HARD DISKS, BUS TYPES AND OPTICAL MEDIA
WHAT IS A HARD DRIVE?
A hard disk commonly known as a HDD (hard disk drive or hard
drive or formerly known as "fixed disk"), is a non-volatile
storage device which stores digitally encoded data on rapidly
rotating platters with magnetic surfaces. Strictly speaking,
"drive" refers to an entire unit containing multiple platters, a
read/write head assembly, driver electronics, and motor while
"hard disk" (sometimes "platter") refers to the storage medium
inside the HDA(Hard Drive Assembly)itself.
Hard disks were originally developed for use with computers.
Nowadays, applications for hard disks have expanded beyond
computers to include digital video recorders, digital audio
players, personal digital assistants (PDAs), and digital
cameras. In 2005 the first mobile phones to include hard disks
were introduced by Samsung Group and Nokia. The need for
large-scale, reliable storage, independent of a particular
device, led to the introduction of software configuration such
as Redundant Array of Inexpensive Disks or RAID, hardware
configuration such as Network Attached Storage (NAS) devices,
and systems such as Storage Area Networks (SANs) for efficient
access to large volumes of data on a network.
The capacity of hard disks has grown dramatically over time. The
first commercial disk, the IBM RAMAC introduced in 1956, stored
5 million characters (about 5 megabytes) on fifty 24-inch
diameter platters. (Huge dimensions compared to today's hard
drive which size 3.5" for desktop hard drives, 2,5" for laptop
hard drives, 1,8" and 1" for ultra portable laptops, PDAs,
digital cameras and other mobile devices.)
With early personal computers in the 1980s, a disk with a 20MB
capacity was considered large. In the latter half of the 1990s,
hard disks with capacities of 1GB (1000MB) and greater became
available. As of 2007, the lowest-capacity desktop computer hard
disk still in production has a capacity of 20GB, while the
largest-capacity internal disks available now are 1TB (1000GB) on
5 platters. The exponential increases in disk space and data
access times for hard disks has enabled the commercial viability
of consumer products that require large storage capacities, such
as the Apple iPod digital music player, the TiVo personal video
recorder, and web-based email programs.
HOW IS A HARD DRIVE OR STORAGE MEDIA IS ACCESSED?
Hard disk drives are generally accessed using one of a number of
available bus types, including ATA (IDE, EIDE), Serial ATA (SATA),
SCSI, SAS, IEEE 1394, USB, and Fibre Channel. (More of this
later.) Most of the world's hard disks are now manufactured by
just a handful of large firms such as Seagate, Maxtor, Western
Digital, Samsung, and the former drive manufacturing division of
IBM, now sold to Hitachi. Fujitsu continues to make specialist
notebook and SCSI drives. Toshiba is a major manufacturer of
2.5-inch and 1.8-inch notebook drives. In 2003, hard disk
pioneer IBM sold the majority of its disk division to Hitachi,
who renamed it Hitachi Global Storage Technologies.
HARD DRIVE BUS TYPES:
Advanced Technology Attachment (ATA) is a standard interface for
connecting storage devices such as hard disks and CD-ROM drives
inside personal computers. Many terms and synonyms for ATA
exist, including abbreviations such as IDE, ATAPI, and UDMA. ATA
standards only allow cable lengths up to 18 inches (up to 450
mm) although cables up to 36 inches (900 mm) can be readily
purchased, so the technology normally appears as an internal
computer storage interface. It provides the most common and the
least expensive interface for this application. Although the
standard has always had the official name "ATA", other names
such as Integrated Drive Electronics (IDE) and Enhanced IDE (EIDE)
have also been adopted for marketing purposes. Although these
new names originated in branding convention and not as an
official standard, the terms EIDE or E-IDE often appear
interchangeably with IDE and ATA.
PATA is actually the same as ATA. With the introduction of
Serial ATA around 2003, this configuration was retroactively
renamed to Parallel ATA (P-ATA),
referring to the method in which data travels over wires in this
interface to distinguish it from Serial ATA (SATA).
