Slide 4.3: A hard disk structure Slide 4.5: The cost of disk accesses Home

A Disk Access

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A disk access can be divided into three distinct physical operations, each with its own cost:

• Seek time: It is the time required to move the access arm to the correct cylinder. The average seek time is around [7.5 msec, 10 msec], where one msec is 10^-3 seconds.


• Rotational delay: It refers to the time it takes for the disk to rotate so the sector we want is under the read/write head. Disks rotate at about 5000 rpm (rotation per minute), which is one revolution per 12 msec. On average, the rotation delay is half a revolution, or about 6 msec.


• Transfer time: Once the data we want is under the read/write head, it can be transferred. The transfer time is given by the formula:
  
  
  Transfer time = Rotation time ×
  ( Number of bytes transferred / Number of bytes on a track )

FAT (File Allocation Table)
FAT describes the files, directories, and free space on a hard disk, or floppy disk. A disk is divided into partitions. Under the FAT file system each partition is divided into clusters, each of which can be one or more sectors, depending on the size of the partition. Each cluster is either allocated to a file or directory or it is free (unused).

A directory lists the name, size, modification time and starting cluster of each file or subdirectory it contains.

At the start of the partition is a table (the FAT) with one entry for each cluster. Each entry gives the number of the next cluster in the same file or a special value for “not allocated” or “this is the last cluster in the chain.” The first few clusters after the FAT contain the root directory.