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Date: Mon, 22 Oct 2012 19:02:16 +0000 (UTC)
From: "Tsz Wo (Nicholas), SZE (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-2802) Support for RW/RO snapshots in HDFS
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    [ https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13481643#comment-13481643 ] 

Tsz Wo (Nicholas), SZE commented on HDFS-2802:
----------------------------------------------

O(1) memory usage in general does not not seem possible since the original files/directories could be modified.  So the best case is O(N) memory usage in general.  However, it is possible to have O(1) memory usage at snapshot creation.

For small subtrees, i.e. when N is small, it does not matter if it is O(1) or O(N).  Such snapshot feature already benefits many applications.  So we are going to implement O(N) snapshot creation in the first phase and then optimization it later.  Then, we could have the snapshot feature out early instead of spending a long time to come up a complicated design and implementation.  A complicated design also increases the risk of bugs in the implementation.
                
> Support for RW/RO snapshots in HDFS
> -----------------------------------
>
>                 Key: HDFS-2802
>                 URL: https://issues.apache.org/jira/browse/HDFS-2802
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: data-node, name-node
>            Reporter: Hari Mankude
>            Assignee: Hari Mankude
>         Attachments: snap.patch, snapshot-one-pager.pdf, Snapshots20121018.pdf
>
>
> Snapshots are point in time images of parts of the filesystem or the entire filesystem. Snapshots can be a read-only or a read-write point in time copy of the filesystem. There are several use cases for snapshots in HDFS. I will post a detailed write-up soon with with more information.

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