Yongjun Zhang commented on HDFS-9820:

Hi [~andrew.wang],

Many thanks again for the review. I uploaded rev 007 which addressed all of 
your comments, except for the documentation.

* replaced all revert with reverse
* added {{testSync9()}} by adding a snapshot in between s1 and s2
* modified {{testSyncWithCurrent()}} to test against "."

About the documentation, can we do it in a follow-up jira after committing this 

I also would like to propose dropping the fallback feature and possibly the 
support of "." in hadoop 3, as follow-up, since they are going to be 
incompatible changes for 2.x.

Thanks a lot.

> 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
>    Affects Versions: 2.6.4
>            Reporter: Yongjun Zhang
>            Assignee: Yongjun Zhang
>         Attachments: HDFS-9820.001.patch, HDFS-9820.002.patch, 
> HDFS-9820.003.patch, HDFS-9820.004.patch, HDFS-9820.005.patch, 
> HDFS-9820.006.patch, HDFS-9820.007.patch
> A common use scenario (scenaio 1): 
> # create snapshot sx in clusterX, 
> # do some experiemnts in clusterX, which creates some files. 
> # throw away the files changed and go back to sx.
> Another scenario (scenario 2) is, there is a production cluster and a backup 
> cluster, we periodically sync up the data from production cluster to the 
> backup cluster with distcp. 
> The cluster in scenario 1 could be the backup cluster in scenario 2.
> For scenario 1:
> HDFS-4167 intends to restore HDFS to the most recent snapshot, and there are 
> some complexity and challenges.  Before that jira is implemented, we count on 
> distcp to copy from snapshot to the current state. However, the performance 
> of this operation could be very bad because we have to go through all files 
> even if we only changed a few files.
> For scenario 2:
> 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 
> snapshot sx of either the source or the target cluster, to restore target 
> cluster's current state to sx. 
> Specifically,
> 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.
> As a native restore feature, HDFS-4167 would still be ideal to have. However, 
>  HDFS-9820 would hopefully be easier to implement, before HDFS-4167 is in 
> place.

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