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https://issues.apache.org/jira/browse/HDFS-3570?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14644814#comment-14644814
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Colin Patrick McCabe commented on HDFS-3570:
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I agree that it would be nice to have an optimized code path assuming a
dedicated partition for HDFS. We could get space used by calling df rather
than du, which would be much more efficient. However, in the past, we've
avoided doing this because MR almost always spills to the same disks that HDFS
is using, so we would have to have 2 partitions on every disk. I'm not sure if
there is a good way around this problem...
> 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 & mover
> Affects Versions: 2.0.0-alpha
> Reporter: Harsh J
> Assignee: Akira AJISAKA
> Priority: Minor
> Attachments: HDFS-3570.003.patch, 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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