rack as the first
> replica.
> I think we should switch to a policy that balances the percent of disk usage
> rather than balancing total block count among the datanodes. This can be done
> by defining the probability of selection of a datanode based on its disk
> percent usage. A formula like 1 - (percent_usage / 100 ) seems reasonable.
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