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Modern Operating Systems by Herbert...
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Page 407
The preamble starts with a certain bit pattern that allows the hardware to rec-
ognize the start of the sector.
It also contains the cylinder and sector numbers and
some other information.
The size of the data portion is determined by the low-
level formatting program. Most disks use 512-byte sectors.
The ECC field con-
tains redundant information that can be used to recover from read errors. The size
and content of this field varies from manufacturer to manufacturer, depending on
how much disk space the designer is willing to give up for higher reliability and
how complex an ECC code the controller can handle.
A 16-byte ECC field is not
unusual. Furthermore, all hard disks have some number of spare sectors allocated
to be used to replace sectors with a manufacturing defect.
The position of sector 0 on each track is offset from the previous track when
the low-level format is laid down. This offset, called
cylinder skew
, is done to im-
prove performance. The idea is to allow the disk to read multiple tracks in one con-
tinuous operation without losing data. The nature of the problem can be seen by
looking at Fig. 5-19(a). Suppose that a request needs 18 sectors starting at sector 0
on the innermost track. Reading the first 16 sectors takes one disk rotation, but a
seek is needed to move outward one track to get the 17th sector.
By the time the
head has moved one track, sector 0 has rotated past the head so an entire rotation is
needed until it comes by again. That problem is eliminated by offsetting the sectors
as shown in Fig. 5-22.
The amount of cylinder skew depends on the drive geometry. For example, a
10,000-RPM (Revolutions Per Minute) drive rotates in 6 msec.
If a track contains
300 sectors, a new sector passes under the head every 20
sec. If the track-to-track
seek time is 800
sec, 40 sectors will pass by during the seek, so the cylinder skew
should be at least 40 sectors, rather than the three sectors shown in Fig. 5-22. It is
worth mentioning that switching between heads also takes a finite time, so there is
head skew
as well as cylinder skew, but head skew is not very large, usually much
less than one sector time.
As a result of the low-level formatting, disk capacity is reduced, depending on
the sizes of the preamble, intersector gap, and ECC, as well as the number of spare
sectors reserved. Often the formatted capacity is 20% lower than the unformatted
The spare sectors do not count toward the formatted capacity, so all disks
of a given type have exactly the same capacity when shipped, independent of how
many bad sectors they actually have (if the number of bad sectors exceeds the
number of spares, the drive will be rejected and not shipped).
There is considerable confusion about disk capacity because some manufact-
urers advertised the unformatted capacity to make their drives look larger than they
in reality are. For example, let us consider a drive whose unformatted capacity is
bytes. This might be sold as a 200-GB disk.
However, after formatting,
posibly only 170
bytes are available for data.
To add to the confusion, the
operating system will probably report this capacity as 158 GB, not 170 GB, be-
cause software considers a memory of 1 GB to be 2
(1,073,741,824) bytes, not
(1,000,000,000) bytes.
It would be better if this were reported as 158 GiB.

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