Disk Storage Devices
l Preferred secondary storage device for high storage
capacity
and low cost.
l Data stored as magnetized areas on magnetic disk
surfaces.
l A disk pack contains several magnetic disks connected
a rotating spindle.
l 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.
l 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.
l 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.
l 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.
l 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).
l Reading or writing a disk block is time consuming
because of
the seek time s and rotational delay (latency) rd.
l Double buffering can be used to speed up the
transfer of
contiguous disk blocks.
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| Diagrams of how to work Disk storage device |
Example of typical disk parameter below
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