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Date: Mon, 22 Oct 2012 20:22:12 +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=13481747#comment-13481747 ] 

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

> ... but it is certainly possible to have an O(1) solution in terms of the number of files/directories that are not modified. ...

I think the meaning of O(1) above is a very special use of the big-O notation.  I would rather keep saying 

- O(N), where N = # files + # directories + # symlinks; and
- O(M), where M = # modified files + # modified directories + # modified symlinks.

I agree that O(M) implementation (i.e. include O(1) snapshot creation) is our end goal.
                
> 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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