In this article I will explain what is High Availability, why it is so important and how SAP HANA provides High Availability.
What is High Availability?
In computing, the term availability is used to describe the period of time when a service is available, as well as the time required by a system to respond to a request made by a user.
High availability is a quality of a system or component that assures a high level of operational performance for a given period of time.
Availability is often expressed as a percentage indicating how much uptime is expected from a particular system or component in a given period of time, where a value of 100% would indicate that the system never fails.
For instance, a system that guarantees 99% of availability in a period of one year can have up to 3.65 days of downtime (1%).
Why High Availability is important for SAP HANA?
High availability is an important subset of reliability engineering, focused towards assuring that a system or component has a high level of operational performance in a given period of time.
When setting up a robust system, minimizing downtime and service interruptions is always a high priority. Highly available systems can recover from server or component failure automatically.
How High Availability is achieved in SAP HANA?
SAP HANA is fully designed for high availability. It supports recovery measures ranging from faults and software errors, to disasters that decommission an entire data center.
High availability is achieved by eliminating single points of failure (fault tolerance), and providing the ability to rapidly resume operations after a system outage with minimal business loss (fault resilience).Fault recovery
is the process of recovering and resuming operations after an outage due to a fault.Disaster recovery
is the process of recovering operations after an outage due to a prolonged data center or site failure. Preparing for disasters may require backing up data across longer distances, and may thus be more complex and costly.
The key to achieving high availability is redundancy, including hardware redundancy, network redundancy and data center redundancy.
SAP HANA provides several levels of defense against failure-related outages:Hardware Redundancy:
SAP HANA appliance vendors offer multiple layers of redundant hardware, software and network components to ensure high availability. For example
- Redundant power supplies and fans
- Enterprise grade error-correcting memories
- Fully redundant network switches and routers
- Uninterrupted power supply (UPS).
Disk storage systems use batteries to guarantee writing even in the presence of power failure, and use striping and mirroring to provide redundancy for automatic recovery from disk failures.
Redundant networks, network equipment and network connectivity is required to avoid network failures from affecting system availability. This is typically accomplished by deploying a completely redundant switch topology, using the Spanning Tree Protocol to avoid loops. Routers can be configured with the Hot Standby Router Protocol (HSRP) for automatic failover. BGP is commonly used to manage dual WAN connectionData Center Redundancy:
Data centers that host SAP HANA solutions are equipped with Uninterrupted Power Supply (UPS) and backup power generators, redundant cooling systems and multi-sourced providers of network connectivity and electricity, achieving operational availability in the presence of individual failures, and significantly reducing the probability of a business-impacting outage.
SAP HANA High Availability Support
As an in-memory database, SAP HANA is not only concerned with maintaining the reliability of its data in the event of failures, but also with resuming operations with most of that data loaded back in memory as quickly as possible.
SAP HANA supports the following recovery measures from failures:Disaster recovery support:
Backups: Periodic saving of database copies in safe place.
Storage replication: Continuous replication (mirroring) between primary storage and backup storage over a network (may be synchronous).
System replication: Continuous update of secondary systems by primary system, including in-memory table loading.Fault recovery support:
- Service auto-restart: Automatic restart of stopped services on host (watchdog).
- Host auto-failover: Automatic failover from crashed host to standby host in the same system.
- System replication: Continuous update of secondary systems by primary system, including in-memory table loading and read-only access on the secondary.
System replication is flexible enough that it can also be used for both fault and disaster recovery to achieve high availability. The data pre-load option can be used for fault recovery to enable a quicker takeover than with Host Auto-Failover.
SAP HANA supports a comprehensive range of High Availability options, designed to satisfy tradeoffs between demanding High Availability and Disaster Recovery requirements, while also considering cost and complexity.
Challenge yourself! Can you pass this quiz on HANA Availability and Scalability?HANA High Availability and Disaster Recover Quiz