Essential System Administration, 3rd Edition

From Disks to Filesystems

As we’ve seen, the basic Unix file storage unit is the disk partition. Filesystems are created on disk partitions, and all of the separate filesystems are combined into a single directory tree. The initial parts of this section discuss the process by which a physical disk becomes one or more filesystems on a Unix system, treating the topic at a conceptual level. Later subsections discuss the mechanics of adding a new disk to the various operating systems we are considering.

Defining Disk Partitions

Traditionally, the Unix operating system organizes disks into fixed-size partitions, whose sizes and locations are determined when the disk is first prepared (as we’ll see). Unix treats disk partitions as logically independent devices, each of which is accessed as if it were a physically separate disk. For example, one physical disk may be divided into four partitions, each of which holds a separate filesystem. Alternatively, a physical disk may be configured to contain only one partition comprising its entire capacity.

Many Unix implementations allow several physical disks to be combined into a single logical device or partition upon which you can build a filesystem. Systems offering a logical volume manager carry this trend to its logical conclusion, allowing multiple physical disks to be combined into a single logical disk, which can then be divided into logical partitions. AIX uses only an LVM and does not use traditional partitions at all.

Physically, adisk ...