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https://issues.apache.org/jira/browse/HDFS-6482?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14081618#comment-14081618
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Colin Patrick McCabe commented on HDFS-6482:
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Why don't we merge this to trunk and then open another JIRA to iron out any
issues with rolling upgrades between different DN layout versions. At minimum,
we should decide whether we support rolling DN upgrades between different
layout versions, and if we don't support it, give a clear failure message to
admins. But this patch is big enough that I don't think cramming all that into
here is a good idea. There also seem to be some issues with rolling DN
downgrade now (for example, HDFS-6005 removed {{datanode \-rollingupgrade
\-rollback}}, but not the usage text for it displayed in {{\-help}}.)
> Use block ID-based block layout on datanodes
> --------------------------------------------
>
> Key: HDFS-6482
> URL: https://issues.apache.org/jira/browse/HDFS-6482
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.5.0
> Reporter: James Thomas
> Assignee: James Thomas
> Attachments: 6482-design.doc, HDFS-6482.1.patch, HDFS-6482.2.patch,
> HDFS-6482.3.patch, HDFS-6482.4.patch, HDFS-6482.5.patch, HDFS-6482.6.patch,
> HDFS-6482.7.patch, HDFS-6482.8.patch, HDFS-6482.9.patch, HDFS-6482.patch,
> hadoop-24-datanode-dir.tgz
>
>
> Right now blocks are placed into directories that are split into many
> subdirectories when capacity is reached. Instead we can use a block's ID to
> determine the path it should go in. This eliminates the need for the LDir
> data structure that facilitates the splitting of directories when they reach
> capacity as well as fields in ReplicaInfo that keep track of a replica's
> location.
> An extension of the work in HDFS-3290.
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