2. Back of the envelope estimation

Three basic concepts should be well understood to give the estimation of your system capacity:

1. Power of two: Allow you to quickly calculate I/O and memory size, such as KB, MB, GB, TB,…
2. Latency numbers every programmer should know: This allows you to compare the latency between systems.
3. Availability numbers: How much downtime per day you guarantee with your customer in SLA (service level agreement)?

Commonly asked back-of-the-envelope estimations: QPS, peak QPS, storage, cache, number of servers, etc. You can practice these calculations when preparing for an interview. Practice makes perfect.