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https://issues.apache.org/jira/browse/HDFS-9820?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15214680#comment-15214680
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Jing Zhao commented on HDFS-9820:
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Thanks for working on this, [~yzhangal]! Some comments on the current patch:
# For the rdiff, We can switch the order of from-snapshot and to-snapshot
before computing the diff report on the target cluster. In this way we can
reuse the original sync code.
# Currently rdiff is a standalone option for distcp. This means we're using
distcp to do the restore. To restore a directory back to a snapshot, this may
not be the most efficient way compared with a local restoring solution
(HDFS-4167), which can avoid most of the unnecessary data copying and can
provide a copy-on-write semantic when supporting restoring appended/truncated
files.
# But before we finish the work in HDFS-4167, maybe we can augment the current
diff-based distcp by allowing the admin to choose to restore the target back to
the latest snapshot. We can still use the implementation in the current patch,
but instead of adding a new rdiff option for distcp, we add a "--force" option
to the current diff-based distcp. What do you think?
> Improve distcp to support efficient restore to an earlier snapshot
> ------------------------------------------------------------------
>
> Key: HDFS-9820
> URL: https://issues.apache.org/jira/browse/HDFS-9820
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: distcp
> Reporter: Yongjun Zhang
> Assignee: Yongjun Zhang
> Attachments: HDFS-9820.001.patch, HDFS-9820.002.patch
>
>
> HDFS-4167 intends to restore HDFS to the most recent snapshot, and there are
> some complexity and challenges.
> HDFS-7535 improved distcp performance by avoiding copying files that changed
> name since last backup.
> On top of HDFS-7535, HDFS-8828 improved distcp performance when copying data
> from source to target cluster, by only copying changed files since last
> backup. The way it works is use snapshot diff to find out all files changed,
> and copy the changed files only.
> See
> https://blog.cloudera.com/blog/2015/12/distcp-performance-improvements-in-apache-hadoop/
> This jira is to propose a variation of HDFS-8828, to find out the files
> changed in target cluster since last snapshot sx, and copy these from the
> source target's same snapshot sx, to restore target cluster to sx.
> If a file/dir is
> - renamed, rename it back
> - created in target cluster, delete it
> - modified, put it to the copy list
> - run distcp with the copy list, copy from the source cluster's corresponding
> snapshot
> This could be a new command line switch -rdiff in distcp.
> HDFS-4167 would still be nice to have. It just seems to me that HDFS-9820
> would hopefully be easier to implement.
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