New storage book

I bought a new book entitled "Information Storage and Management", which is written by EMC² experts. I'm up to the second chapter, and it's all very dry so far. However... I think it's going to start getting interesting because it goes from the basics of how hard disk drives work and how their components fit together to... 2.4 Fundamental Laws Governing Disk Performance. Which tells me the following about average queue size:

N_Q = N - U
= a × R - U (from eq. 1)
= a × (R_S / (1 - U)) - U (from eq. 5)
= (R_S / R_a) / (1 - U) - U (from eq. 3)
= U / (1 - U) - U (from eq. 4)
= U (1 / (1 - U) - 1)
= U² / (1-U)      (6)

Whew! Glad I knew that :-)

Sounds like some interesting reading.