In WT-4447 we introduce a new configuration setting io_capacity=(total=<size>). Internally we then assign a percentage of that size for checkpoints, eviction, logging and reads. Although the summed percentages for each of those subsystems totals more than 100 to allow some fluctuation between subsystems, we don't allow it to exceed the total size.

We arrived at the percentages we're using through some trial and error. We think it is a decent general purpose setting.

It may be desired to have other preset policies for write heavy or read heavy workloads. Consider extending the API to include io_capacity=(policy=reading|writing|general).