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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 402
SEC. 5.4
DISKS
371
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Figure 5-19.
(a) Physical geometry of a disk with two zones. (b) A possible vir-
tual geometry for this disk.
The controller then remaps a request for (
x
,
y
,
z
) onto the real cylinder, head, and
sector.
A possible virtual geometry for the physical disk of Fig. 5-19(a) is shown
in Fig. 5-19(b). In both cases the disk has 192 sectors, only the published arrange-
ment is different than the real one.
For PCs, the maximum values for these three parameters are often (65535, 16,
and 63), due to the need to be backward compatible with the limitations of the
original IBM PC.
On this machine, 16-, 4-, and 6-bit fields were used to specify
these numbers, with cylinders and sectors numbered starting at 1 and heads num-
bered starting at 0.
With these parameters and 512 bytes per sector, the largest pos-
sible disk is 31.5 GB.
To get around this limit, all modern disks now support a sys-
tem called
logical block addressing
, in which disk sectors are just numbered con-
secutively starting at 0, without regard to the disk geometry.
RAID
CPU performance has been increasing exponentially over the past decade,
roughly doubling every 18 months. Not so with disk performance.
In the 1970s,
average seek times on minicomputer disks were 50 to 100 msec. Now seek times
are still a few msec. In most technical industries (say, automobiles or aviation), a
factor of 5 to 10 performance improvement in two decades would be major news
(imagine 300-MPG cars), but in the computer industry it is an embarrassment.
Thus the gap between CPU performance and (hard) disk performance has become
much larger over time. Can anything be done to help?
Page 403
372
INPUT/OUTPUT
CHAP. 5
Yes! As we have seen, parallel processing is increasingly being used to speed
up CPU performance.
It has occurred to various people over the years that parallel
I/O might be a good idea, too.
In their 1988 paper, Patterson et al. suggested six
specific disk organizations that could be used to improve disk performance, re-
liability, or both (Patterson et al., 1988).
These ideas were quickly adopted by in-
dustry and have led to a new class of I/O device called a
RAID
.
Patterson et al.
defined RAID as
Redundant Array of Inexpensive Disks
, but industry redefined
the I to be ‘‘Independent’’ rather than ‘‘Inexpensive’’ (maybe so they could charge
more?). Since a villain was also needed (as in RISC vs. CISC, also due to Patter-
son), the bad guy here was the
SLED
(
Single Large Expensive Disk
).
The fundamental idea behind a RAID is to install a box full of disks next to the
computer, typically a large server, replace the disk controller card with a RAID
controller, copy the data over to the RAID, and then continue normal operation.
In
other words, a RAID should look like a SLED to the operating system but have
better performance and better reliability. In the past, RAIDs consisted almost ex-
clusively of a RAID SCSI controller plus a box of SCSI disks, because the per-
formance was good and modern SCSI supports up to 15 disks on a single con-
troller. Nowadays, many manufacturers also offer (less expensive) RAIDs based on
SATA.
In this way, no software changes are required to use the RAID, a big sell-
ing point for many system administrators.
In addition to appearing like a single disk to the software, all RAIDs have the
property that the data are distributed over the drives, to allow parallel operation.
Several different schemes for doing this were defined by Patterson et al.
Now-
adays, most manufacturers refer to the seven standard configurations as RAID
level 0 through RAID level 6.
In addition, there are a few other minor levels that
we will not discuss. The term ‘‘level’’ is something of a misnomer since no hier-
archy is involved; there are simply seven different organizations possible.
RAID level 0 is illustrated in Fig. 5-20(a). It consists of viewing the virtual
single disk simulated by the RAID as being divided up into strips of
k
sectors each,
with sectors 0 to
k
−
1 being strip 0, sectors
k
to 2
k
−
1 strip 1, and so on. For
k
=
1, each strip is a sector; for
k
=
2 a strip is two sectors, etc. The RAID level 0
organization writes consecutive strips over the drives in round-robin fashion, as
depicted in Fig. 5-20(a) for a RAID with four disk drives.
Distributing data over multiple drives like this is called
striping
.
For example,
if the software issues a command to read a data block consisting of four consecu-
tive strips starting at a strip boundary, the RAID controller will break this com-
mand up into four separate commands, one for each of the four disks, and have
them operate in parallel. Thus we have parallel I/O without the software knowing
about it.
RAID level 0 works best with large requests, the bigger the better.
If a request
is larger than the number of drives times the strip size, some drives will get multi-
ple requests, so that when they finish the first request they start the second one.
It
is up to the controller to split the request up and feed the proper commands to the
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