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. Background and Motivation Disk drives are probably one of the biggest headaches administrators will ever run across. Our experience has been that the number of disk drives increases the support effort significantly. One of the reasons is the amount of time involved in installation and reconfiguration of disk drives. Figuring out how to configure the drives and backing up and restoring the files on the existing drives can take a lot of time. Standard hardware interfaces (de facto as well as official) have helped create a market where users can pick and choose different combinations of disk drives and controllers. The Small Computer System Interface (SCSI) is the most common type of bus found on workstations today. Vendors typically use a proprietary, high-speed bus to connect CPUs with memory and SCSI to connect I/O devices and peripherals. The SCSI specification is published and vendors have used this specification to manufacture hundreds of SCSI devices. This allows customers to select the device that best meets their needs and the computer vendors can focus on their processor and operating-system technologies. Many computer vendors supply default configuration information for configuring disks drives. However, these defaults are not optimized for each disk and by taking the time to perform the calculations yourself, you can maximize the amount of disk space available after configuring the disk.