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https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480205#comment-13480205
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Colin Patrick McCabe commented on HDFS-2802:
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It's good to see that this design carefully considers how to separate the
metadata of a snapshotted file (or directory) from the metadata of a later
version of that file.
bq. When there are one or more objects (either the original file or snaplinks)
under a sub-tree, the occupied space is counted as the max file size times the
max replication of these object (the max calculations include only the objects
under the sub-tree but exclude the objects outside the sub-tree.) Note that it
is easy to determine if a given INode is under a sub-tree by traversing up with
the parent references.
In some of the most commercially popular systems which implement snapshots,
snapshots do not count against the disk quotas. I think system administrators
might expect this behavior by now. Some other filesystems have two kinds of
quotas-- one which counts snapshots, and another which does not. This could be
a good way to go.
> 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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