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From "Sean Mackrory (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10797) Disk usage summary of snapshots causes renamed blocks to get counted twice
Date Thu, 01 Sep 2016 21:50:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15456729#comment-15456729
] 

Sean Mackrory commented on HDFS-10797:
--------------------------------------

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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