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