Million service units

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A million service units (MSU) is a measurement of the amount of processing work a computer can perform in one hour. The term is most commonly associated with IBM mainframes.

One “service unit” originally related to an actual hardware performance measurement (a specific model’s instruction performance). However, that relationship disappeared many years ago as hardware and software evolved. MSUs are now like other common (but physically imprecise) measurements, such as “cans of coffee” or “tubes of toothpaste.” (Cans and tubes can vary in physical size depending on brand, market, and other factors. Some coffee cans contain 13 ounces and others 500 grams, for example.)

Most mainframe software vendors charge by MSUs consumed (“workload-based charging”) or by total MSU system capacity. Thus, while MSU is an artificial construction, it does have a direct financial implication. In fact, software charges are why the MSU measurement exists at all.

IBM publishes MSU ratings for every mainframe server model, including the zSeries and System z9 ranges. For example, a zSeries z890 Model 110 is a 4 MSU system. MSU ratings are always rounded to whole numbers. IBM enforces an MSU rule called the “technology dividend”: each new mainframe model has a 10% lower MSU rating for the same level of system capacity. For example, when IBM introduced the System z9-109 in 2005, if a particular z9 configuration could process the same number of transactions per second as its predecessor (a particular z990 configuration) then it would do so with 10% fewer MSUs. The lower MSU rating means lower software costs, providing an incentive for customers to upgrade.

[edit] Sample usage

“Our accounting department will need 6 MSUs on a System z9-109 from 10 p.m. to 1:00 a.m. each night in order make sure our quarterly financial statements arrive on time.”
“You will need 8 more DB2 MSUs for your z900 to handle this year’s Christmas sales rush. Since your current z900 configuration doesn’t have enough capacity to add 8 DB2 MSUs, you need to add another engine. It might be less expensive to upgrade to a z9 because of the double technology dividend.”