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From "Aaron T. Myers (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2802) Support for RW/RO snapshots in HDFS
Date Mon, 05 Nov 2012 21:44:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13490960#comment-13490960

Aaron T. Myers commented on HDFS-2802:

bq. Eli, the memory usage in ATM's proposal is not O(1) since it adds tags for every INode,
therefore, it is O(N), where N is the number of INodes...

I believe the distinction here is how much memory is used _at snapshot creation time_. Both
of the current proposals are constant in terms of time/space at snapshot creation time, but
you're right that the most recent proposal I posted would add some overhead per INode.

In any case, I agree that the solution described in the most recent document posted by Suresh
will be more space efficient than the latest proposal I posted. No disagreement there. This
is what I was referring to when I said "This [the design posted by Suresh] has the advantage
of saving NN memory space for files/directories which have never had snapshots created of
> 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: HDFS-2802.20121101.patch, HDFS-2802-meeting-minutes-121101.txt,
HDFSSnapshotsDesign.pdf, snap.patch, snapshot-design.pdf, snapshot-design.tex, snapshot-one-pager.pdf,
Snapshots20121018.pdf, Snapshots20121030.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

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