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Date: Wed, 9 May 2012 15:54:06 +0000 (UTC)
From: "Eli Collins (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-2802) Support for RW/RO snapshots in HDFS
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    [ https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13271497#comment-13271497 ] 

Eli Collins commented on HDFS-2802:
-----------------------------------

Hey Hari,

Could you elaborate some on scenario #3?  The motivation is test, so I assume use cases are things like (1) giving an experimental Hive/Pig user safe access /datasets/xyz or (2) running a test/dev HBase instance from an existing /hbase directory. Use case #1 seems like it could be accomplished in practice with external tables (or, in general, analysis programs that can separate their input data from directories they create/mutate). Use case #2 seems unlikely to be used in practice. Could be useful if /hbase is production (ie big) but in that case you wouldn't want to run a dev/test HBase build against a production HDFS cluster (as the production HBase instance runs there as well). Ie the usual case here is that you have a separate test/dev HBase/HDFS instance and you need/want to copy the datasets.

What's your plan for handling append and truncate? The design document does not discuss whether you're punting on these, or if not the semantics (eg for a snapshot taken while a block is being written) or the implementation. There's relevant design and implementation discussion in HDFS-233 and [Snapshots in Hadoop Distributed File System|http://www.cs.berkeley.edu/~sameerag/hdfs-snapshots.pdf] that's worth referencing.

Looking forward to checking out your prototype patch.

Thanks,
Eli
                
> 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
>    Affects Versions: 0.24.0
>            Reporter: Hari Mankude
>            Assignee: Hari Mankude
>         Attachments: snapshot-one-pager.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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