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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3637) Support for snapshots
Date Wed, 25 Jun 2008 00:43:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3637?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12607820#action_12607820

dhruba borthakur commented on HADOOP-3637:

A few Primary requirements:

1. Latency of creating a snapshot be small. This is important for a data-warehousing installation
where decision-analysis-software would like to create frequent snapshots of production data.
2. A system should be able to support a few thousand snapshots concurrently.
3. Existence of a snapshot should not unduly tax the memory requirements of the namenode.
4. Only a few of those snapshots will be mounted simultaneously.
5. A write to one snapshot should not affect data in another snapshot.
6. The guarantees are such that files/directories that were created before the snapshot was
created will be part  of the snapshot. Files that were created after the snapshot was taken
will not be part of the snapshot.
7. Blocks that were *allocated* before the snapshot was created will be part of the snapshot.
No cluster-wide freeze-thaw mechanism is needed for data blocks. A client can continue to
write data to a block that was allocated to a snapshot without needing to trigger copy-on-write.

Secondary Requirements:
1. It would be good to have snapshotting support for a subtree rather than the entire namespace.
2. It would be good to have some sort of "search feature" for files in snaphots without mounting
the snapshot. This will help in finding out if a file/dir exists in the snapshot before mounting

> Support for snapshots
> ---------------------
>                 Key: HADOOP-3637
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3637
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
> Support HDFS snapshots. It should support creating snapshots without shutting down the
file system. Snapshot creation should be lightweight and a typical system should be able to
support a few thousands concurrent snapshots. There should be a way to surface (i.e. mount)
a few of these snapshots simultaneously.

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