In computer hardware, Serial ATA (also SATA or S-ATA) is a
computer bus primarily designed for transfer of data to and from
a hard disk. It is the successor to the legacy Advanced
Technology Attachment standard (ATA, also known as IDE). This
older technology is now known as Parallel ATA (PATA) to
distinguish it from Serial ATA.
First-generation Serial ATA interfaces, also known as SATA150,
run at 1.5 gigahertz. Because Serial ATA uses 8B/10B encoding at
the physical layer, this results in an actual data transfer rate
of 1.2 gigabits per second (Gbit/s), or 150 megabytes per
second. This transfer rate is only slightly higher than that
provided by the fastest Parallel ATA mode, UDMA-133. However,
further increasing PATA bandwidth is somewhat impractical, but
the relative simplicity of a serial link and the use of LVDS
have allowed Serial ATA to scale easily.
With the release of the NVIDIA nForce4 chipset in 2004, the
maximum throughput has been doubled to 300 MB/s (2.4 Gbit/s).
This increased data rate specification is very widely referred
to as “Serial ATA II” (“SATA II”); however, the official website
for the SATA standard states that this is a misnomer, SATA II
being the name of the organization formed to author the Serial
ATA specifications. Indeed, the increased data rate capability
was only one of many that were defined by the SATA II committee.
The Serial ATA standard organization has since changed names,
and is now “The Serial ATA International Organization”, or SATA-IO.
SATA-IO plans to further increase the maximum throughput of
Serial ATA to 600 MB/s around 2007. Physically, the cables used
are the most noticeable change from Parallel ATA. The Serial ATA
standard defines a data cable using seven conductors and 8 mm
wide wafer connectors on each end. SATA cables can be up to 1 m
(40 in.) long. PATA ribbon cables, in comparison, carry either
40- or 80-conductor wires and are limited to 45 cm (18 in.) in
length. Serial ATA drops the master/slave shared bus of PATA,
giving each device a dedicated cable and dedicated bandwidth.
Unlike early PATA connectors, SATA connectors are keyed — it is
not possible to install cable connectors upside down. The Serial
ATA standard also specifies a power connector sharply differing
from the four-pin Molex connector used by PATA drives and many
other computer components. Like the data cable, it is wafer
based, but its wider 15-pin shape should prevent confusion
between the two. The seemingly large number of pins are used to
supply three different voltages if necessary — 3.3 V, 5 V, and
12 V. The same physical connections are used on 3.5-in. and
2.5-in. (notebook) hard disks.
Features allowed for by SATA but not by PATA include
hot-swapping and native command queueing. To ease their
transition to Serial ATA, many manufacturers have produced
drives which use controllers largely identical to those on their
PATA drives and include a bridge chip on the logic board.
Bridged drives have a SATA connector, may include either or both
kinds of power connectors, and generally perform identically to
native drives. They may, however, lack support for some SATA-specific
features. As of
2004, all major hard drive manufacturers produce either bridged
or native SATA drives.
SATA drives may be plugged into Serial Attached SCSI (SAS)
controllers and communicate on the same physical cable as native
SAS disks. SAS disks however may not be plugged into a SATA
Initially SATA was designed as an internal or inside-the-box
interface technology, bringing improved performance and new
features to internal PC or consumer storage. Creative designers
quickly realized the innovative interface could reliably be
expanded outside the PC, bringing the same performance and
features to external storage needs instead of relying on USB or
FireWire (IEEE 1394) interfaces. Called external SATA or eSATA,
customers can now utilize shielded cable lengths up to two
meters outside the PC to take advantage of the benefits the SATA
interface brings to storage. SATA is now out of the box as an
external standard, with specifically defined cables, connectors,
and signal requirements released as new standards in mid-2004.
eSATA provides more performance than existing solutions and is
Key benefits of eSATA:
-Up to six times faster than existing external storage
solutions: USB 2.0 and FireWire.
