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OceanStor 18500, 18800, and 18800F Storage System V100R001C30 Remote Replication Feature Guide 09

This document describes the remote replication feature, in terms of the implementation principle, application scenario, configuration process, and reference information.
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Related Concepts

Related Concepts

This section describes concepts related to the remote replication feature, including pair, consistency group, synchronization, splitting, primary/secondary switchover, data status and writable secondary LUN.

Full implementation of remote replication consists of five phases: creating a remote replication pair, replicating data, switching services, restoring data, and restoring the pair, as shown in Figure 1-1.
Figure 1-1  Implementation of remote replication

Pair

A pair refers to the relationship between a primary logical unit number (LUN) and a secondary LUN of a remote replication session. In a pair, data can be replicated only from the primary LUN to the secondary LUN. People can speak over two phones only after a connection is set up between the phones by dialup. Similarly, data can be replicated from the primary LUN to the secondary LUN only after a pair is set up between the LUNs. The primary and secondary LUNs of a pair must come from different storage systems.

A pair may be in different running status at different phases of a remote replication session. By viewing the running status of a pair, you can determine whether you need to perform an operation for that pair, and what operation should be performed if you need. After you perform an operation, you can view the pair running status to determine whether the operation succeeded. Table 1-4 describes the pair running status involved in a remote replication session.
Table 1-4  Pair running status of a remote replication session

Running Status

Description

Normal

Data synchronization between the primary LUN and the secondary LUN is complete.

Split

Data replication between the primary LUN and the secondary LUN is suspended. The pair running status changes to Split after the primary and secondary LUNs are split from each other manually by the administrator.

Interrupted

The pair running status changes to Interrupted after the pair between the primary and secondary LUNs is interrupted because the links used by the remote replication are interrupted or any of the two LUNs fails.

To be recovery

If a remote replication session needs to be restored using a Manual policy after the fault that caused a pair interruption is removed, the pair running status changes to To be recovery. This status reminds users of manual synchronization between the original primary LUN and the secondary LUN to restore the pair.

Invalid

If the original basic pair properties of the primary or secondary LUN are changed (for example, the secondary LUN configured at the secondary site is deleted) after the pair is interrupted, configuration becomes inconsistent between the primary and secondary sites and the pair running status changes to Invalid.

Synchronizing

Data is being copied from the primary LUN to the secondary LUN. When a pair is in this state, data on the secondary LUN is unusable.

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, which respectively store service data, log, 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 these LUNs, you can create a consistency group, and add the remote replication sessions of the LUNs to the consistency group.

Synchronization

Synchronization refers to a process during which data is copied from the primary LUN to the secondary LUN of a pair. Synchronization can be performed for a single remote replication session or multiple remote replication sessions in a consistency group at one time.

Splitting

At some time, for example when the bandwidth is not enough to support critical services, you probably do not want to copy data from the primary LUN to the secondary LUN. In such cases, you can split the secondary LUN from the primary LUN to suspend data synchronization.

Splitting refers to the action of stopping data replication from the primary LUN to the secondary LUN. This operation can be performed only by the administrator. Splitting can be performed for a single remote replication session or multiple remote replication sessions 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.

Users can effectively control a remote replication process by performing synchronization and splitting.

Primary/Secondary Switchover

A primary/secondary switchover is a process during which the primary and secondary LUNs in a pair exchange their roles. A primary/secondary switchover changes the original relationship between the LUNs in a remote replication session (a new pair is formed). You can perform a primary/secondary switchover for a single remote replication session or multiple remote replication sessions in a consistency group at one time. Usually, a primary/secondary switchover needs to be performed in the following scenarios:
  • The remote replication links are reestablished and data is synchronized between the primary and secondary sites after the primary site recovers from the disaster. In such a case, the administrator needs to perform a primary/secondary switchover for the remote replication session at the primary or secondary site.
  • When a device in the primary storage system needs to be replaced, the administrator performs primary/secondary switchover for the remote replication session at the primary or secondary site.

After a primary/secondary switchover, services are running on the secondary storage system. Therefore, it is required that data on the secondary LUN is available before a primary/secondary switchover is initiated. You can determine whether data on the secondary LUN is available by viewing the data status of the secondary LUN.

Data Status

The remote replication feature identifies the data status of a pair based on the data difference between the primary LUN and the secondary LUN. When a disaster occurs, the remote replication feature determines whether to allow a primary/secondary switchover for a pair based on the data status of the pair. There are two pair data states: Consistent and Inconsistent.

Writable secondary LUN

A writable secondary LUN can receive data from hosts. After a remote replication is configured, its secondary LUN state is read-only. If a fault occurs on the primary LUN the administrator needs to set the secondary LUN state to writable through the action of Disable Write Protection For Secondary LUN Of Remote Replication so that the secondary storage system can take over host services to ensure service continuity. The two usual scenarios for setting a writable secondary LUN are as follows:
  • When a fault occurs on the primary LUN, and the remote replication is linked down, the administrator needs to set the secondary LUN state to writable on the secondary storage system.
  • When a fault occurs on the primary LUN, and the remote replication is linked up, the administrator needs to split pair before setting the secondary LUN state to writable on the primary or secondary storage system.
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Updated: 2019-01-28

Document ID: EDOC1000077758

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