Our calculations always assume worst-case scenarios. This is both expensive and in many cases, overzealous. We ignore RAM caching of disk blocks, we don't account for application frontend caches, and the PostgreSQL shared buffers are also not included.
Why? Crashes are always a concern. If a database crashes, buffers are forfeit. If the application frontend cache gets emptied or has problems, reads will be served directly from the database. Until caches are rebuilt, query results can be multiple orders of magnitude slower than normal for minutes or hours. We will discuss methods of circumventing these effects, but these IOPS numbers give us a baseline.
The number of necessary IOPS, and hence disk requirements, are subject ...
