A second class of field problems with appliance systems arise because of their poor handling of capacity overloads2. Most commonly-used general-purpose operating systems, and many appliance operating systems, perform well when the request load to which the system is being subjected lies within the capacity of system, but poorly when the offered load exceeds the capacity of the system [20,7]. Historically, the problem of poor overload performance of computer systems is well known, but has been deemed of somewhat marginal importance. In most circumstances it is not desirable to operate a system under overload conditions for any length of time; instead, the focus so far has been to avoid overload by trying to ensure that there are always sufficient hardware resources available in order to handle the maximum offered load.
In the filer appliance market, systems are often purchased by customers with a certain client load in mind. The number and types of systems purchased is chosen based on rated capacities of the filers, by in-house benchmarking, or from knowledge based on prior-use of filers. Filers are usually assigned rated capacities based on their performance under some standardized benchmark, e.g. the SpecFS (SFS) benchmark [30]. For many customers' sites, however, the request load profile is significantly different from the SFS profile, and the real capacity of a filer in operation may be very different from its rated capacity. When offered load does exceed real capacity, poor performance and a field problem results.