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https://issues.apache.org/jira/browse/HDFS-2802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480561#comment-13480561
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Aaron T. Myers commented on HDFS-2802:
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bq. I agree with you on this. We wanted to begin with this approach and then
optimize it further in memory. The initial patch uploaded here tried premature
optimization both for memory and snapshot creation time and thus made the code
really complicated. But this is a definite goal and that part of the design we
will update as we continue to work. This is covered in open issues/future work
section.
My concern is that we're going about this wrong if the plan is to implement a
O(# of files + # of directories) solution and then optimize this via
multi-threading, offloading to disk, etc., as the design document suggests. We
should work on coming up with a design which is O(1) or something which is
roughly O(log(# of files + # of directories)), both in terms of time and space
efficiency. This sort of fundamental design decision is not something that can
be easily improved incrementally. I hope you will be open to reconsidering a
reworking of this design along these lines.
> 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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