The DASD and
Evolution of Storage Devices
Rule
1: “Data processing requires data
storage”. – I just made that up.
Data
were originally stored on paper media, first as written documents but fairly soon
(Hollerith, late 19th century) the storage medium was machine–readable.
In
the 1950’s, New York Life Insurance Company was devoting an entire floor of its
main building to the storage of punched cards.
Something had to change.
IBM
quickly came out with two magnetic media for storing data
the
magnetic tape
the
DASD (disk)
The
acronym “DASD” stands for Direct Access Storage Device.
Until
recently, the standard disk drive was the only commercially viable example.
We
now have another very popular example, these USB “flash drives”. While different from standard disk drives,
these are managed as if they were disk drives and are considered disk drives.
Structure of
a Large Disk Drive
The
typical large–capacity (and physically small) disk drive has a number of glass
platters with magnetic coating. These
spin at a high rate (7,200 rpm or 120 / second)
This
drawing shows a disk with three platters and six surfaces. In general, a disk drive with N platters will
have 2·N surfaces, the top and
bottom of each platter.
On
early disk drives, before the introduction of sealed drives, the top and bottom
surfaces would not be used because they would become dirty.
More on Disk
Drive Structure
Each
surface is divided into a number of concentric tracks.
Each
track has a number of sectors.
A
sector usually contains 512 bytes of data, along with a header and trailer
part.
Seek Time
and Rotational Latency
In
order to read from a disk track, the read/write heads must be moved to the
track.
This
is a mechanical action, as the read/write heads are physical devices.
There
are two seek times typically quoted for a disk.
Track–to–track: the time to move the heads to the next track
over
Average: the average time to move the
heads to any track.
The
rotational delay is due to the fact
that the disk is spinning at a fixed high speed.
It takes a certain time for a specific sector to rotate under the read/write
heads.
Suppose
a disk rotating at 12,000 RPM. That is
200 revolutions per second.
Each sector moves under the read/write heads 200 times a second,
once every 0.005 second or every 5 milliseconds.
The
rotational latency, or average
rotational delay, is one half of the time for a complete revolution of the
disk. Here it would be 2.50
milliseconds.
The Idea of
a Cylinder
Fixed head disks have one head per track. The last time I heard of such a device was
1977, when working with a 1 MB fixed head disk on a PDP–11/45.
I
claim that fixed head disks are obsolete.
Revisit the picture of a typical disk.
Question:
How many tracks can be read before the read/write heads must be moved?
Answer:
One track per surface can be read without moving the heads. Here it is 6.
Definition: A cylinder
is that set of tracks that can be read without moving the disk
read write heads. A disk has as many cylinders as a surface has
tracks.
A cylinder has as many tracks as
the disk has surfaces.