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https://issues.apache.org/jira/browse/HDFS-4758?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13642256#comment-13642256
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Todd Lipcon commented on HDFS-4758:
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bq. For your first example, we may set snapshottable for all users. Then taking
snapshots of /user can be done by taking snapshots for all user dirs.
But then the snapshot is not atomic across users, which makes it difficult to
do a "point in time backup" of a cluster by snapshot+distcp
bq. For root, I think we should not encourage taking snapshots since all the
tmp files will sit in a snapshot forever. Instead, we should encourage taking
sanpshots in subdirectories.
A common use case for snapshots is a short-lived snapshot which is then used as
the source for distcp. The distcp can explicitly exclude copying tmp files.
Once the distcp is complete, then the snapshot can be removed, so the space
usage of the tmp files is only temporary.
> Disallow nested snapshottable directories
> -----------------------------------------
>
> Key: HDFS-4758
> URL: https://issues.apache.org/jira/browse/HDFS-4758
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: namenode
> Reporter: Tsz Wo (Nicholas), SZE
> Assignee: Tsz Wo (Nicholas), SZE
>
> Nested snapshottable directories are supported by the current implementation.
> However, it seems that there are no good use cases for nested snapshottable
> directories. So we disable it for now until someone has a valid use case for
> it.
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