|Red Hat Linux 8.0: The Official Red Hat Linux System Administration Primer|
Chapter 5. Managing Storage
If there is one thing that takes up the majority of a system
administrator's day, it would have to be storage management. It seems
that disks are always running out of free space, becoming overloaded with
too much I/O activity, or failing unexpectedly. Therefore, it is vital to
have a solid working knowledge of disk storage in order to be a successful
To start, let us see how disk devices are named under Red Hat Linux.
Device Naming Conventions
As with all Linux-like operating systems, Red Hat Linux uses device files to
access all hardware (including disk drives). However, most of these
operating systems use slightly different naming conventions to identify
any attached storage devices. Here is how these device files are named
under Red Hat Linux.
Under Red Hat Linux, the device files for disk drives appear in the
/dev/ directory. The format for each file name
depends on several aspects of the actual hardware, and how it has been
configured. Here are these aspects:
We will now explore each of these aspects in more detail.
The first two letters of the device file name refer to the
specific type of device. For disk drives, there are two device
types that are most common:
SCSI and IDE are two different industry standards that define
methods for attaching devices to a computer system. The following
sections briefly describe the characteristics of these two different
Formally known as the Small Computer System Interface, the
SCSI standard defines a bus along which multiple devices may be
connected. A SCSI bus is a parallel bus, meaning that there is a
single set of parallel wires that go from device to device.
Because these wires are shared by all devices, it is necessary to
have a way of uniquely identifying and communicating with an
individual device. This is done by assigning each device on a
SCSI bus a unique numeric address or SCSI
The number of devices that are supported on a SCSI bus
depends on the width of the bus. Regular SCSI supports 8
uniquely-addressed devices, while wide
SCSI supports 16. In either case, you must make
sure that all devices are set to use a unique SCSI ID. Two
devices sharing a single ID will cause problems that could lead
to data corruption before it can be resolved.
One other thing to keep in mind is that
every device on the bus uses an ID.
This includes the SCSI controller. Quite
often system administrators forget this, and unwittingly set a
device to use the same SCSI ID as the bus's controller.
This also means that, in practice, only 7 (or 15, for wide SCSI)
devices may be present on a single bus, as each bus must
include its own controller.
As technological advances have taken place, the SCSI standard
has been amended to support them. For instance, the number of
wires that carried data along the bus went from 8 (known simply as
SCSI) to 16 (known as wide SCSI). As it became possible to build
faster hardware, and the speed at which data could be transferred
increased, the bus speed increased from 5MB/sec to as much as
160MB/sec. The different bus speeds are identified by adding
words like "fast", "ultra", and "ultra-3" to the name of the SCSI
environment being supported.
Because of SCSI's bus-oriented architecture, it is necessary
to properly terminate both ends of the bus.
Termination is accomplished by placing a load of the correct
impedance on each conductor comprising the SCSI bus. Termination
is an electrical requirement; without it, the various signals
present on the bus would be reflected off the ends of the bus,
garbling all communication.
Many (but not all) SCSI devices come with internal terminators
that can be enabled or disabled using jumpers or switches.
External terminators are also available.
IDE stands for Integrated Drive Electronics. A later version
of the standard — known as EIDE (the
extra "E" standing for "Enhanced") has been almost universally
adopted in place of IDE. However, in normal conversation both are
known as IDE. Like SCSI, IDE is an interface standard used to
connect devices to computer systems. Like SCSI, IDE implements a
However, there are differences between the two standards. The
most important is that IDE cannot match SCSI's expandability, with
each IDE bus supporting only two devices (known as a
master and a
Following the two-letter device type (sd,
for example) are one or two letters denoting the specific unit. The
unit designator starts with "a" for the first unit, "b" for the
second, and so on. Therefore, the first hard drive on your system
may appear as hda or
SCSI's ability to address large numbers of devices
necessitated the addition of a second unit character to support
systems with more than 26 SCSI devices attached. Therefore, the
first 26 SCSI hard drives would be named sda
through sdz, with the 27th named
sdaa, the 28th named
sdab, and so on through to
The final part of the device file name is a number representing a
specific partition on the device, starting with "1". The number may
be one or two digits in length, depending on the number of
partitions written to the specific device.
Once the format for device file names is known, it is easy to
understand what each refers to:
/dev/hda1 — The first partition on the
first IDE drive
/dev/sdb12 — The twelfth partition on
the second SCSI drive
/dev/sdad4 — The fourth partition on
the thirtieth SCSI drive
There are instances where it is necessary to access the entire
device, and not just a specific partition. This is normally done
when the device is not partitioned, or does not support standard
partitions (such as a CD-ROM drive). In these cases, the partition
number is omitted:
However, most disk drives use partitions; the next section will
take a closer look at this method of storage division.
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