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From "Hari Mankude (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2802) Support for RW/RO snapshots in HDFS
Date Sat, 20 Oct 2012 17:26:15 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480784#comment-13480784
] 

Hari Mankude commented on HDFS-2802:
------------------------------------

Todd,

I do not agree that your solution will be any beneficial to hbase than what is being proposed.
Any type of txid information in DNs will be at the beginning of the transaction. If the client
is writing in the middle of block, there is no way to know the exact size when snap was taken.
Querying inodesUnderConstruction will give the block length at the time of the query. It is
not possible to take an application consistent snapshot (one which does not require recovery)
without coordination with the application.  

In fact, communication with DNs when snapshots are being taken will make the process of taking
snapshots very slow while giving very little additional benefit. 
                
> 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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