• Answer: No one (in my experience) knows the disk and network mixture of block or packet sizes, the read and write rates for each size or the periods of time, which cause a peak in workload. In new workloads, it is all guess work anyway - lets be generous that would be + or - 25%.
• If you do know, IBM can do some maths to estimate the CPU cycles at the peak period.
• But most of the time that peak sizing would be total overkill.

• Trick 1 - "Use the PowerVM, Luke!"
  • Use PowerVM to reuse unused VIOS CPU cycles in the client virtual machines
  • VIOS = Micro-partition Shared CPU, Uncapped, high weight factor, with virtual processor minimum +1 or +2 headroom (virtual processor would be better called spreading factor)
  • PowerVM allows for workload peaks but doesn't waste CPU resources

• Trick 2 – Don’t worry about the tea bags!
  • No one calculates the number of tea bags they need per year
  • In my house, we simply have some in reserve and monitor the use of tea bags and then purchase more when needed
  • Likewise, start with sensible VIOS resources and monitor the situation

• Trick 3 - Go dual VIOS
  • Use a pair of VIOS to allow VIOS upgrades
  • In practice, we have low failure rates in the VIOS - mostly because systems administrators are recommended NOT to fiddle!
  • If the VIOS is using high-speed adapters like 10+ Gbps Ethernet or 16+ Gbps SAN, then VIOS buffering space is needed, so double the RAM to 4 GB.
    • Ignore this rule, if you have these adapters but are only likely to use a fraction of the bandwidth like 1 Gbps.
  • If you know that your applications are I/O intensive (that is likely to stress the high-speed adapters) then go to 6 GB or 8 GB of RAM.
  • If you are using large numbers of tiny virtual machines (lots of virtual connections) also go to 6 GB or 8GB.
  • On large machines, like 32 processor (cores) or more, many customers use a pair of VIOS for production and further a pair for other workloads.

• Trick 4 - the actual "Rule of Thumb"
  • VIOS base memory = 4 GB
  • Each VIOS: for every 16 CPUs – 1.0 Shared CPU and 2 GB of memory
  • This assumes that your virtual machines are roughly 1 to 2 CPUs each and not extremely I/O intensive (more CPU limited)

• Trick 5 - Check VIOS performance regularly
  • As workloads are added to a machine in the first few months, monitor VIOS CPU and memory use & tune as necessary
  • See other AIXpert blog entries for monitoring tools - whole machine and including VIOS

• Trick 6 - Driving system utilisation beyond, say, 70%
  • As you drive system utilisation up by adding more workloads, you need more pro-active monitoring
  • Implement some tools for Automatic Alerting of VIOS stress

Update for 2015 and assuming POWER8 and beyond

• WLE is IBM’s Official answer to calculate the VIOS size based on your configuration, number of virtual machines and adapters.
  • https://ibm.biz/workload-estimator  

• Tiny machine (~4 CPU cores)
  • Entitlement = 0.5 probably one VIOS
  • Virtual Processors = 1    
  • RAM = 4 GB
  • Uncapped, weight=200
• Small machine (12-16 CPU cores)
  • Entitlement = 1 each VIOS (more than 1, if using high-speed adapters that are used at high speed)
  • Virtual Processors = 2   
  • RAM = 6 GB
  • Uncapped, weight=200
• Larger
  • Entitlement = 2 to 8 each VIOS
  • Virtual Processors = E+1  
  • RAM = 8 GB to 16 GB (depends on high-speed adapters being using at high speed)
  • Uncapped, weight=200