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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 412
SEC. 5.4
DISKS
381
Figure 5-25 shows the elevator algorithm using the same seven requests as
Fig. 5-24, assuming the direction bit was initially
UP
.
The order in which the cyl-
inders are serviced is 12, 16, 34, 36, 9, and 1, which yields arm motions of 1, 4, 18,
2, 27, and 8, for a total of 60 cylinders. In this case the elevator algorithm is slight-
ly better than SSF, although it is usually worse. One nice property the elevator al-
gorithm has is that given any collection of requests, the upper bound on the total
motion is fixed: it is just twice the number of cylinders.
Initial
position
Cylinder
X
X
XX
X
X
X
0
5
10
15
20
25
30
35
Time
Sequence of seeks
Figure 5-25.
The elevator algorithm for scheduling disk requests.
A slight modification of this algorithm that has a smaller variance in response
times (Teory, 1972) is to always scan in the same direction. When the highest-num-
bered cylinder with a pending request has been serviced, the arm goes to the
lowest-numbered cylinder with a pending request and then continues moving in an
upward direction. In effect, the lowest-numbered cylinder is thought of as being
just above the highest-numbered cylinder.
Some disk controllers provide a way for the software to inspect the current sec-
tor number under the head. With such a controller, another optimization is pos-
sible. If two or more requests for the same cylinder are pending, the driver can
issue a request for the sector that will pass under the head next. Note that when
multiple tracks are present in a cylinder, consecutive requests can be for different
tracks with no penalty. The controller can select any of its heads almost in-
stantaneously (head selection involves neither arm motion nor rotational delay).
If the disk has the property that seek time is much faster than the rotational
delay, then a different optimization should be used. Pending requests should be
sorted by sector number, and as soon as the next sector is about to pass under the
head, the arm should be zipped over to the right track to read or write it.
With a modern hard disk, the seek and rotational delays so dominate per-
formance that reading one or two sectors at a time is very inefficient. For this rea-
son, many disk controllers always read and cache multiple sectors, even when only
one is requested. Typically any request to read a sector will cause that sector and
much or all the rest of the current track to be read, depending upon how much
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