Basic Concepts

Pair

A pair is the relationship between a primary logical unit number (LUN) and a secondary LUN in a remote replication task. In remote replication, data can be synchronized only from the primary LUN to the secondary LUN through a remote replication link. Before data synchronization, a pair must be established between two LUNs. To be paired, the primary and secondary LUNs must be in different storage systems, namely, primary storage system and secondary storage system.

The running status of a pair may change throughout the implementation of a remote replication task. By viewing the running status of a pair, you can determine whether the pair requires further actions and, if so, what operation is required. After performing an operation, you can view the running status of the pair to check whether the operation has succeeded. Table 1-4 describes the running status of a pair involved in a remote replication task.

Data Status

By determining data differences between the primary and secondary LUNs in a remote replication pair, HyperReplication identifies the data status of the pair. If a disaster occurs, HyperReplication determines whether a primary/secondary switchover is allowed based on the data status of the pair. The data status values are Consistent and Inconsistent.

Writable Secondary LUN

A writable secondary LUN refers to a secondary LUN to which host data can be written. After HyperReplication is configured, the secondary LUN is read-only by default. If the primary LUN is faulty, the administrator can cancel write protection for the secondary LUN and set the secondary LUN to writable. In this way, the secondary storage system can take over host services, ensuring service continuity. The secondary LUN can be set to writable in the following scenarios:

• The primary LUN fails and the remote replication links are in disconnected state.
• The primary LUN fails but the remote replication links are in normal state. The pair must be split before you enable the secondary LUN to be writable.

Consistency Group

A consistency group is a collection of pairs that have a service relationship with each other. For example, the primary storage system has three primary LUNs that respectively store service data, logs, and change tracking information of a database. If data on any of the three LUNs becomes invalid, all data on the three LUNs becomes unusable. For the pairs in which these LUNs exist, you can create a consistency group. Upon actual configuration, you need to create a consistency group and then manually add pairs to the consistency group.

Synchronization

Synchronization is a process of copying data from the primary LUN to the secondary LUN. Synchronization can be performed for a single remote replication pair or for multiple remote replication pairs in a consistency group at the same time.

Synchronization of a remote replication pair involves initial synchronization and incremental synchronization.

• After an asynchronous remote replication pair is created, initial synchronization is performed to copy all data from the primary LUN to the secondary LUN. After the initial synchronization is complete, if the remote replication pair is in normal state, incremental data will be synchronized from the primary LUN to the secondary LUN based on the specified synchronization mode (manual or automatic). If the remote replication pair is interrupted due to a fault, incremental data will be synchronized from the primary LUN to the secondary LUN based on the specified recovery policy (manual or automatic) after the fault is rectified.
• After a synchronous remote replication pair is created, initial synchronization is performed to copy all data from the primary LUN to the secondary LUN. After the initial synchronization is complete, if the remote replication pair is in normal state, host I/Os will be written into both the primary and secondary LUNs, not requiring data synchronization. If the remote replication pair is interrupted due to a fault, incremental data will be synchronized from the primary LUN to the secondary LUN based on the specified recovery policy (manual or automatic) after the fault is rectified.

Splitting

Splitting is a process of stopping data synchronization between primary and secondary LUNs. This operation can be performed only by an administrator. Splitting can be performed for a single remote replication pair or multiple remote replication pairs in a consistency group at one time. After the splitting, the pair relationship between the primary LUN and the secondary LUN still exists and the access permission of hosts for the primary and secondary LUNs remains unchanged.

At some time, for example when the bandwidth is insufficient to support critical services, you probably do not want to synchronize data from the primary LUN to the secondary LUN in a remote replication pair. In such cases, you can split the remote replication pair to suspend data synchronization.

You can effectively control the data synchronization process of HyperReplication by performing synchronization and splitting.

Primary/Secondary Switchover

A primary/secondary switchover is a process of exchanging the roles of the primary and secondary LUNs in a pair relationship. You can perform a primary/secondary switchover for a single remote replication pair or for multiple remote replication pairs in a consistency group at the same time. A primary/secondary switchover is typically performed in the following scenarios:

• After the primary site recovers from a disaster, the remote replication links are re-established and data is synchronized between the primary and secondary sites.
• When the primary storage system requires maintenance or an upgrade, services at the primary site must be stopped, and the secondary site takes over the services.

Link Compression

Link compression is an inline compression technology. In an asynchronous remote replication task, data is compressed on the primary storage system before transfer. Then the data is decompressed on the secondary storage system, reducing bandwidth consumption in data transfer. Link compression has the following highlights:

• Inline data compression

  Data is compressed when being transferred through links.

• Intelligent compression

  The system preemptively determines whether data can be compressed, preventing unnecessary compression and improving transfer efficiency.

• High reliability and security

  The lossless compression technology is used to ensure data security. Multiple check methods are used to ensure data reliability. After receiving data, the secondary storage system verifies data correctness and checks data consistency after the data is decompressed.

• User unawareness

  Link compression does not affect services running on the hosts and is transparent to users.

• Compatibility with full backup and incremental backup

  Link compression compresses all data that is transferred over the network regardless of upper-layer services.

Protected Object

For customers, the protected objects are LUNs or protection groups. That is, HyperReplication is configured for LUNs or protection groups for data backup and disaster recovery.

• Data protection can be implemented for each individual LUN.
• Data protection can be implemented for a protection group that consists of multiple independent LUNs or a LUN group.

How to distinguish a protection group and a LUN group:

A LUN group applies to mapping scenarios in which the LUN group can be directly mapped to a host. You can group LUNs for different hosts or applications.

A protection group applies to data protection of consistency groups. You can plan data protection policies for different applications and components in the applications. In addition, you can enable the LUNs used by multiple applications in the same protection scenario to be protected in a unified manner. For example, you can group the LUNs to form a LUN group, map the LUN group to a host or host group, and create a protection group for the LUN group to implement unified data protection of the LUNs used by multiple applications in the same protection scenario.