A Tale of Two LEDs

Fri, 06 Sep 2019 18:23:45 +0000

It was the brightest of LEDs, it was the darkest of LEDs, it was the age of data links, it was the age of AHCI enclosure services, …​

Today, I’d like to talk about two aspects of a project that I worked on a little while back under the aegis of RFD 89 Project Tiresias. This project covered improving the infrastructure for how we gathered and used information about various components in the system. So, let’s talk about LEDs for a moment.

LEDs are strewn across systems and show up on disks, networking cards, or just to tell us the system is powered on. In many cases, we rely on the blinking lights of a NIC, switch, hard drive, or another component to see that data is flowing. The activity LED is a mainstay of many devices. However, there’s another reason that we want to be able to control the LEDs: for identification purposes. If you have a rack of servers and you’re trying to make sure you pull the right networking cable, it can be helpful to be able to turn a LED on, off, or blink it with a regular pattern. So without further ado, let’s talk about how we control LEDs for network interface cards (NICs or data links) and a class of SATA hard drives.