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HDFS-11040. Add documentation for HDFS-9820 distcp improvement. Contributed by Yongjun Zhang.

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Yongjun Zhang 2016-10-24 16:29:43 -07:00
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hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm
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@ -233,7 +233,8 @@ Flag | Description | Notes
`-bandwidth` | Specify bandwidth per map, in MB/second. | Each map will be restricted to consume only the specified bandwidth. This is not always exact. The map throttles back its bandwidth consumption during a copy, such that the **net** bandwidth used tends towards the specified value.
`-atomic {-tmp <tmp_dir>}` | Specify atomic commit, with optional tmp directory. | `-atomic` instructs DistCp to copy the source data to a temporary target location, and then move the temporary target to the final-location atomically. Data will either be available at final target in a complete and consistent form, or not at all. Optionally, `-tmp` may be used to specify the location of the tmp-target. If not specified, a default is chosen. **Note:** tmp_dir must be on the final target cluster.
`-async` | Run DistCp asynchronously. Quits as soon as the Hadoop Job is launched. | The Hadoop Job-id is logged, for tracking.
`-diff <fromSnapshot> <toSnapshot>` | Use snapshot diff report between given two snapshots to identify the difference between source and target. | This option is valid only with `-update` option and the following conditions should be satisfied. 1. Both the source and target FileSystem must be DistributedFileSystem. 2. Two snapshots (e.g., s1 and s2) have been created on the source FS. The diff between these two snapshots will be copied to the target FS. 3. The target has the same snapshot s1. No changes have been made on the target since s1. All the files/directories in the target are the same with source.s1. |
`-diff <oldSnapshot> <newSnapshot>` | Use snapshot diff report between given two snapshots to identify the difference between source and target, and apply the diff to the target to make it in sync with source. | This option is valid only with `-update` option and the following conditions should be satisfied. <ol><li> Both the source and the target FileSystem must be DistributedFileSystem.</li> <li> Two snapshots `<oldSnapshot>` and `<newSnapshot>` have been created on the source FS, and `<oldSnapshot>` is older than `<newSnapshot>`. </li> <li> The target has the same snapshot `<oldSnapshot>`. No changes have been made on the target since `<oldSnapshot>` was created, thus `<oldSnapshot>` has the same content as the current state of the target. All the files/directories in the target are the same with source's `<oldSnapshot>`.</li></ol> |
`-rdiff <newSnapshot> <oldSnapshot>` | Use snapshot diff report between given two snapshots to identify what has been changed on the target since the snapshot `<oldSnapshot>` was created on the target, and apply the diff reversely to the target, and copy modified files from the source's `<oldSnapshot>`, to make the target the same as `<oldSnapshot>`. | This option is valid only with `-update` option and the following conditions should be satisfied. <ol><li>Both the source and the target FileSystem must be DistributedFileSystem. The source and the target can be two different clusters/paths, or they can be exactly the same cluster/path. In the latter case, modified files are copied from target's `<oldSnapshot>` to target's current state).</li> <li> Two snapshots `<newSnapshot>` and `<oldSnapshot>` have been created on the target FS, and `<oldSnapshot>` is older than `<newSnapshot>`. No change has been made on target since `<newSnapshot>` was created on the target. </li> <li> The source has the same snapshot `<oldSnapshot>`, which has the same content as the `<oldSnapshot>` on the target. All the files/directories in the target's `<oldSnapshot>` are the same with source's `<oldSnapshot>`.</li> </ol> |
`-numListstatusThreads` | Number of threads to use for building file listing | At most 40 threads.
`-skipcrccheck` | Whether to skip CRC checks between source and target paths. |