-Robust and user friendly external connection
-High performance, cost effective expansion storage
-Up to 2-m shielded cables and connectors
Applications of eSATA include External Direct Attached Storage
for notebooks, desktop, consumer electronics and entry
servers.Many existing external hard drives use USB and/or
FireWire. These interfaces are not nearly as fast as SATA when
compared using peak values, and can compromise drive
USB and IEEE 1394 external drives are ATA drives with a bridge
chip that translates from the ATA protocol to USB or IEEE 1394
protocol used for the connection. These interfaces require
encapsulation or conversion of the transmit data and then
de-capsulation after the data is received. This protocol
overhead reduces the efficiency of these host buses, increases
the host CPU utilization or requires a special chip to off-load
the host. The results of eSATA are dramatic and with no protocol
overhead issues as with USB or IEEE 1394. The eSATA storage bus
delivers as much as 37 times more performance. This ability is
perfect for using an array of drives with performance striping
behind the eSATA host port.
The typical cable length is two meters (six feet); long enough
to reach from a floor-mounted PC to a drive placed on the
desktop. The compliance is defined in the SATA II: Electrical
Specification, as the Gen1m and Gen2m specifications for 1.5 Gb/s
and 3.0 Gb/s respectively.
Currently (November 2007), most PC motherboards do not have an
eSATA connector. eSATA is readily enabled, however, through the
addition of an eSATA host bus adapter (HBA) or bracket connector
for desktop systems or with a Cardbus or ExpressCard for
notebooks. New motherboards introduced in 2005 will start to
incorporate e-SATA connectors directly, making the addition of
external storage an easy option.
SCSI stands for "Small Computer System Interface", and is a
standard interface and command set for transferring data between
devices on a computer bus. SCSI is pronounced "scuzzy" when
spoken aloud, while occasional attempts to promulgate the more
flattering pronunciation "sexy" have never succeeded. SCSI is
most commonly used for hard disks and tape storage devices, but
also connects a wide range of other devices, including scanners,
CD-ROM drives, CD recorders, and DVD drives. In fact, the entire
standard promotes device independence, which means that
theoretically anything can be made SCSI (SCSI printers actually
exist). In the past, SCSI was very popular on all kinds of
computers. SCSI remains popular on high-performance
workstations, servers, and high-end peripherals. Desktop
computers and notebooks more typically use the ATA/IDE
interfaces for hard disks and USB (which uses a subset of the
SCSI command set for hard disks and floppy drives) for other
As of 2003, there have only been three SCSI standards: SCSI-1,
SCSI-2, and SCSI-3. All SCSI standards have been modular,
defining various capabilities which manufacturers can include or
not. Individual vendors and SCSITA have given names to specific
combinations of capabilities. For example, the term "Ultra SCSI"
is not defined anywhere in the standard, but is used to refer to
SCSI implementations that signal at twice the rate of "Fast
SCSI." Such a signalling rate is not compliant with SCSI-2 but
is one option allowed by SCSI-3. Similarly, no version of the
standard requires low-voltage-differential (LVD) signalling, but
products called Ultra-2 SCSI include this capability. This
terminology is helpful to consumers, because "Ultra-2 SCSI"
device has a better-defined set of capabilities than simply
identifying it as "SCSI-3."
Starting with SCSI-3, the SCSI standard has been maintained as a
loose collection of standards, each defining a certain piece of
the SCSI architecture, and bound together by the SCSI
Architectural Model. This change divorces SCSI's various
interfaces from the command set, allowing devices that support
SCSI commands to use any interface (including ones not otherwise
specified by T10), and also allowing the interfaces that are
defined by T10 to develop on their own terms. This change is
also why there is no "SCSI-4".
No version of the standard has ever specified what kind of
connector should be used. The connectors used by vendors have
tended to evolve over time. Although SCSI-1 devices typically
used bulky Blue Ribbon ("Centronics") connectors, and SCSI-2
devices typically "Mini-D" connectors, it is not correct to
refer to these as "SCSI-1" and "SCSI-2" connectors.
