Hardware Reference
In-Depth Information
control signals to delineate the phases and arbitrate bus access when multiple de-
vices are trying to use the bus at the same time. This arbitration is important be-
cause SCSI allows all the devices to run at once, potentially greatly improving per-
formance in an environment with multiple processes active at once. IDE and EIDE
allow only one active device at a time.
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 10 msec. In most technical industries (say, automobiles or aviation), a factor of
5 to 10 performance improvement in two decades would be major news, but in the
computer industry it is an embarrassment. Thus the gap between CPU per-
formance and disk performance has become much larger over time.
As we have seen, parallel processing is often used to speed up CPU per-
formance. 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, reliability, or both
(Patterson et al., 1988). These ideas were quickly adopted by industry 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 ''Inde-
pendent'' rather than ''Inexpensive'' (maybe so they could use expensive disks?).
Since a villain was also needed (as in RISC versus CISC, also due to Patterson),
the bad guy here was the
SLED
(
Single Large Expensive Disk
).
The 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. Since SCSI disks have good performance, low
price, and the ability to have up to 7 drives on a single controller (15 for wide
SCSI), it is natural that many RAIDs consist of a RAID SCSI controller plus a box
of SCSI disks that appear to the operating system as a single large disk. In this
way, no software changes are required to use the RAID, a big selling 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., and they
are now known as RAID level 0 through RAID level 5. In addition, there are a few
other minor levels that we will not discuss. The term ''level'' is something of a
misnomer since there is no hierarchy involved; there are simply six different organ-
izations, each with a different mix of reliability and performance characteristics.
