All Meters Have Problems

You need metering data to do any performance work but metering data never perfectly adds up, never aligns 100%, and typically contains many numbers that are meaningless to you. If you are a type-A detail-oriented person, this can drive you nuts. I urge you to relax just a little bit.

When comparing different meters that look at the same general thing, remember they might be sampling at different points in the operating system, or sampling at different frequencies. They may be reporting different units – for example, a disk read can be reported in bytes, file records, disk blocks, logical IO, or physical IO. They may be counting directly, or sampling indirectly, the values they are reporting. If you don’t understand exactly what the meter is metering, then you are missing a lot of its value.

There is also a non-zero probability that a meter might just be wrong. Just about the last thing that gets added into any system or application are performance meters. Mostly they are added in a hurry, to solve a problem, and most of their output might be utterly useless to anyone who doesn’t work in the vendor’s engineering department. Typically they are not part of the vendor’s quality assurance (QA) work and so, unless someone notices, they can start telling lies as the years pass.

Meters can also lie due to changes in technology over time. For example, in 1800 you might meter the utilization of a road by metering the horses that pass by per hour. That meter might still be around today and working perfectly, but will give the erroneous impression that the metered stretch of road has a rush hour utilization of zero.

Even beautifully crafted third-party performance metering tools can have a hidden problem that can bite you hard in a crisis. If you don’t know the name of the low level meter that is the source of the beautifully presented graphic data before you, then the only people you can discuss this data with are other users of this tool. Most likely the people in support (or the external vendor wizards) have never seen this tool and will ask for the basic operating system meters instead. This can make for an awkward and slow conversation in a time of a performance crisis.