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Date: Thu, 25 Apr 2013 21:44:16 +0000 (UTC)
From: "Todd Lipcon (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Message-ID: <JIRA.12644685.1366924617968.227069.1366926256721@arcas>
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Subject: [jira] [Commented] (HDFS-4758) Disallow nested snapshottable
 directories
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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 ] 

Todd Lipcon commented on HDFS-4758:
-----------------------------------

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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