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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 408
SEC. 5.4
DISKS
377
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Direction of disk
rotation
Figure 5-22.
An illustration of cylinder skew.
To make things even worse, in the world of data communications, 1 Gbps
means 1,000,000,000 bits/sec because the prefix
giga
really does mean 10
9
(a kilo-
meter is 1000 meters, not 1024 meters, after all).
Only with memory and disk
sizes do kilo, mega, giga, and tera mean 2
10
,2
20
,2
30
, and 2
40
, respectively.
To avoid confusion, some authors use the prefixes kilo, mega, giga, and tera to
mean 10
3
, 10
6
, 10
9
, and 10
12
respectively, while using kibi, mebi, gibi, and tebi to
mean 2
10
,2
20
,2
30
, and 2
40
, respectively. However, the use of the ‘‘b’’ prefixes is
relatively rare. Just in case you like really big numbers, the prefixes following tebi
are pebi, exbi, zebi, and yobi, so a yobibyte is a whole bunch of bytes (2
80
to be
precise).
Formatting also affects performance.
If a 10,000-RPM disk has 300 sectors
per track of 512 bytes each, it takes 6 msec to read the 153,600 bytes on a track for
a data rate of 25,600,000 bytes/sec or 24.4 MB/sec.
It is not possible to go faster
than this, no matter what kind of interface is present, even if it is a SCSI interface
at 80 MB/sec or 160 MB/sec.
Actually reading continuously at this rate requires a large buffer in the con-
troller. Consider, for example, a controller with a one-sector buffer that has been
given a command to read two consecutive sectors. After reading the first sector
from the disk and doing the ECC calculation, the data must be transferred to main
Page 409
378
INPUT/OUTPUT
CHAP. 5
memory. While this transfer is taking place, the next sector will fly by the head.
When the copy to memory is complete, the controller will have to wait almost an
entire rotation time for the second sector to come around again.
This problem can be eliminated by numbering the sectors in an interleaved
fashion when formatting the disk.
In Fig. 5-23(a), we see the usual numbering pat-
tern (ignoring cylinder skew here). In Fig. 5-23(b), we see
single interleaving
,
which gives the controller some breathing space between consecutive sectors in
order to copy the buffer to main memory.
(a)
0
7
3
4
1
2
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5
(b)
0
7
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4
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(c)
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Figure 5-23.
(a) No interleaving. (b) Single interleaving. (c) Double interleaving.
If the copying process is very slow, the
double interleaving
of Fig. 5-24(c)
may be needed.
If the controller has a buffer of only one sector, it does not matter
whether the copying from the buffer to main memory is done by the controller, the
main CPU, or a DMA chip; it still takes some time.
To avoid the need for inter-
leaving, the controller should be able to buffer an entire track. Most modern con-
trollers can buffer many entire tracks.
After low-level formatting is completed, the disk is partitioned. Logically, each
partition is like a separate disk.
Partitions are needed to allow multiple operating
systems to coexist. Also, in some cases, a partition can be used for swapping. In
the x86 and most other computers, sector 0 contains the
MBR
(
Master Boot
Record
), which contains some boot code plus the partition table at the end.
The
MBR, and thus support for partition tables, first appeared in IBM PCs in 1983 to
support the then-massive 10-MB hard drive in the PC XT.
Disks have grown a bit
since then. As MBR partition entries in most systems are limited to 32 bits, the
maximum disk size that can be supported with 512 B sectors is 2 TB. For this rea-
son, most operating since now also support the new
GPT
(
GUID Partition Table
),
which supports disk sizes up to 9.4 ZB (9,444,732,965,739,290,426,880 bytes).
At
the time this book went to press, this was considered a lot of bytes.
The partition table gives the starting sector and size of each partition.
On the
x86, the MBR partition table has room for four partitions.
If all of them are for
Windows, they will be called C:, D:, E:, and F: and treated as separate drives. If
three of them are for Windows and one is for UNIX, then Windows will call its
partitions C:, D:, and E:.
If a USB drive is added, it will be F:.
To be able to boot
from the hard disk, one partition must be marked as active in the partition table.
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