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https://issues.apache.org/jira/browse/HDFS-9820?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15241868#comment-15241868
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Yongjun Zhang commented on HDFS-9820:
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Thanks a lot [~jingzhao]!
My thoughts to share:
1.
{quote}
Let's say we first have snapshot s1 both both source and target (and the source
and the target have been synced). Then we make some changes on the source, do a
forward incremental distcp copy to apply the changes to the target. Based on
our assumption, before the next incremental copy, we will create a snapshot s2
on both the source and the target.
{quote}
This is HDFS-7535/HDFS-8828. One small correction: before we do the incremental
copy, we create a snapshot s2 on source cluster first, find snapshot diff
between s1 and s2, and apply this diff to target cluster, then finally create
s2 on target cluster. We assume that no changes have been made at target
cluster after s1 was created before we do incremental copy in this case
(*assumption I*).
2. Do you mean if {{""}} ever appear as one parameter of {{-diff}}, then it's a
revert operation, otherwise it's forward operation?
In theory, we could copy incremental changes from source cluster to destination
cluster without creating a new snapshot (s2 in our example). Say, after s1 is
made in source cluster, and s1 is sync-ed to target cluster, and s1 is also
created in target cluster, we could interpret
{{distcp -diff s1 "" source target}}.
as to incrementally copy changes made after s1 in source cluster to target,
right? Because {{""}} is just an alias of current state "snapshot",
I personally feel it's more intuitive to count on the parameter order, and let
({{-diff s1 s2}} mean the forward change from s1 to s2, {{-diff s2 s1}} mean
the revert change from s2 to s1. Say, assume a cluster is already at state s2,
and we do {{-diff s1 s2}}, it would be a non-op; If we do {{-diff s2 s1}}, it
means to go back to s1. In other words, {{-diff <fromState> <toState>}} is what
I feel more intuitive.
But if this is what you prefer, we can relax the order requirement, and let
{{""}} means revert operation. Would you please confirm?
And would you please let me know whether my comment #1 in my previous reply
makes sense to you?
3. Not quite follow what you meant by "bypass DistCpSync#prepareDiffList. ".
Some more details would help.
Many thanks.
> 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-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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