Because the heads of the disk move together on all surfaces we can increase

Because the heads of the disk move together on all

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Because the heads of the disk move together on all surfaces, we can increase read-write efficiency by allocating blocks in parallel across all surfaces. Thus, if a file is stored in consecutive blocks, on a disk with surfaces and heads, it could read sectors sectors-per-track without any head movement. When a disk is supplied by a manufacturer, the physical properties of the disk (number of tracks, number of heads, sectors per track, speed of revolution) are provided with the disk. An operating system must be able to adjust to different types of disk. Clearly sectors per track is not a constant, nor is necessarily the number of tracks. The numbers given are just a convention used to work out a consistent set of addresses on a disk and may not have anything to do with the hard and fast physical limits of the disk. To address any portion of a disk, we need a three component address consisting of ( surface, track, sector ). 5.3.2 Device drivers and IDs A hard-disk is a device , and as such, an operating system must use a device controller to talk to it. Some device controllers are simple microprocessors which translate numerical addresses into head motor movements, while others contain small decision making computers of their own. The most popular type of drive for larger personal computers and workstations is the SCSI drive. SCSI (pronounced scuzzy) (Small Computer System Interface) is a protocol and now exists in four variants SCSI 1, SCSI 2, and fast SCSI 2, SCSI 3. SCSI disks live on a data bus which is a fast parallel data link to the CPU and memory, rather like a very short network. Each drive coupled to the bus identifies itself by a SCSI address () and each SCSI controller can address up to seven units. If more disks are required, a second controller must be added. SCSI is more efficient at multiple access sharing than other disk types for microcomputers. In order to talk to a SCSI disk, an operating system must have a SCSI device driver. This is a layer of software which translates disk requests from the operating system's abstract command-layer into the language of signals which the SCSI controller understands. The operating system generally provides two logical devices for each SCSI address: a raw device and a buffered device . On BSD UNIX systems these are referred to as /dev/?? and /dev/r?? . 5.3.3 Checking data consistency and formatting Hard drives are not perfect: they develop defects due to magnetic dropout and imperfect manufacturing. On more primitive disks, this is checked when the disk is formatted and these damaged sectors are avoided. If sector becomes damaged under operation, the structure of the disk must be patched up by some repair program. Usually the data are lost. On more intelligent drives, like the SCSI drives, the disk itself keeps a defect list which contains a list of all bad sectors. A new disk from the manufacturer contains a starting list and this is updated as time goes by if more defects occur.
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