Virtual I/O Server sizing
The contents of this web page solely reflect the personal views of the authors and do not necessarily represent the views, positions, strategies or opinions of IBM or IBM management. Please use the 
Add Comment link at the bottom of the page to provide feedback. Note: Until you log in (using the link in the upper right corner of this web page), you will not see the Add Comment link and you can not add a comment. If you do not already have an IBM ID, use the Register Now link on the sign in page to obtain one. Registration is quick and easy.
This web page is meant to supplement, not replace, other documentation which has been published:
The articles cited above specify the clock cycles required on a 1.65 GHz processor to perform a variety of Virtual I/O operations. The articles suggest using clock speed to extrapolate from a server with a 1.65 GHz clock speed to servers with other clock speeds. Such extrapolation may be accurate when extrapolating to other servers with POWER5 processors, but can be very inaccurate when extrapolating to servers with POWER5+ and POWER6 processors.
A better methodology is to estimate the number of 1.65 GHz cores which will be required and to then use rPerf metrics in Section 2, AIX Multiuser Performance in the Power Systems Performance Report
to extrapolate to servers with other clock speeds. For example, if a sizing suggests that three 1.65 GHz cores will be required, observe that the rPerf metric for a p520 with two 1.65 GHz POWER5 cores is 9.86. So 3x9.86/2 = 14.79 rPerfs are required. Assuming the sizing is for a VIO Server running on a p6 595 with sixteen 4.2 GHz POWER6 cores, observe that the p6 595 has an rPerf metric of 142.90. On that p6 595, 14.79*16/142.90 = 1.6 cores will be consumed by a VIO Server handling the I/O operations on which the sizing was based.
Regarding memory required by a VIO Server, the Memory allocation article (linked to by the Planning for Shared Ethernet Adapters article) says that 512 MB should be sufficient for a VIO Server. But please note that regarding memory, Section 5.4.1, Virtual I/O Server memory planning of the IBM System p Advanced POWER Virtualization Best Practices Redpaper says:
"The Virtual I/O Server, similar to other workloads, requires system memory to operate. Some of this memory is used to support network communications. This network memory is used to buffer network traffic, so detailed planning will involve knowing such factors as the messages sizes (MTU or jumbo frames) and also how many Shared Ethernet Adapters to use.
"The easiest rule to follow is that if you have a simple Virtual I/O Server that will only be bridging a couple of networks and will never be using jumbo frames or a larger MTU, 512 MB will be needed by the Virtual I/O Server. System configurations with 20-30 clients, many LUNs, and a generous vhost configurations might need increased memory to suit performance expectations.
"If there is a possibility that you might start bridging more networks and use jumbo frames, use 1 GB of RAM for the Virtual I/O Servers. If you have enough memory to spare, the best situation is to use 1 GB for the Virtual I/O Servers to allow the maximum flexibility, but this is generally the maximum. When 10 Gbps Ethernet adapters are commonly implemented, this memory requirement can require revision."
And please note that the recommendation in the Redpaper does not consider Integrated Virtual Ethernet adapters (IVEs/HEAs). If a VIO Server LPAR is using IVE/HEA as an SEA, the LPAR will need more than 1 GB. IBMers can see PMR 52098,033 for more information. As of 6/2009, an update to the Redpaper is being discussed which would revise the recommendation cited above to consider IVEs/HEAs.
