Wednesday, August 1, 2012

Little's Law and IO Performance

Next Tuesday, August 7th, I'll be presenting at the Northern California CMG meeting*. My talk will be about Little's law and its implications for storage IO performance.

As a performance analyst or capacity planner, you already know all about Little's law—it's elementary. Right? Therefore, you completely understand:

  1. How Little's law relates inventory and manufacturing cycle time
  2. John Little (now 84) is not a performance analyst
  3. John Little did not invent Little's law
  4. Little's law was known to A. K. Erlang more than 100 years ago
  5. That there are actually two three versions of Little's law
  6. Little's law is not based on queueing theory
  7. Little's law expresses the fact that response time decreases with increasing throughput
  8. However, on the SPEC website you'll see that response time increases with increasing throughput. WTF !!!?

If you're feeling slightly bewildered about all this, you really should come along to my talk (assuming you're in the area). Otherwise, you can read the slide deck embedded below.

3-dimensional view of Little's law

I'll show you how I discovered the resolution to the apparent contradiction between items 7 and 8 (above) by representing Little's law in 3-dimensions. It's very cool! Even John Little doesn't know about this.

Oh yeah, and I'll also explain how Little's law reveals why it's possible to make your application IOs go 10x to 100x faster. IOPS bandwidth has become irrelevant.

Some of these conclusions are based on recent work I've been doing for Fusion-io. You might've heard of their billion IOPS benchmark, and more recently by association with SSDAlloc software from Princeton University.

* If you're not a ncCMG member, it's a one-time $25 entry fee, which then makes you a life member. See the bottom of their web page for payment and contact details.