An Operating Systems Vade Mecum

150 Transput Chapter 5be made. By the Hysteresis Principle, the motor is not turned off again immediately, butis left on for a few seconds in the expectation that another transfer will be needed soon.In addition to a block of data, a sector may also hold addressing information(which sector it is), a bit that indicates whether the sector is usable (if one sector is damaged,the rest of the disk pack may still be usable), and some error-checking informationsuch as a checksum. (We will discuss checksums shortly.)The operating system usually does not need to be aware of this extra sector information,but it must know how the disk is formatted — that is, how many sectors thereare in a track and how many bytes may be stored in a sector. On many disk units, theformatting is performed under software control when the disk pack is brought into service.Formatting also discovers bad sectors and marks them. Extra tracks may be availableto make up for bad parts of the disk. Some sophisticated disk controllers automaticallyremap good sectors from these extra tracks onto sectors that were discovered to bebad during formatting so that the operating system does not need to remember which sectorsto avoid.The disk is only able to read or write from the track where the read/write headcurrently sits. This head can move in and out when necessary to access other tracks; thismotion is called seeking. It takes on the order of 20 to 40 milliseconds to complete aseek. Of course, it takes longer to seek across many tracks than to seek across just a few.After a disk has been in use for a while, a seek operation may fail to move to exactly theright track. In this case, the software must ask the disk head to recalibrate itself by movingall the way out to a well-known position.The read/write heads for all the disk surfaces are usually physically tied together sothey all access the same track number at the same time. This combination of similarlylocated tracks on different surfaces is called a cylinder.Three types of delay, or latency, affect how long it takes to read data from or writedata to the disk. Seek latency is the amount of time needed to seek to the desiredcylinder. Rotational latency is the amount of time needed for the desired sector toarrive under the read/write head. Transfer latency is the amount of time needed forthose sectors with data to be completely scanned. (A single transput operation caninvolve more than one contiguous sector.)1.2 Magnetic tapeMagnetic tape has three major uses: archival store, data transfer between computers, andintermediate storage of large amounts of data. Magnetic tape is the medium of choice forarchival storage of large amounts of data. Periodic backups of disk are typically madeonto tape, which can then be stored for long periods of time. Even though computer networkslink many installations, massive amounts of data are still most economicallytransmitted by magnetic tape. A tape written at one installation with one brand of tapedrive can usually be read at a different installation with a different brand of tape drive. Infact, the two installations may have different operating systems and different brands ofcomputer. Such standardization makes it possible to transfer data practically anywhereby tape. Many programs of importance in the business world take a large number ofinput records, extract some data from each one, and then generate an output record.