Where Did All The Bytes Go?

Configuring, installing, re-configuring, and re-installing disk drives can be time consuming. Understanding the physical disk and how UNIX file systems are laid out on disk drives can not only help an administrator troubleshoot problems, but can allow him/her to maximize the amount of disk space available. Standard formatting utilities tend to configure disks with similar geometry identically. By using different geometry parameters or third party formatting programs, you can get the maximum amount of disk space from your drives. This paper is a tutorial on understanding SCSI disk drive geometry (and how it differs from more traditional drives) and the steps required to get a disk drive to the point where it is usable (e.g., with a UNIX file system on it.) It includes practical tips such as questions to ask vendors so you know how much usable space you will have when you purchase a disk drive. It also include a case study where we show how you can squeeze additional usable disk space from a drive. This paper is targeted at novice system administrators, but experienced administrators who want to learn more about disk drives will hopefully learn something from reading this paper. The examples we use will be based on a BSD system, but the concepts apply to other types of UNIX systems as well.