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Date: Wed, 16 Mar 2011 10:28:30 +0000 (UTC)
From: "Steve Loughran (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] Commented: (HDFS-1312) Re-balance disks within a Datanode
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    [ https://issues.apache.org/jira/browse/HDFS-1312?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13007403#comment-13007403 ] 

Steve Loughran commented on HDFS-1312:
--------------------------------------

Another driver for rebalancing is a cluster set up with mapred temp space allocated to the same partitions as HDFS. This delivers best IO rate for temp storage, but once that temp space is reclaimed, the disks can get unbalanced.

> 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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