MBR Partitions

In Chapter 3 we discussed OpenBSD partitions and briefly mentioned MBR partitions. You didn't need to worry about MBR partitions unless you wanted to have multiple operating systems on one computer. The Master Boot Record includes some basic disk partitioning information, marking the locations of up to four partitions.

These partitions are used for different purposes in different operating systems. Windows-based operating systems use these as "logical drives." If you're old enough, you'll remember when a Windows 95 computer could only have a 2GB C: drive, and if you had a larger disk you needed to split it up into logical drives. At one point I had Windows C:, D:, E:, and F: drives on one 8GB hard drive!

OpenBSD, Linux, and FreeBSD all like to support more than four partitions on a drive. They have to have their own partitioning scheme within a single MBR partition. For example, when you have a dedicated OpenBSD machine with five partitions (/, swap, /tmp, /var, /usr, and home), these partitions are all subdivisions of a single MBR partition. That MBR partition just happens to fill the whole disk. OpenBSD partitions need to go within a single MBR partition.

Dedicate a single MBR partition to each operating system on a hard drive. (If you need more than four operating systems, you need to invest in a commercial product that will let you do so.)

A Dozen Different fdisks

Every operating system includes tools to manage MBR partitions. Unfortunately, every operating system handles MBR partitions in a slightly different manner. More unfortunately, most of these tools are named "fdisk." Each tool operates differently, and while some of them look similar or have a common heritage they are not guaranteed to be interoperable. The upshot of all this is that you should create the MBR partitions used by an operating system by the operating system's native tools. When you're installing a dual-boot OpenBSD/XP system, use OpenBSD's fdisk(8) to create the MBR partitions only for OpenBSD and use the Windows XP installer to create the partition for Windows XP. Do not use the OpenBSD fdisk tool to create Windows XP partitions, and do not try to create OpenBSD partitions with XP! This might look workable, but minor differences between fdisk implementations might make the partition unusable.

Because this is an OpenBSD book, we'll focus on OpenBSD's fdisk. Check the documentation for the other operating systems you're using for details on their fdisk implementations. The concepts are the same, but many of the details differ.