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https://issues.apache.org/jira/browse/HDFS-4461?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13568989#comment-13568989
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Todd Lipcon commented on HDFS-4461:
-----------------------------------
Looks like we can cut the memory usage in half again -- storing both the
metafile path and the block file path is redundant, since you can always
compute the block path from the meta path by chopping off the
"_<genstamp>.meta" prefix.
Suresh -- we routinely see users with millions of replicas per DN now that
48TB+ configurations have become commodity. Sure, we should also encourage
users to use things like HAR to coalesce into larger blocks, but easy wins on
DN memory usage are a no-brainer IMO.
> 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,
> 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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