Cloudera Docs

Creating an HDFS replication policy

You can create an HDFS replication policy in CDP Private Cloud Data Services Replication Manager to replicate HDFS data between CDP Private Cloud Base 7.1.8 or higher clusters.

  1. Log into CDP Private Cloud Data Services, and click Replication Manager.
  2. Click Replication Policies.
  3. On the Replication Policies page, click Create Policy.
  4. On the General page, choose or enter the following information:
    Option Action to take
    HDFS Choose to create an HDFS replication policy.
    Policy Name Enter a unique name for the replication policy.
    Description Optional. Enter a brief description about the replication policy.
  5. Click Next.
  6. On the Select Source page, choose or enter the following information:
    Option Action to take
    Source cluster Choose the source cluster.
    Source Path Choose one of the following methods to determine the directory where source data resides on the source cluster:
    • Enter the complete directory path.
    • Click File Browser to view and navigate the existing directory list on the selected cluster. Select the required directory that you want to replicate.
    Run As Username (on source) Optional. The replication policy uses the Default username to replicate HDFS data.

    If you are using a kerberized cluster, enter the required username. The replication policy uses this username to replicate the data in the kerberized cluster.
  7. Click Next.
  8. On the Select Destination page, choose or enter the following information:
    Option Action to take
    Destination cluster Choose the target cluster.
    Destination Path Enter the directory path on the target cluster to which the replication policy replicates HDFS data.
    Run as Username Optional. The replication policy uses the Default username to replicate HDFS data.

    Enter another username if you want the replication policy to use it to replicate data.
  9. Click Validate Policy.
    Replication Manager verifies whether the details provided are correct.
  10. Click Next.
  11. On the Schedule page, choose or enter the following information:
    Option Action to take
    Run Now Choose to initiate data replication after the replication policy creation is complete. Choose the frequency to replicate data periodically, if required.
    Schedule Run Choose to run the replication policy to replicate data at a later time. Choose the date and time for the first run, and then choose the frequency to replicate data periodically.

    Replication Manager ensures that the same number of seconds elapse between the runs. For example, if you choose the Start Time as January 19, 2022 11.06 AM and Interval as 1 day, Replication Manager runs the replication policy for the first time at the specified time in the timezone the replication policy was created in, and then runs it exactly after 1 day that is, after 24 hours or 86400 seconds.
    Frequency Choose one of the following options:

    • Does Not Repeat.

    • Custom - In the Custom Recurrence dialog box, choose the time, date, and the frequency to run the policy.
  12. Click Next.
  13. On the Additional Settings page, enter the values as necessary for the attributes:
    Option Description
    YARN Queue Name Enter the name of the YARN queue for the cluster to which the replication job is submitted if you are using Capacity Scheduler queues to limit resource consumption. The default value for this field is default.
    Maximum Maps Slots Set the maximum number of map tasks (simultaneous copies) per replication job. The default value is 20.
    Maximum Bandwidth Adjust this setting so that each map task is throttled to consume only the specified bandwidth. The default value for the bandwidth is 100 MB per second for each mapper.

    Each map task (simultaneous copy) is restricted to consume only the specified bandwidth. This is not always exact. The map throttles back its bandwidth consumption during a copy in such a way that the net used bandwidth tends towards the specified value. You can adjust this setting so that each map task is throttled to consume only the specified bandwidth so that the net used bandwidth tends towards the specified value.

    Path Exclusion Enter one or more regular expressions separated by comma. Replication Manager does not copy the subdirectories or files from the source that matches one of the specified regular expressions, to the target cluster.
    Replication Strategy Choose one of the following replication strategies:
    • Static distributes file replication tasks among the mappers up front to achieve an uniform distribution based on the file sizes.
    • Dynamic distributes the file replication tasks in small sets to the mappers, and as each mapper completes its tasks, it dynamically acquires and processes the next set of unallocated tasks.
    The default replication strategy is Dynamic.
    MapReduce Service Choose the MapReduce or YARN service.
    Log Path Enter an alternate path for the logs, if required.
    Error Handling Select the following options as necessary:
    • Skip Checksum Checks - Determines whether to skip checksum checks on the copied files. If selected, checksums are not validated. Checksums are checked by default.
      Checksums are used for two purposes:
      • To skip replication of files that have already been copied. If Skip Checksum Checks is selected, the replication job skips copying a file if the file lengths and modification times are identical between the source and destination clusters. Otherwise, the job copies the file from the source to the destination.
      • To redundantly verify the integrity of data. However, checksums are not required to guarantee accurate transfers between clusters. HDFS data transfers are protected by checksums during transfer and storage hardware also uses checksums to ensure that data is accurately stored. These two mechanisms work together to validate the integrity of the copied data.
    • Skip Listing Checksum Checks - 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 - 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, the replication policy uses a complete copy to replicate data. If you select this option, the policy aborts the replication when it encounters an error instead.
    Preserve Choose the required options 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 selected, and both the source and destination clusters support ACLs, replication preserves ACLs. Otherwise, ACLs are not replicated. When Extended attributes is selected, 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.)

    If you select one or more of the Preserve options and you are replicating to S3 or ADLS, the values of all of these items are saved in metadata files on S3 or ADLS. When you replicate from S3 or ADLS to HDFS, you can select which of these options you want to preserve.
    Delete Policy Choose the required options to determine whether the 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 option.
    • Delete to Trash - If the HDFS trash is enabled, files are moved to the trash folder. This is not supported when replicating to S3 or ADLS.
    • Delete Permanently - Uses the least amount of space; use with caution.
    Alerts Choose to generate alerts for various state changes in the replication workflow. You can choose to generate an alert On Failure, On Start, On Success, or On Abort of the replication job.

    You can configure alerts to be delivered by email or sent as SNMP traps. If alerts are enabled for events, you can search for and view the alerts on the Events tab, even if you do not have email notification configured. For example, if you choose Command Result that contains the Failed filter on the Diagnostics > Events page, the alerts related to the On Failure alert for all the replication policies for which you have set the alert appear. For more information, see Managing Alerts and Configuring Alert Delivery.
  14. Click Create.

    If you selected Run Now on the Schedule page, the replication job starts data replication after you click Create.

The replication task appears on the Replication Policies page.

If the replication job takes a long time to complete, and files change before the replication finishes, the replication may fail. Consider making the directories snapshottable, so that the replication job creates snapshots of the directories before copying the files and then copies files from these snapshottable directories when running the replication jobs. For more information, see Guidelines to consider for HDFS replication policies.