9.3. Disk drives

Several permanent mass storage devices or disks are available with UNICOS and UNICOS/mk systems. A disk system for UNICOS and UNICOS/mk systems consists of I/O processors, disk controller units, and disk storage units.

A sector is the smallest unit of allocation for a file in the file system. It is also the smallest unit of allocation; all I/O is performed in sectors.

In each disk storage unit, the recording surface available to a read/write head group is called a disk track. Each track contains a number of sectors in which data can be recorded and read back. The data in one sector is called a data block; the size of the data block varies with the disk type. The number of sectors per track, the number of tracks per cylinder, and the number of cylinders per drive also vary according to the type of disk storage unit. For example, a DD-49 disk storage unit contains 886 cylinders with 8 tracks per cylinder and 42 sectors per track. See the dsk(4), disksipn, disksfcn(7), and disksmpn(7) man pages for complete details.

The following table lists sector size, track size, and tracks per cylinder for a variety of disks:

This information is useful when you must determine an efficient buffer size.

Disk-based storage under the UNICOS operating system is divided into logical devices. A logical disk device is a collection of blocks on one or more physical disks or other logical disk devices. These blocks are collected into partitions to be used as file system entities. A block is a sector.

An optional striping capability exists for all disk drives. Striping allows a group of physical devices to be treated as one large device with a potential I/O rate of a single device multiplied by the number of devices in the striped group. Striped devices must consist of physical devices that are all of the same type. I/O requests using striping should be in multiples of n × ts bytes; n is the number of devices in the group and ts is the track size of the disk in bytes (not in words or sectors).

For most disks this figure will be n × 4096 bytes. For DD-60 disks, n must be rounded to the nearest multiple of 4 because its sector size is 16 Kbytes.

Disk striping on some systems can enhance effective transfer rates to and from disks.