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https://issues.apache.org/jira/browse/HDFS-4103?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13508428#comment-13508428
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Suresh Srinivas commented on HDFS-4103:
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Some more comments for InodeDirectoryWithSnapshot.java:
# SnapshotDiff class
#* A detailed javadoc for the class would be very useful. A nice ascii diagram
would be even better :-)
#* SnapshotDiff#size could be named childrenSize? Also should we indicate it is
the size of children list when snapshot was created?
#* "The snapshot will obtain after applied this diff" this does not read
correctly.
#* "Posterior diff is the diff happened after this diff" - shoudl it be "after
this snapshot"
#* Should the javadoc indiciate - the snapshot state of this snapshot =
(current state) + all the posterior diffs over the current state?
# Javadoc for many of the InodeDirectoryWithSnapshot methods added in this
change would be good
> 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
> Attachments: h4103_20121129.patch, h4103_20121202.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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