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https://issues.apache.org/jira/browse/HDFS-3290?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13256783#comment-13256783
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Matt Foley commented on HDFS-3290:
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Hi Colin,
I think you've misunderstood the block storage. In each data sub-directory, it
stores the next 64 blocks (by default) and their metadata (128 files
altogether), then spawns up to 64 new subdirectories and starts filling those.
Recurses as necessary. Result is a directory tree where each sub-directory has
a max of 192 objects, and the leaf directories have 128 or less.
Please see org.apache.hadoop.hdfs.server.datanode.FSDataset.FSDir.addBlock() in
the hadoop-1 branch.
> Use a better local directory layout for the datanode
> ----------------------------------------------------
>
> Key: HDFS-3290
> URL: https://issues.apache.org/jira/browse/HDFS-3290
> Project: Hadoop HDFS
> Issue Type: Improvement
> Affects Versions: 0.23.0
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Priority: Minor
>
> When the HDFS DataNode stores chunks in a local directory, it currently puts
> all of the chunk files into one big directory. As the number of files
> increases, this does not work well at all. Local filesystems are not
> optimized for the case where there are hundreds of thousands of files in the
> same directory. It also makes inspecting directories with standard UNIX
> tools difficult.
> Similar to the git version control system, HDFS should create a few different
> top level directories keyed off of a few bits in the chunk ID. Git uses 8
> bits. This substantially cuts down on the number of chunk files in the same
> directory and gives increased performance.
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