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Availability and Scalability

System Replication - Disaster Recovery Solution in SAP HANA

In previous article SAP HANA Disaster recovery we explained what is disaster recovery and HANA supports it.
In this article, we will focus on System Replication - A solution for High Availability and Disaster Recovery.

What is System Replication?

System Replication is an alternative, high availability solution for SAP HANA platform, providing an extremely short RTO.
Usually system replication is set up so that a secondary standby system is configured as an exact copy of the active primary system, with the same number of active hosts in each system. The number of standby hosts need not be identical.


System Replication - Disaster Recovery Solution in SAP HANA

The secondary system can be located near the primary system to serve as a rapid failover solution for planned downtime, or to handle storage corruption or other local faults, or, it can be installed in a remote site to be used in a disaster recovery scenario.

Important point about System Replication:
  • Compatible with all SAP HANA hardware partner solutions
  • Secondary system has the same number of active nodes as the active, primary system
  • The instances in the secondary system operate in live replication mode.
  • In this mode, all secondary system services constantly communicate with their primary counterparts.
  • System replication replicates data and persists data/logs, and finally loads data to memory.
  • The logs and data can be compressed before shipping.

Different Log Replication Modes

Synchronous in-memory (default):
The primary system commits the transaction after it receives a reply confirming that the log was received by the secondary system, but before it has been persisted. The transaction delay in the primary system is shorter, because it only includes the data transmission time.

Synchronous with full sync:

System Replication - Disaster Recovery Solution in SAP HANA

This option means that the log write is successful when the log buffer has been written to the log file of the primary and the secondary instance.
In addition, when the secondary system is disconnected (for example, because of network failure) the primary systems suspends transaction processing until the connection to the secondary system is re-established. No data loss occurs in this scenario

Synchronous:
The primary system does not commit a transaction until it receives confirmation that the log has been persisted in the secondary system. This mode guarantees immediate consistency between both systems, however, the transaction is delayed by the time it takes to transmit the data to and persist it in the secondary system.

Asynchronous:

System Replication - Disaster Recovery Solution in SAP HANA

The primary system sends redo log buffers to the secondary system asynchronously. The primary system commits a transaction when it has been written to the log file of the primary system and sent to the secondary system through the network. It does not wait for confirmation from the secondary system.

Different Operation modes

Depending on the system replication configuration, there are two operation modes available:
Delta data shipping:

System Replication - Disaster Recovery Solution in SAP HANA

In this operation mode the secondary system persists but does not immediately replay the received logs.

Logreplay (available from SPS11):

System Replication - Disaster Recovery Solution in SAP HANA

With this operation mode configured, the received log entries are replayed immediately in the secondary system. The takeover time is reduced significantly because the log no longer has to be replayed.

System Replication in SAP HANA multitenant database containers

Important points for System replication in case of SAP HANA Multitenant Database Containers
  • SAP HANA system replication can be used to replicate the whole system.
  • The replication process treats the complete collection of tenant containers as one.
  • Primary and secondary systems must be identical at the time of system replication setup, including tenant databases.
  • Replication of a single tenant to an individual location is not possible.

System Replication - Disaster Recovery Solution in SAP HANA

What's Next?

Challenge yourself! Can you pass this quiz on HANA Availability and Scalability?
System Replication - Disaster Recovery Solution in SAP HANA
HANA High Availability and Disaster Recover Quiz




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