This preview shows page 1. Sign up to view the full content.
Unformatted text preview: number. Before a disk drive can access a piece
of data (a record) stored on a disk, it must specify the record's disk address. The disk
address is comprised of the sector number, track number, and surface number (when
double-sided disks are used). That is, the disk address represents the physical location of
the record on the disk.170
We saw that disk drives are designed to access only whole sectors at a time (a sector
being the smallest unit of data access from a disk). Since a file usually is not of a size that
is an even multiple of sector size (usually 512 bytes), some part of the last sector
allocated to the file may be unused. Thus, on an average half of the sector size is wasted
for each file stored on the disk. Hence it is recommended that the sector size for a disk
should not be too large. The sector size should not even be too small, otherwise the size of the mapping table used to map disk addresses to data records will become too large.
The DOS operating system goes one step further of combining two or more sectors to
form a cluster. In this case, the smallest unit of data access from a disk becomes a cluster
-not a sector. That is, read/write operations read/write a whole cluster at a time. Cluster
sizes vary, depending on the size and type of the disk, but they can range from 2 to 64
sectors per cluster. Notice that a cluster-based disk organization may lead to more
wastage of disk space as compared to sector-based organization, because in case of
cluster-based systems, on an average, half of the cluster size is wasted for each file stored
on the disk. However, cluster-based organization, leads to less management overhead for
mapping of data records to physical disk locations.
Often multiple disks are stacked and used together to create large capacity disk storage
systems. In this case, a set of magnetic disks is fixed to a central shaft one below the
other to form a disk pack (see Figure 8.16). The disk pack is sealed and mounted on a
disk drive, which consists of a motor to rotate the disk pack about its axis. The disk drive
also has an access arm assembly that has separate read/write heads for each surface of the
View Full Document
This document was uploaded on 04/07/2014.
- Spring '14