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HyperReplication Feature Guide for Block

OceanStor V5 Series V500R007

This document is applicable to OceanStor 5110 V5, 5110F V5, 5300 V5, 5300F V5, 5500 V5, 5500F V5, 5600 V5, 5600F V5, 5800 V5, 5800F V5, 6800 V5, 6800F V5, 18500 V5, 18500F V5, 18800 V5, and 18800F V5. It describes the working principle and application scenarios of the HyperReplication feature (for block). Also, it explains how to configure and manage the feature.
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Basic Concepts

Basic Concepts

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

As shown in Figure 1-1, a remote replication task is implemented in five phases.

Figure 1-1 Implementation of a remote replication task

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.

Table 1-4 Running status of a remote replication pair

Running Status

Description

Normal

Data synchronization between the primary and secondary LUNs is complete.

NOTE:

If Initial Synchronization is set to The data on primary and secondary resources is consistent and data synchronization is not required, a newly created remote replication pair is in the Normal state.

Split

Data replication between the primary and secondary LUNs is suspended. The running status of a pair changes to Split after the primary and secondary LUNs are manually split.

Interrupted

The running status of a pair changes to Interrupted after the pair between the primary and secondary LUNs is interrupted. This occurs when the links used by a remote replication task are down or either LUN fails.

To be recovered

If a remote replication task requires restoration using a Manual policy after the fault that caused a pair interruption is rectified, the pair running status changes to To be recovered. This status reminds users of manual data synchronization between the original primary LUN and the secondary LUN to restore the pair.

Invalid

If the original properties of a primary or secondary LUN change while a pair is Interrupted, the pair running status changes to Invalid because the primary and secondary system configurations become inconsistent.

Synchronizing

The running status of a pair is Synchronizing when data is being synchronized from the primary LUN to the secondary LUN. In this state, data on the secondary LUN is unavailable and cannot be used for data recovery if a disaster occurs. Only the secondary LUN in the Consistent state can be used to recover data of the primary LUN.

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 task 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 task 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.

Synchronization can be executed in the following three scenarios based on the types of primary and secondary LUNs:

  • The primary LUN is a thin LUN not configured with SmartDedupe and SmartCompression and the secondary LUN is a thin LUN with no space allocated. During initial synchronization, data for which space is not allocated on the primary LUN will not be read and synchronized to the secondary LUN.
  • The primary LUN is a thick LUN or a thin LUN configured with SmartDedupe and SmartCompression and the secondary LUN is a thin LUN with no space allocated. During initial synchronization, data in the primary LUN is read in sequence and the consecutive 64 KB all-zero data will not be synchronized to the secondary LUN.
  • In other scenarios, during initial synchronization, the system reads data from the primary LUN in sequence. The 64 KB all-zero data is compressed to 4 KB (16:1), synchronized to the secondary LUN, decompressed to 64 KB, and then written to the secondary LUN. Other data is written to the secondary LUN in sequence.

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 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.

You can effectively control a remote replication process 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 such as parts replacement, the secondary storage system must take control of services.

After a primary/secondary switchover, services run on the secondary storage system instead of the primary storage system. Data on the secondary LUN must be available prior to the primary/secondary switchover. Determine whether data on the secondary LUN is available by viewing its data status.

Data Status

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

Writable Secondary LUN

Hosts can send data to secondary LUNs. After the remote replication feature 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.

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

    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.

Optimizing Data Synchronization Performance

OceanStor storage systems support optimization of data synchronization performance. After optimization, data is transferred to the remote storage system through the optimal path. This avoids the impact of I/O forwarding on performance and improves data synchronization efficiency.

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Updated: 2019-07-11

Document ID: EDOC1000181492

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