- Connecting
disks to
a computer system:
- By using one or more
disk controllers
- 1 disk controller can
control
n disks
-
Function of the
disk controller:
- Control movement of
disk heads
(seek to correct track on
disk)
- Select a sector on a
track
- Transfers data between disk surface and main memory
- Control movement of
disk heads
(seek to correct track on
disk)
- By using one or more
disk controllers
- Disk controller has
internal buffers to
store data
being transfered
to/from
disk
(from/to memory (RAM))
- Steps to
access data on a
disk
- Move the
disk heads to the
desired cylinder
- Time to
seek a cylinder =
seek time
(~ 0 - 10 msec)
- Time to
seek a cylinder =
seek time
(~ 0 - 10 msec)
- Wait for
the
desired sector to
arrive under the
disk head
- Time to
wait for a sector =
rotational delay
(~ 0 - 10 msec)
- Time to
wait for a sector =
rotational delay
(~ 0 - 10 msec)
- Transfer the data from
sector to
main memory
(through the disk controller)
Graphically:
- Move the
disk heads to the
desired cylinder
- Conclussion:
- Time to read
1 sector from disk ~=
20 msec
- Very long compare to nsec CPU cycle
- Time to read
1 sector from disk ~=
20 msec
- Next sector:
- Seek time = 0
(unless next sector is located on a
different cylinder
- Rotational delay ~= 0
(unless next sector is located on a
different cylinder
- Transfer time unchange (< 1 msec)
- Seek time = 0
(unless next sector is located on a
different cylinder
- Conclusion:
- Store related data that are
used together
on nearby
sectors
(Sectors of one blocks are near each other)
- Store related data that are
used together
on nearby
sectors