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https://issues.apache.org/jira/browse/HDFS-4461?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13687122#comment-13687122
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Todd Lipcon commented on HDFS-4461:
-----------------------------------
FWIW we are running into this again with another customer who has a large
number of blocks per DN. The heap usage is constant at around 4GB until the
directory scanner runs. This causes an doubling of the required heap in the DN,
which caused OOM. We've worked around it by bumping the heap even higher, but
we should address this issue. The fact that the usage is short-lived doesn't
matter, since the heap size has to be set according to the max heap, not the
average heap.
> DirectoryScanner: volume path prefix takes up memory for every block that is
> scanned
> -------------------------------------------------------------------------------------
>
> Key: HDFS-4461
> URL: https://issues.apache.org/jira/browse/HDFS-4461
> Project: Hadoop HDFS
> Issue Type: Improvement
> Affects Versions: 2.0.3-alpha
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Priority: Minor
> Attachments: HDFS-4461.002.patch, HDFS-4461.003.patch,
> HDFS-4461.004.patch, memory-analysis.png
>
>
> In the {{DirectoryScanner}}, we create a class {{ScanInfo}} for every block.
> This object contains two File objects-- one for the metadata file, and one
> for the block file. Since those File objects contain full paths, users who
> pick a lengthly path for their volume roots will end up using an extra
> N_blocks * path_prefix bytes per block scanned. We also don't really need to
> store File objects-- storing strings and then creating File objects as needed
> would be cheaper. This would be a nice efficiency improvement.
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