MBR partitions, also known as primary partitions, are universally understood by operating systems that run on i386 hardware. Every hard drive has four MBR partitions. In most cases, only one partition has any space allocated to it; the other three partitions have zero size. If you want to install multiple operating systems on a single disk, then each operating system needs its own MBR partition.

Most operating systems manage MBR partitions with a program called fdisk. It’s not the same program, mind you—OpenBSD’s fdisk(8) is not the same as Microsoft’s Fdisk, which is different from the program for Linux, FreeBSD, OpenSolaris, and so on. Any operating system’s fdisk can see MBR partitions that belong to other operating systems, and while they might not recognize what’s on the MBR partition, they will recognize that space has been allocated for something and will warn you about overwriting it. Unfortunately, not all fdisk programs play nicely with each other. Do not partition disks for one operating system with another operating system’s tools.^[6]

With the advent of cheap virtualization, installing multiple operating systems on a single disk is no longer advisable. Assign each disk a single MBR partition that fills the entire disk, and give the other three MBR partitions zero size. You will see an example of how to do this in Chapter 3.

BSD did not originate on i386 hardware; it had its own disk-partitioning system, based on labeling the disk’s partitions. When BSD was ported to i386, the disklabel was nailed up inside an MBR partition. When someone speaks of “partitions” in OpenBSD, they almost certainly mean disklabel partitions.

One disklabel can support 16 partitions. If you need more than 16 partitions, you must create a second MBR partition and add more disklabels. I would suggest that if you need more than 16 partitions on a single disk, you took a wrong turn somewhere in your decision-making process. Step back and reassess what you want to accomplish and how you’re going about it.

Foreign operating systems do not recognize OpenBSD disklabels. BSD-based operating systems might appear to understand them, but the disklabel formats used on the various BSD-derived systems have diverged in the past 20 years. Use only OpenBSD disk tools to manage OpenBSD partitions.