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From "Aaron T. Myers (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4103) Support O(1) snapshot creation
Date Wed, 26 Dec 2012 09:44:14 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4103?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13539504#comment-13539504
] 

Aaron T. Myers commented on HDFS-4103:
--------------------------------------

Fine by me. Like I said, most of these I considered less important than #16. I think in general
I'm more concerned with code clarity than you are, but that shouldn't affect correctness so
have at. 
                
> Support O(1) snapshot creation
> ------------------------------
>
>                 Key: HDFS-4103
>                 URL: https://issues.apache.org/jira/browse/HDFS-4103
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: namenode
>    Affects Versions: Snapshot (HDFS-2802)
>            Reporter: Tsz Wo (Nicholas), SZE
>            Assignee: Tsz Wo (Nicholas), SZE
>             Fix For: Snapshot (HDFS-2802)
>
>         Attachments: h4103_20121129.patch, h4103_20121202b.patch, h4103_20121202.patch,
h4103_20121209b.patch, h4103_20121209.patch, h4103_20121210b.patch, h4103_20121210.patch,
h4103_20121211.patch, h4103_20121212b.patch, h4103_20121212.patch, h4103_20121213.patch, h4103_20121215b.patch,
h4103_20121215.patch, h4103_20121216.patch, h4103_20121217b.patch, h4103_20121217.patch, h4103_20121218.patch,
h4103_20121219.patch, h4103_20121220.patch
>
>
> In our first snapshot implementation, snapshot creation runs in O(N) and occupies O(N)
memory space, where N = # files + # directories + # symlinks in the snapshot.  The advantages
of the implementation are that there is no additional cost for the modifications after snapshots
are created, and it leads to a simple implementation.
> In this JIRA, we optimize snapshot creation to O(1) although it introduces additional
cost in the modifications after snapshots are created.  Note that the INode is given as an
assumption, otherwise, there is a non-constant cost to find the INode.

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