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https://issues.apache.org/jira/browse/HDFS-3570?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13894196#comment-13894196
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Andrew Ash commented on HDFS-3570:
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I applied this patch to cdh4.4.0 source and swapped out the hadoop-hdfs-*jar in
my hadoop install's lib/ directory. Running hadoop balancer shows the new
option for -policy now and running with that option on my 5-node cluster is
progressing. The blocks are being shifted in a way that's the same for both
the datanode and datanode-actual policies though, so it will be a bit before I
can determine that the end state is the proper one.
> Balancer shouldn't rely on "DFS Space Used %" as that ignores non-DFS used
> space
> --------------------------------------------------------------------------------
>
> Key: HDFS-3570
> URL: https://issues.apache.org/jira/browse/HDFS-3570
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: balancer
> Affects Versions: 2.0.0-alpha
> Reporter: Harsh J
> Assignee: Akira AJISAKA
> Priority: Minor
> Attachments: HDFS-3570.2.patch, HDFS-3570.aash.1.patch
>
>
> Report from a user here:
> https://groups.google.com/a/cloudera.org/d/msg/cdh-user/pIhNyDVxdVY/b7ENZmEvBjIJ,
> post archived at http://pastebin.com/eVFkk0A0
> This user had a specific DN that had a large non-DFS usage among
> dfs.data.dirs, and very little DFS usage (which is computed against total
> possible capacity).
> Balancer apparently only looks at the usage, and ignores to consider that
> non-DFS usage may also be high on a DN/cluster. Hence, it thinks that if a
> DFS Usage report from DN is 8% only, its got a lot of free space to write
> more blocks, when that isn't true as shown by the case of this user. It went
> on scheduling writes to the DN to balance it out, but the DN simply can't
> accept any more blocks as a result of its disks' state.
> I think it would be better if we _computed_ the actual utilization based on
> {{(100-(actual remaining space))/(capacity)}}, as opposed to the current
> {{(dfs used)/(capacity)}}. Thoughts?
> This isn't very critical, however, cause it is very rare to see DN space
> being used for non DN data, but it does expose a valid bug.
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