Formula to Determine Availability of SAP running on an VMware ESXi Cluster
When we architect SAP on VMware deployments an important topic is how we design for high availability. We have options in the VMware environment from VMware HA, VMware FT and use of in-guest clustering software like Microsoft Cluster Services or Linux-HA. So can we determine a numerical availability for our design expressed as a fraction/percentage (same metric used to define uptime Service Level Agreements like 99.9% )? Yes, there are ways to estimate this value and one method is explained in the following paper http://www.availabilitydigest.com/public_articles/0712/sap_vmware.pdf . This paper develops an equation to estimate the availability of SAP running on an ESXi cluster expressed as a fraction/percentage. The concepts are taken from other papers at http://www.availabilitydigest.com (a digest of topics on high availability) and are based on mathematical algebra and probability theory that have been previously used in the IT industry for availability calculations. The availability metric (e.g. 99.9% or 0.999) is essentially a probability hence we use mathematical probability techniques to calculate the overall availability of a system.
The final general equation calculates the availability of an “n” node ESXi cluster sized with “s” number of spares i.e. an “n+s” cluster. It also factors in the software failover times of the single-points-of-failure (SPOF) in the SAP application architecture (database and Central Services). The failover time refers to the time taken for the SPOF to failover and restart on another ESXi host or other virtual machine in the event of an ESXi host failure – this period is important as it corresponds to downtime for the SAP system. The final equation gets a bit heavy on the algebra, but that’s because it models a generic use case. Once you replace the variables with practical “real-world” values, the equation gets easier and that’s when the algebra stops and spread sheeting takes over.
To learn more and to read the entire article at its source, please refer to the following page, Virtualize Business Critical Applications Blog: Formula to Determine Availability of SAP running on an ESXi Cluster- VMware Blogs