Multiple OS Partitioning

Many people need to run multiple operating systems on one computer, and OpenBSD allows you to do that. By far, the easiest way to do this is to install a hard drive in your computer for each operating system. This allows you to use each OS's native disk tools without risking tramping on your other operating system. In this day of dirt-cheap hundred-gig hard drives, however, this is an added complication for many people who simply want to divide up their hard disk appropriately.

When you divide up a single hard disk between multiple operating systems, you fall into another level of partitioning, known as MBR (Master Boot Record) partitions. The boundaries of these partitions are stored in the Master Boot Record on a disk, and are managed by tools such as UNIX fdisk(8), DOS fdisk, or Microsoft's Disk Administrator. Any operating system can see MBR partitions; they may not recognize that one of these partitions is designated for OpenBSD, but they realize that this is a discrete section of disk. Within these large partitions, you create smaller OpenBSD-specific partitions for /home, /usr, and so on.

The fdisk tools allow you to, say, take your 80GB disk and designate the first 20GB for OpenBSD, the second 20GB for Microsoft Windows XP, the third 20GB for FreeBSD, and the last chunk for Linux, should you wish. You then use each OS's native tools to manage those chunks of disk space. You would then use a separate "boot manager" to choose between operating systems at boot time.

When you decide where to put disk space for any one OS, you need to allow for OpenBSD's boot limitations. Just because you have multiple operating systems on a hard drive doesn't mean that you can ignore the 504MB limit or the 8GB limit. If you have enough disk space to install more than one operating system, chances are your system does not suffer from the 504MB limit. Still, the OpenBSD root partition must be contained entirely within the first 8GB of disk, not the first 8GB of disk space assigned to OpenBSD. In most cases, this means that OpenBSD must be the first operating system on your disk. Also, OpenBSD on a hard disk must be a single contiguous section; you cannot dedicate the first 20GB of your hard drive to OpenBSD, have a 20GB Microsoft partition, and have a 40GB OpenBSD partition to round out your disk.

Put your OpenBSD partition first on the disk, and you won't have any problems. We discuss multiboot partitioning and installation at length in Chapter 4.