Cloudera Docs

Creating HDFS replication policy to replicate HDFS data

You must set up your clusters before you create an HDFS replication policy.

  1. Verify whether your cluster conforms to one of the supported replication scenarios. For more information, see Support matrix for Replication Manager on CDP Private Cloud Base
  2. If you are using different Kerberos principals for the source and destination clusters, add the destination principal as a proxy user on the source cluster. For example, if you are using the hdfssrc principal on the source cluster and the hdfsdest principal on the destination cluster, add the following properties to the HDFS service Cluster-wide Advanced Configuration Snippet (Safety Valve) for core-site.xml property on the source cluster: 
    <property>
    <name>hadoop.proxyuser.hdfsdest.groups</name>
    <value>*</value>
</property>
<property>
    <name>hadoop.proxyuser.hdfsdest.hosts</name>
    <value>*</value>
</property>

    Deploy the client configuration and restart all services on the source cluster, if the source cluster is managed by a different Cloudera Manager server than the destination cluster.

  3. Go to the Cloudera Manager > Replication > Replication Policies page, click Create Replication Policy.
  4. Select HDFS Replication Policy.

    The Create HDFS Replication Policy wizard appears.

  5. Configure the following options on the General page:
    Option Description
    Name Enter a unique name for the replication policy.
    Source Select the source HDFS service.

    You can select HDFS services managed by a peer Cloudera Manager Server, local HDFS services (managed by the Cloudera Manager Server for the Admin Console you are logged into).
    Source Path Enter the directory (or file) you want to replicate.
    Destination Select the destination HDFS service from the HDFS services managed by the Cloudera Manager Server for the Admin Console you are logged into.
    Destination Path Enter the destination path to save the source files.
    Schedule Choose:
    • Immediate to run the schedule immediately.
    • Once to run the schedule one time in the future. Set the date and time.
    • Recurring to run the schedule periodically in the future. Set the date, time, and interval between runs.
    Run As Username Enter the user to run the replication job in the field. By default this is hdfs.

    If you want to run the job as a different user, enter the user name. If you are using Kerberos, you must provide a user name here, and it must be one with an ID greater than 1000. (You can also configure the minimum user ID number with the min.user.id property in the YARN or MapReduce service.) Verify whether the user running the job has a home directory, /user/username, owned by username:supergroup in HDFS. This user must have permissions to read from the source directory and write to the destination directory.

    Note the following:
    • The user must not be present in the list of banned users specified with the Banned System Users property in the YARN configuration. For security purposes, the hdfs user is banned by default from running YARN containers.
    • The requirement for a user ID that is greater than 1000 can be overridden by adding the user to the "white list" of users that is specified with the Allowed System Users property.
    To view the properties, go to the YARN service and search for the properties on the Configuration tab.
  6. Configure the following options on the Resources page:
    Option Description
    Scheduler Pool (Optional) Enter the name of a resource pool in the field. The value you enter is used by the MapReduce Service you specified when Cloudera Manager executes the MapReduce job for the replication. The job specifies the value using one of these properties:
    • MapReduce – Fair scheduler: mapred.fairscheduler.pool
    • MapReduce – Capacity scheduler: queue.name
    • YARN – mapreduce.job.queuename
    Maximum Map Slots Enter the number of map slots per mapper, as required. The default value is 20.
    Maximum Bandwidth Enter the bandwidth per mapper, as required. The default is 100 MB.
    Replication Strategy Choose Static or Dynamic. Determines whether the file replication tasks must be distributed among the mappers statically or dynamically. The default is Dynamic.

    Static replication distributes file replication tasks among the mappers up front to achieve a uniform distribution based on the file sizes. Dynamic replication distributes file replication tasks in small sets to the mappers, and as each mapper completes its tasks, it dynamically acquires and processes the next unallocated set of tasks.
  7. Configure the following options on the Advanced Options tab:
    Option Description
    Add Exclusion Click the link to exclude one or more paths from the replication. Enter a regular expression-based path in the Regular Expression-Based Path Exclusion field.

    When you add an exclusion, include the snapshotted relative path for the regex. For example, to exclude the /user/bdr directory, use the following regular expression, which includes the snapshots for the bdr directory:

    .*/user/\.snapshot/.+/bdr.*

    To exclude top-level directories from replication in a globbed source path, specify the relative path for the regex without including .snapshot in the path. For example, to exclude the bdr directory from replication, use the following regular expression:

    .*/user+/bdr.*

    You can add more than one regular expression to exclude.
    MapReduce Service Select the MapReduce or YARN service to use.
    Log path Enter an alternate path for the logs.
    Description Enter a description of the replication policy.
    Error Handling Select the following option based on your requirements:
    • Skip Checksum Checks - Determines whether to skip checksum checks on the copied files. If selected, checksums are not validated. Checksums are checked by default.
    • Skip Listing Checksum Checks - Determines whether to skip checksum check when comparing two files to determine whether they are same or not. If skipped, the file size and last modified time are used to determine if files are the same or not. Skipping the check improves performance during the mapper phase. Note that if you select the Skip Checksum Checks option, this check is also skipped.
    • Abort on Error - Determines whether to abort the job on an error. If selected, files copied up to that point remain on the destination, but no additional files are copied. Abort on Error is not selected by default.
    • Abort on Snapshot Diff Failures - If a snapshot diff fails during replication, Replication Manager uses a complete copy to replicate data. If you select this option, the Replication Manager aborts the replication when it encounters an error instead.
    Preserve Whether to preserve the block size, replication count, permissions (including ACLs), and extended attributes (XAttrs) as they exist on the source file system, or to use the settings as configured on the destination file system. By default source system settings are preserved. When Permission is checked, and both the source and destination clusters support ACLs, replication preserves ACLs. Otherwise, ACLs are not replicated. When Extended attributes is checked, and both the source and destination clusters support extended attributes, replication preserves them. (This option only displays when both source and destination clusters support extended attributes.)
    Delete Policy Determines whether files that were deleted on the source should also be deleted from the destination directory. This policy also determines the handling of files in the destination location that are unrelated to the source. Options include:
    • Keep Deleted Files - Retains the destination files even when they no longer exist at the source. (This is the default.).
    • Delete to Trash - If the HDFS trash is enabled, files are moved to the trash folder.
    • Delete Permanently - Uses the least amount of space; use with caution. This option does not delete the files and directories in the top level directory. This is in line with rsync/Hadoop DistCp behavior.
    Alerts Whether to generate alerts for various state changes in the replication workflow. You can alert on failure, on start, on success, or when the replication workflow is aborted.
  8. Click Save Policy.
    The replication policy appears in the Replication Policies table. It can take up to 15 seconds for the task to appear.

    If you selected Immediate in the Schedule field, the replication job starts replicating after you click Save Policy.