Creating HDFS replication policy to replicate HDFS data

You must set up your clusters before you create an HDFS replication policy. You can also use CDP Private Cloud Base Replication Manager to replicate HDFS data to and from cloud, however you cannot replicate data from one cloud instance to another using Replication Manager.

To replicate HDFS data to and from cloud, you must have the appropriate credentials to access the cloud account. Additionally, you must create buckets in S3 and GCP or data lake store in ADLS. Replication Manager backs up file metadata, including extended attributes and ACLs when you replicate data to cloud storage.

Verify whether your cluster conforms to one of the supported replication scenarios. For more information, see Support matrix for CDP Private Cloud Base Replication Manager.
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.
Add the required credentials in Cloudera Manager to access the cloud storage to replicate HDFS to and from cloud storage.
1. To add AWS credentials, see How to Configure AWS Credentials.
  Ensure that the following basic permissions are available to provide read-write access to S3 through the S3A connector:
```
s3:Get*
s3:Delete*
s3:Put*
s3:ListBucket
s3:ListBucketMultipartUploads
s3:AbortMultipartUpload
```
2. To add ADLS credentials, perform the following steps:
  1. Click Add AD Service Principal on the Cloudera Manager Admin Console > Administration > External Accounts > Azure Credentials page for the source cluster.
  2. Enter the Name, Client ID, Client Secret Key, and Tenant Identity for the credential in the Add AD Service Principal modal window.
  3. Click Add.
Click Create Replication Policy on the Cloudera Manager > Replication > Replication Policies page.
Select HDFS Replication Policy.

The Create HDFS Replication Policy wizard appears.

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 one of the following values depending on your source cluster: Directory (or file) on the on-premises cluster. s3a://`[*BUCKET NAME]`/`[PATH]` path to replicate from Amazon S3. adl://`[ACCOUNT NAME]`.azuredatalakestore.net/`[PATH]`path to replicate from ADLS Gen 1. abfs[s]://`[FILESYSTEM]`@`[ACCOUNT NAME]`.dfs.core.windows.net/`[PATH*]`/ path to replicate from ADLS Gen 2. You can also use a glob path to specify more than one path for replication.
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 one of the following values to save the source files: Directory (or file) on the on-premises cluster. s3a://`[*BUCKET NAME]`/`[PATH]` path to replicate to Amazon S3. adl://`[ACCOUNT NAME]`.azuredatalakestore.net/`[PATH]`path to replicate to ADLS Gen 1. abfs[s]://`[FILESYSTEM]`@`[ACCOUNT NAME]`.dfs.core.windows.net/`[PATH*]`/ path to replicate to ADLS Gen 2.
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. 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.
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.

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 tasks that the DistCp MapReduce job can use for the replication policy. Default is 20.
Maximum Bandwidth	Enter the bandwidth limit for each mapper. Default is 100 MB. The total bandwidth used by the replication policy is equal to Maximum Bandwidth multiplied by Maximum Map Slots. Therefore, you must ensure that the bandwidth and map slots you choose do not impact other tasks or network resources in the target cluster. tip The Throughput field on the Cloudera Manager > Replication Policies page shows the maximum bandwidth set for the replication policy during replication policy creation.
File listing threads	Choose the Override DistCp default option and configure the number of threads (a maximum of 128 threads) that the HDFS replication policy must use during the copylisting phase of replication. By default, Replication Manager uses the default value of 20 threads for the copylisting phase of replication. The default number of threads for the copylisting phase of replication (using replication policies) can be set in the core-site.xml or hdfs-site.xml file for the HDFS service. You can set a maximum of 128 threads only. important Increasing this value increases the load on the HDFS NameNode of the source cluster which in turn increases the network bandwidth used by the replication jobs.
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.

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. important You must skip checksum checks to prevent replication failure due to non-matching checksums in the following cases: Replications from an encrypted zone on the source cluster to an encrypted zone on a destination cluster. Replications from an encryption zone on the source cluster to an unencrypted zone on the destination cluster. Replications from an unencrypted zone on the source cluster to an encrypted zone on the destination cluster. 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 - 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. Restart replication using non-incremental (bootstrap) replication on replication failure - Select to run the next replication job as a bootstrap replication if the replication job fails. Replication Manager replicates all the specified directories and files in the first HDFS replication policy job. This is also called bootstrap replication or non-incremental replication. Subsequent replication jobs are snapshot-based incremental replication. important When you choose this option for HDFS replication policies that are replicating huge amounts of data (otherwise called large policies), Replication Manager takes a long time to complete the bootstrap replication. This operation overwrites existing data and might consume more network resources to replicate the large amount of data.
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. To preserve permissions to HDFS, you must be running as a superuser on the destination cluster. Use the Run As Username option to ensure that is the case. When Extended attributes is checked, and both the source and destination clusters support extended attributes, replication preserves them. This option appears when both the source and destination clusters support extended attributes. When you preserve the attributes on the destination cluster, the HDF replication factor is also preserved.
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	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.

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.

If your replication job takes a long time to complete, see Improve network latency during replication job run to improve network latency.
If files change before the replication finishes, the replication might fail. For more information, see Guidelines to add or delete source data during replication job run.
For efficient replication, consider making the directories snapshottable. For more information, see Guidelines to use snapshot diff-based replication.
If your cluster has clients installed on hosts with limited resources, HDFS replication may use these hosts to run commands for the replication, which might cause performance degradation. To limit HDFS replication to run only on selected DataNodes, you can specify a "whitelist" of DataNode hosts. For more information, see Specifying hosts to improve HDFS replication policy performance.