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From "Steve Hoffman (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-1312) Re-balance disks within a Datanode
Date Fri, 19 Oct 2012 14:32:16 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1312?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480055#comment-13480055
] 

Steve Hoffman commented on HDFS-1312:
-------------------------------------

Given the general nature of HDFS and its many uses (HBase, M/R, etc) as much as I'd like it
to "just work", it is clear it always depends on the use.  Maybe one day we won't need a balancer
script for disks (or for the cluster).

I'm totally OK with having a machine-level balancer script.  We use the HDFS balancer to fix
inter-machine imbalances when they crop up (again, for a variety of reasons).  It makes sense
to have a manual script for intra-machine imbalances for people who DO have issues and make
it part of the standard install (like the HDFS balancer).
                
> Re-balance disks within a Datanode
> ----------------------------------
>
>                 Key: HDFS-1312
>                 URL: https://issues.apache.org/jira/browse/HDFS-1312
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: data-node
>            Reporter: Travis Crawford
>
> Filing this issue in response to ``full disk woes`` on hdfs-user.
> Datanodes fill their storage directories unevenly, leading to situations where certain
disks are full while others are significantly less used. Users at many different sites have
experienced this issue, and HDFS administrators are taking steps like:
> - Manually rebalancing blocks in storage directories
> - Decomissioning nodes & later readding them
> There's a tradeoff between making use of all available spindles, and filling disks at
the sameish rate. Possible solutions include:
> - Weighting less-used disks heavier when placing new blocks on the datanode. In write-heavy
environments this will still make use of all spindles, equalizing disk use over time.
> - Rebalancing blocks locally. This would help equalize disk use as disks are added/replaced
in older cluster nodes.
> Datanodes should actively manage their local disk so operator intervention is not needed.

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