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https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Suresh Srinivas updated HDFS-2802:
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Attachment: Snapshots20121030.pdf
Attaching the updated design document. Hopefully it addresses the issues that
have been rasied. This should give sufficient details about the implementation
we are currently working on. To summarize:
# *Snapshot allowed only at the root vs snapshot at the subdirectories* -
Ability to snapshot a sub-directory is very important requirement for many
Hadoop users. Please see the requirements in the document posted for more
details. The alternate proposal to allow snapshots only at the root is a
non-starter in this regard.
# *Efficiency of snapshot creation and management* - The current design
addresses the concerns raised. To summarize the creation of snapshot is O(1).
The design uses copy-on-modify approach so that the cost of snapshot is zero
when there is no modification and is proportional the modifications when they
are made. Please provide feedback.
# *Snapshot of being written files and consistency* - Our document describes
several design choices - some easy, some complicated. Please see the proposed
choice in the document. We could continue this discussion in HDFS-3960.
I took a look at the alternate proposal. It is too high level without
sufficient details to evaluate. From my limited understanding of the alternate
proposal, the design document we have posted here has several significant
advantages over it:
# It supports sub-directory snapshots, an important use case for many Hadoop
users.
# It supports on-demand and user managed snapshots.
# When snapshots are not created, there is no cost incurred in terms of memory.
The alternate proposal has O(N) memory cost for storing tags.
# Our design can also be extended to do RW snapshots, if we feel a need for it.
Hopefully this clarifies the design better. We would like continue focus on
implementing it. Any feedback provided will be incorporated into the design and
the implementation.
> 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: HDFSSnapshotsDesign.pdf, snap.patch,
> snapshot-one-pager.pdf, Snapshots20121018.pdf, Snapshots20121030.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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