Performing Concurrent Operations on HDFS Files

Scenario

Performing this operation can concurrently modify file and directory permissions and access control tools in a cluster.

This section applies to MRS 3.x or later.

Impact on the System

Performing concurrent file modification operations in a cluster has adverse impacts on the cluster performance. Therefore, you are advised to do so when the cluster is idle.

Prerequisites

• The HDFS client or a client that contains the HDFS client has been installed. For example, the installation directory is /opt/client.
• Service users of each component are created by the MRS cluster administrator based on service requirements. In security mode, machine-machine users need to download the keytab file. A human-machine user needs to change the password upon the first login. (This operation is not required in normal mode.)

Procedure

1. Log in to the node where the client is installed as the client installation user.
2. Run the following command to go to the client installation directory:

   cd /opt/client

3. Run the following command to configure environment variables:

   source bigdata_env

4. If the cluster is in security mode, the user executing the DistCp command must belong to the supergroup group and run the following command to perform user authentication. In normal mode, user authentication is not required.

   kinit Component service user

5. Increase the JVM size of the client to prevent out of memory (OOM). (32 GB is recommended for 100 million files.)
   

   The HDFS client exits abnormally and the error message "java.lang.OutOfMemoryError" is displayed after the HDFS client command is executed.

   This problem occurs because the memory required for running the HDFS client exceeds the preset upper limit (128 MB by default). You can change the memory upper limit of the client by modifying CLIENT_GC_OPTS in <Client installation path>/HDFS/component_env. For example, if you want to set the upper limit to 1 GB, run the following command:

   CLIENT_GC_OPTS="-Xmx1G"

   Run the following command to apply the modifications.

   source <Client installation path>//bigdata_env

6. Run the concurrent commands shown in the following table.

   Command	Description	Function
   hdfs quickcmds [-t threadsNumber] [-p principal] [-k keytab] -setrep <rep> <path> ...	threadsNumber indicates the number of concurrent threads. The default value is the number of vCPUs of the local host. principal indicates the Kerberos user. keytab indicates the Keytab file. rep indicates the number of replicas. path indicates the HDFS directory.	Used to concurrently set the number of copies of all files in a directory.
   hdfs quickcmds [-t threadsNumber] [-p principal] [-k keytab] -chown [owner][:[group]] <path> ...	threadsNumber indicates the number of concurrent threads. The default value is the number of vCPUs of the local host. principal indicates the Kerberos user. keytab indicates the Keytab file. owner indicates the owner. group indicates the group to which the user belongs. path indicates the HDFS directory.	Used to concurrently set the owner group of all files in the directory.
   hdfs quickcmds [-t threadsNumber] [-p principal] [-k keytab] -chmod <mode> <path> ...	threadsNumber indicates the number of concurrent threads. The default value is the number of vCPUs of the local host. principal indicates the Kerberos user. keytab indicates the Keytab file. mode indicates the permission (for example, 754). path indicates the HDFS directory.	Used to concurrently set permissions for all files in a directory.
   hdfs quickcmds [-t threadsNumber] [-p principal] [-k keytab] -setfacl [{-b|-k} {-m|-x <acl_spec>} <path> ...]|[--set <acl_spec> <path> ...]	threadsNumber indicates the number of concurrent threads. The default value is the number of vCPUs of the local host. principal indicates the Kerberos user. keytab indicates the Keytab file. acl_spec indicates the ACL list separated by commas (,). path indicates the HDFS directory.	Used to concurrently set ACL information for all files in a directory.