FreeBSD Foundationals: The Boot Process - From the Loader to Boot Environments

The two previous articles in this series covered the things that store your data and the things that run your workloads. This one covers the part that nobody thinks about until it’s three in the morning and the machine won’t come back up: how FreeBSD actually boots, and - more importantly - how to make a bad boot recoverable. This is the third article in the FreeBSD Foundationals series. The first covered Jails, the second covered ZFS. We’re covering the boot process now because everything else depends on it and because, compared to other UNIX-like systems, FreeBSD keeps the early boot chain unusually small and inspectable. There’s a small, documented chain of stages, a text file you edit by hand, and - if you’re on root-on-ZFS - a recovery mechanism that is genuinely one of the best reasons to run FreeBSD on a server. By the end of this article you’ll understand what each boot stage does, where to put a setting and why, the difference between a tunable and a sysctl (people get this…