Constraints

Before using MRS, ensure that you have read and understand the following restrictions.

• MRS clusters must be created in VPC subnets.
• You are advised to use any of the following browsers to access MRS:
  • Google Chrome: 36.0 or later
  • Internet Explorer: 9.0 or later

    If you use Internet Explorer 9.0, you may fail to log in to the MRS management console because user Administrator is disabled by default in some systems. The browser automatically selects a system user for installation. As a result, the browser cannot access the management console. Reinstall the browser or a later version (recommended) or run the browser as user Administrator.

  • Microsoft Edge is updated with the Windows operating system.
• When you create an MRS cluster, you can select Auto create from Security Group to create a security group or select an existing security group. After the MRS cluster is created, do not delete or modify the used security group. Otherwise, a cluster exception may occur.
• To prevent illegal access, only assign access permission for security groups used by MRS where necessary.
• Do not perform the following operations because they will cause cluster exceptions:
  • Shutting down, restarting, or deleting MRS cluster nodes on the ECS console, changing or reinstalling their OS, or modifying their specifications.
  • Deleting the existing processes, applications or files on cluster nodes.
  • Deleting MRS cluster nodes. Clusters with deleted nodes are still be charged.
• If a cluster exception occurs when no incorrect operations have been performed, contact technical support engineers. They will ask you for your password and then perform troubleshooting.
• Keep the initial password for logging in to the master node properly because MRS will not save it. Use a complex password to avoid malicious attacks.
• MRS clusters are still charged during exceptions. Contact technical support engineers to handle cluster exceptions.
• Plan disks of cluster nodes based on service requirements. If you want to store a large volume of service data, add EVS disks or storage space to prevent insufficient storage space from affecting node running.
• The cluster nodes store only users' service data. Non-service data can be stored in the OBS or other ECS nodes.
• The cluster nodes only run MRS cluster programs. Other client applications or user service programs are deployed on separate ECS nodes.
• The capacity (including storage and computing capabilities) of an MRS cluster can be expanded by adding core or task nodes.
• If a Master node in an MRS cluster is shut down and the cluster is still used to execute jobs or modify component configurations, you must start the stopped Master node before stopping other nodes. Otherwise, data may be lost due to an active/standby switchover.
• If all nodes in an MRS cluster have been stopped, start them in the reverse order of node shutdown.
• The Capacity and Superior scheduler switchover is complete when the MRS cluster is used, while configuration synchronization is not complete. Configure synchronization again based on the new scheduler if necessary.
• Do not enable the MOB feature for the HBase service in the MRS cluster. Using this feature may lead to table data read failure and JVM crash.

  For existing HBase tables, run the following command in hbase shell to check whether the table description contains the keyword MOB. If it is contained, contact O&M engineers to set the table non-MOB.

  desc 'Table name'

  For example, if the value of IS_MOB is true in the following command output, the HBase MOB feature is enabled:

  hbase:009:0> desc 't3'
  t3
  COLUMN FAMILIES DESCRIPTION
  {NAME => 'd', MOB_THRESHOLD => '102400', VERSIONS => '1', KEEP_DELETED_CELLS => 'FALSE', DATA_BLOCK_ENCODING => 'NONE', TTL => 'FOREVER', MIN_VERSIONS => '0', REPLICATION_SCOPE => '0', BLOO
  MFILTER => 'ROW', IN_MEMORY => 'false', IS_MOB => 'true', COMPRESSION => 'NONE', BLOCKCACHE => 'true', BLOCKSIZE => '65536'}

• Spark and other engines in the MRS cluster are forbidden to execute self-read and self-write SQL statements.

  The following is an example:

  insert overwrite table test select * from test;

  insert overwrite table test select a.* from test a join test2 b on a.name=b.name;