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https://issues.apache.org/jira/browse/HDFS-9820?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15543394#comment-15543394
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Yongjun Zhang commented on HDFS-9820:
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Copied from
https://issues.apache.org/jira/browse/HDFS-10314?focusedCommentId=15510391&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15510391
For clarity, and as a recap, here is a comparison table between -diff and the
proposed -rdiff, which shows the symmetricity:
||Comparison||-diff s1 s2 <src> <tgt>||-rdiff s2 s1 <src> <tgt>||
|Current feature state|Existing in distcp|Proposed Addition |
|Functionality| Given <tgt>'s current state is s1, make <tgt>'s current state
the same as newer snapshot s2 | Given <tgt>'s current state is s2, make <tgt>'s
current state the same as older snapshot s1 |
|Requirements| # <src> and <tgt> need to be different paths
# both <src> and <tgt> have snapshot s1 with exact same content
# <src> has snapshot s2
# s2 is newer than s1
# <tgt>'s current state is the same as s1
# <tgt> doesn't have snapshot s2 | # <src> and <tgt> can be the same or
different paths
# both <src> and <tgt> have snapshot s1 with exact same content
# <tgt> has snapshot s2
# s2 is newer than s1
# <tgt>'s current state is the same as s2
# <src> may or may not have snapshot s2 |
|Steps|# calculate snapshotDiff<s1,s2> at <src>
# apply rename/delete part of snapshotDiff on <tgt>
# copy modified part of snapshotDiff from s2 of <src> to <tgt> | # calculate
snapshotDiff<s2,s1> at <tgt>
# apply rename/delete part of snapshotDiff on <tgt>
# copy modified part of snapshotDiff from s1 of <src> to <tgt> |
> 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-9820.003.patch, HDFS-9820.004.patch, HDFS-9820.005.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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