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https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480745#comment-13480745
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Hari Mankude commented on HDFS-2802:
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Todd, another option is to look at the inodesUnderConstruction in the NN and
query the DNs for the exact filesize at the time of taking snapshot. Even with
this, the filesize that is obtained will be at the instant. Applications like
hbase will have to deal with hlogs that could have incomplete log entries when
an un-cordinated snapshot is taken at the hdfs. A better approach is to have
the application reach a quiesce point and then take a snap. This is normally
done for oracle (hot backup mode) and sqlserver so that an application
consistent snapshot can be taken.
Also, createSnap()/removeSnap() has the writeLock() on the FSNamesystem which
will ensure that there are no other metadata updates when snap is being taken.
> 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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