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https://issues.apache.org/jira/browse/MAPREDUCE-2765?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13082503#comment-13082503
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Mithun Radhakrishnan commented on MAPREDUCE-2765:
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A couple of notes on the comments:
0. In its current state, DistCpV2 is maven-ized. I'm considering not switching
to ant and then switching back.
1. CopyMapper behaviour: The quoted check is for overwriting a file when the
source-path and the target-path are both complete paths to pre-existing files.
"distcp /source/file.txt /target/file.txt" doesn't skip file if the target-file
exists. This behaviour is consistent with old-distcp (which in turn adopts the
behaviour of Unix cp.)
2. DynamicInputChunk isn't public since it's specific to the implementation of
DynamicInputFormat, and is as such not meant to be consumed except in
conjunction with the DynamicInputFormat. I will change the FileSplit-size as
you suggest, and remove the commented code.
3. CopyCommitter:
* You're right. Atomic-commit removing existing directories is not intuitive.
I've changed that. Testing.
* preserveFileAttributes() should have been named
preserveFileAttributesOnDirectory(). I'll change that.
* Will fix up the javadoc.
I'll post a revised patch shortly.
> DistCp Rewrite
> --------------
>
> Key: MAPREDUCE-2765
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-2765
> Project: Hadoop Map/Reduce
> Issue Type: New Feature
> Components: distcp
> Affects Versions: 0.20.203.0
> Reporter: Mithun Radhakrishnan
> Assignee: Mithun Radhakrishnan
> Attachments: distcpv2.20.203.patch
>
>
> This is a slightly modified version of the DistCp rewrite that Yahoo uses in
> production today. The rewrite was ground-up, with specific focus on:
> 1. improved startup time (postponing as much work as possible to the MR job)
> 2. support for multiple copy-strategies
> 3. new features (e.g. -atomic, -async, -bandwidth.)
> 4. improved programmatic use
> Some effort has gone into refactoring what used to be achieved by a single
> large (1.7 KLOC) source file, into a design that (hopefully) reads better too.
> The proposed DistCpV2 preserves command-line-compatibility with the old
> version, and should be a drop-in replacement.
> New to v2:
> 1. Copy-strategies and the DynamicInputFormat:
> A copy-strategy determines the policy by which source-file-paths are
> distributed between map-tasks. (These boil down to the choice of the
> input-format.)
> If no strategy is explicitly specified on the command-line, the policy
> chosen is "uniform size", where v2 behaves identically to old-DistCp. (The
> number of bytes transferred by each map-task is roughly equal, at a per-file
> granularity.)
> Alternatively, v2 ships with a "dynamic" copy-strategy (in the
> DynamicInputFormat). This policy acknowledges that
> (a) dividing files based only on file-size might not be an
> even distribution (E.g. if some datanodes are slower than others, or if some
> files are skipped.)
> (b) a "static" association of a source-path to a map increases
> the likelihood of long-tails during copy.
> The "dynamic" strategy divides the list-of-source-paths into a number
> (> nMaps) of smaller parts. When each map completes its current list of
> paths, it picks up a new list to process, if available. So if a map-task is
> stuck on a slow (and not necessarily large) file, other maps can pick up the
> slack. The thinner the file-list is sliced, the greater the parallelism (and
> the lower the chances of long-tails). Within reason, of course: the number of
> these short-lived list-files is capped at an overridable maximum.
> Internal benchmarks against source/target clusters with some slow(ish)
> datanodes have indicated significant performance gains when using the
> dynamic-strategy. Gains are most pronounced when nFiles greatly exceeds nMaps.
> Please note that the DynamicInputFormat might prove useful outside of
> DistCp. It is hence available as a mapred/lib, unfettered to DistCpV2. Also
> note that the copy-strategies have no bearing on the CopyMapper.map()
> implementation.
>
> 2. Improved startup-time and programmatic use:
> When the old-DistCp runs with -update, and creates the
> list-of-source-paths, it attempts to filter out files that might be skipped
> (by comparing file-sizes, checksums, etc.) This significantly increases the
> startup time (or the time spent in serial processing till the MR job is
> launched), blocking the calling-thread. This becomes pronounced as nFiles
> increases. (Internal benchmarks have seen situations where more time is spent
> setting up the job than on the actual transfer.)
> DistCpV2 postpones as much work as possible to the MR job. The
> file-listing isn't filtered until the map-task runs (at which time, identical
> files are skipped). DistCpV2 can now be run "asynchronously". The program
> quits at job-launch, logging the job-id for tracking. Programmatically, the
> DistCp.execute() returns a Job instance for progress-tracking.
>
> 3. New features:
> (a) -async: As described in #2.
> (b) -atomic: Data is copied to a (user-specifiable) tmp-location, and
> then moved atomically to destination.
> (c) -bandwidth: Enforces a limit on the bandwidth consumed per map.
> (d) -strategy: As above.
>
> A more comprehensive description the newer features, how the dynamic-strategy
> works, etc. is available in src/site/xdoc/, and in the pdf that's generated
> therefrom, during the build.
> High on the list of things to do is support to parallelize copies on a
> per-block level. (i.e. Incorporation of HDFS-222.)
> I look forward to comments, suggestions and discussion that will hopefully
> ensue. I have this running against Hadoop 0.20.203.0. I also have a port to
> 0.23.0 (complete with unit-tests).
> P.S.
> A tip of the hat to Srikanth (Sundarrajan) and Venkatesh (Seetharamaiah), for
> ideas, code, reviews and guidance. Although much of the code is mine, the
> idea to use the DFS to implement "dynamic" input-splits wasn't.
>
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