We have received some requests from SAP Basis colleagues on how to go about designing SAP systems on VMware. Now that vSphere 5 can support up to 32-way virtual machines it is possible to fit larger SAP systems into one single virtual machine (VM) so should we go with 2-tier versus 3-tier? Here are some guidelines.
Sizing
First let’s cover sizing as this will impact the final VM architecture. SAP sizing is conducted in the SAP metric “SAPS” (http://www.sap.com/solutions/benchmark/measuring/index.epx ). All SAP on VMware sizing is officially conducted by the server vendor SAP practice. VMware partners with the server vendors so we can help but we are not ultimately responsible for sizing. The background behind this is as follows:
The hardware vendor can conduct the sizing and provide the number of ESX servers required to fulfill the business requirements. Once this is available VMware can work with the hardware vendor and customer to jointly fine-tune the VM size and layout.
We recommend starting conservatively for business critical workloads. An initial sizing option could be to allocate number of vCPUs = number of cores on the ESX server – we would do this even for hyper-threaded systems.
To achieve higher utilizations, the total amount of vCPUs running on an ESX server can be higher than the total amount of physical cores. The ESX hypervisor is designed to optimally schedule the workload amongst the available CPUs. Additionally, it can be configured to give more important virtual machines a higher priority. Hardware supported features like hyper-threading will increase the CPU scheduling efficiency. No general statement can be made regarding the optimal CPU over-commitment ratio, as this always depends on individual utilization patterns of the workload.
2-tier versus 3-tier
The architecture of a single SAP system consists of: a database instance; application server instances; Central Instance (CI - includes locking and message services and other SAP processes). In newer SAP releases the Central Instance is replaced with Central Services (CS - locking and messaging only) and the Primary Application Server instance (PAS). 2-tier refers to all these components running in the same guest-OS/ VM. 3-tier refers to the situation where, for a single SAP system these components are spread out onto at least two VMs. Each of the components can be deployed into a separate VM.
Advantage of 2-tier systems is that there are less VMs to manage and there is no network latency between the SAP components.
3-tier has the following advantages:
In the physical world some customers run batch jobs on the CI which is on the same physical server as the DB instance. The advantage - the jobs run quicker as there is no network hop between the app and the DB. In virtual a similar setup would require a large VM with the DB and app server/CI instance installed in the same guest-OS. The only downside is if there are long periods where the batch jobs are not run – we end up with an oversized VM with low utilization. Some datacenters may have a security requirement to separate the DB in its own guest-OS in which case your options are limited. VMware supports hot-add vCPU for the latest Linux and Windows versions but hot-remove is not supported. One solution, if the batch job is designed to run in parallel threads (many SAP ABAP batch jobs have this ability), increase the degree of parallelism and distribute the batch workload across more app server VMs to decrease the overall runtime of the job (this assumes you have available CPU) – the latter can be provisioned and de-provisioned based on the cyclical nature of the workload.
Regards
Matthias Schlarb (SAP Technical Alliance Engineer)
Michael Hesse (SAP Technical Alliance Manager)
Vas Mitra (SAP Solutions Architect)
(original blog post: http://blogs.vmware.com/apps/2012/04/sap-on-vmware-design-guidelines.html)
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