The original standard that was derived from SASI and formally
adopted in 1986 by ANSI. SCSI-1 features an 8-bit bus (with
parity), running asynchronously at 3.5 MB/s or 5 MB/s in
synchronous mode, and a maximum bus cable length of 6 meters
(just under 20 feet -- compare that to the 18 inch (0.45 meter)
limit of the ATA interface). A variation on the original
standard included a high-voltage differential (HVD)
implementation whose maximum cable length was many times that of
the single-ended versions.
This standard was introduced in 1989 and gave rise to the Fast
SCSI and Wide SCSI variants. Fast SCSI doubled the maximum
transfer rate to 10 MB/s and Wide SCSI doubled the bus width to
16 bits on top of that (to reach 20 MB/s). However, these
improvements came at the minor cost of a reduced maximum cable
length to 3 meters. SCSI-2 also specified a 32-bit version of
Wide SCSI, which used 2 16-bit cables per bus; this was largely
ignored by SCSI device makers because it was expensive and
unnecessary, and was officially retired in SCSI-3.
Before Adaptec and later SCSITA codified the terminology, the
first parallel SCSI devices that exceeded the SCSI-2
capabilities were simply designated SCSI-3. These devices, also
known as Ultra SCSI and fast-20 SCSI, were introduced in 1992.
The bus speed doubled again to 20 MB/s for narrow (8 bit)
systems and 40 MB/s for wide. The maximum cable length stayed at
3 meters but ultra SCSI developed an undeserved reputation for
extreme sensitivity to cable length and condition (faulty
cables, connectors or terminators were often to blame for
This standard was introduced c. 1997 and featured a low voltage
differential (LVD) bus. For this reason ultra-2 is sometimes
referred to as LVD SCSI. Using LVD technology, it became
possible to allow a maximum bus cable length of 12 meters
(almost 40 feet!), with much greater noise immunity. At the same
time, the data transfer rate was increased to 80 MB/s. Ultra-2
SCSI actually had a relatively short lifespan, as it was soon
superseded by ultra-3 (ultra-160) SCSI.
Also known as Ultra-160 SCSI and introduced toward the end of
1999, this version was basically an improvement on the ultra-2
standard, in that the transfer rate was doubled once more to 160
MB/s by the use of double transition clocking. Ultra-160 SCSI
offered new features like cyclic redundancy check (CRC), an
error correcting process, and domain validation.
This is the ultra-160 standard with the data transfer rate
doubled to 320 MB/s. Nearly all new SCSI hard drives being
manufactured at the time of this writing (October 2003) are
actually ultra-320 devices.
Ultra-640 (otherwise known as Fast-320) was promulgated as a
standard (INCITS 367-2003 or SPI-5) in early 2003. Ultra-640
doubles the interface speed yet again, this time to 640 MB/s.
Ultra640 pushes the limits of LVD signaling; the speed limits
cable lengths drastically, making it impractical for more than
one or two devices. Because of this, most manufacturers have
skipped over Ultra640 and are developing for Serial Attached
iSCSI preserves the basic SCSI paradigm, especially the command
set, almost unchanged. iSCSI advocates project the iSCSI
standard, an embedding of SCSI-3 over TCP/IP, as displacing
Fibre Channel in the long run, arguing that Ethernet data rates
are currently increasing faster than data rates for Fibre
Channel and similar disk-attachment technologies. iSCSI could
thus address both the low-end and high-end markets with a single
Three recent versions of SCSI SSA, FC-AL and Serial Attached
SCSI break from the traditional parallel SCSI standards and
perform data transfer via serial communications.
SCSI COMPATIBILITY ISSUES:
Note: Ultra-2, ultra-160 and ultra-320 devices may be freely
mixed on the LVD bus with no compromise in performance, as the
host adapter will negotiate the operating speed and bus
management requirements for each device. