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Saturday, February 13, 2010

How does the operating system does its job when the storage media are removable ?

Major OS jobs are to manage physical devices and to present a virtual machine abstraction to applications.Most operating system handles removable disks almost exactly as they do the fixed disks.A new cartridge is formatted and an empty file system is generated on the disk.

Tapes are often handled differently. Tapes are presented as a raw storage medium, i.e., an application does not not open a file on the tape, it opens the whole tape drive as a raw device.Usually the tape drive is reserved for the exclusive use of that application. The operating system does not provide file-system services when tape drive is presented as a raw device.Since the OS does not provide file system services, the application must decide how to use the array of blocks. Since every application makes up its own rules for how to organize a tape, a tape full of data can generally only be used by the program that created it.

The basic operations of tape drives differ from the operations of disk drive.
* locate positions the tape to a specific logical block, not an entire track (corresponds to seek).
* read position operation returns the logical block number where the tape head is.
* The space operation enables relative motion.

Tape drives are “append-only” devices; updating a block in the middle of the tape also effectively erases everything beyond that block. An EOT (End of tape) mark is placed after a block that is written.

The issue of naming files on removable media is especially difficult when we want to write data on a removable cartridge on one computer, and then use the cartridge in another computer. Contemporary OS generally leave the name space problem
unsolved for removable media, and depend on applications and users to figure out how to access and interpret the data. Some kinds of removable media (e.g., CDs) are so well standardized that all computers use them the same way.

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