Although a single disk that is large enough can hold all slices and their corresponding file systems, two or more disks are often used to hold a system's slices and file systems.
A slice cannot be split between two or more disks. However, multiple swap slices on separate disks are allowed.
For instance, a single disk might hold the root (/) file system, a swap area, and the /usr file system, while a separate disk is provided for the /export/home file system and other file systems containing user data.
In a multiple disk arrangement, the disk containing the operating system software and swap space (that is, the disk holding the root (/) or /usr file systems or the slice for swap space) is called the system disk. Disks other than the system disk are called secondary disks or non-system disks.
Locating a system's file systems on multiple disks allows you to modify file systems and slices on the secondary disks without having to shut down the system or reload operating system software.
Having more than one disk also increases input-output (I/O) volume. By distributing disk load across multiple disks, you can avoid I/O bottlenecks.