- Preferred secondary storage device for high storage capacity
and low cost.
2. Data stored as magnetized areas on magnetic disk surfaces.
3. A disk pack contains several magnetic disks connected
a rotating spindle.
4. Disks are divided into concentric circular tracks on each
disk surface. Track capacities vary typically from 4 to 50
Kbytes. Because a track usually contains a large amount of
information, it is divided into smaller blocks or sectors.
5. The division of a track into sectors is hard-coded on the disk
surface and cannot be changed. One type of sector organization
calls a portion of a track that subtends a fixed angle at the
center as a sector.
6. A track is divided into blocks. The block size B is fixed for each
system. Typical block sizes range from B=512 bytes to B=4096
bytes. Whole blocks are transferred between disk and main
memory for processing.
7. A read-write head moves to the track that contains the block to
be transferred. Disk rotation moves the block under the read-
write head for reading or writing.
8) A physical disk block (hardware) address consists of a cylinder
number (imaginary collection of tracks of same radius from all
recorded surfaces), the track number or surface number (within
the cylinder), and block number (within track).
9. Reading or writing a disk block is time consuming because of
the seek time s and rotational delay (latency) rd.
l0. Double buffering can be used to speed up the transfer of
contiguous disk blocks.
Example of typical disk parameter below
