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https://issues.apache.org/jira/browse/HDFS-10797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15456729#comment-15456729
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Sean Mackrory commented on HDFS-10797:
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Just a note for anyone reading my comments above, I learned that snapshots
don't really work by maintaining references to immutable blocks, they work by
maintaining a diff from the current state. So it's not a matter of counting up
the correct blocks, it's a matter of ensuring renames, appends, etc. or
properly accounted for from the diff. That can get tricky, since a file *could*
get renamed multiple times, etc.
> Disk usage summary of snapshots causes renamed blocks to get counted twice
> --------------------------------------------------------------------------
>
> Key: HDFS-10797
> URL: https://issues.apache.org/jira/browse/HDFS-10797
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Sean Mackrory
>
> DirectoryWithSnapshotFeature.computeContentSummary4Snapshot calculates how
> much disk usage is used by a snapshot by tallying up the files in the
> snapshot that have since been deleted (that way it won't overlap with regular
> files whose disk usage is computed separately). However that is determined
> from a diff that shows moved (to Trash or otherwise) or renamed files as a
> deletion and a creation operation that may overlap with the list of blocks.
> Only the deletion operation is taken into consideration, and this causes
> those blocks to get represented twice in the disk usage tallying.
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