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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4103) Support O(1) snapshot creation
Date Mon, 29 Oct 2012 01:19:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4103?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13485771#comment-13485771
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Tsz Wo (Nicholas), SZE commented on HDFS-4103:
----------------------------------------------

There is a well-known data structure supportting
- O(1) snapshot creation,
- O(1) access slowdown, and
- O(1) modification space and time.

See http://www.cs.cmu.edu/~sleator/papers/Persistence.htm

The data structure collides with the ideas Suresh, Jing and I were discussing.  We probably
will use it in our implementation.
                
> 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: name-node
>            Reporter: Tsz Wo (Nicholas), SZE
>            Assignee: Tsz Wo (Nicholas), SZE
>
> 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.

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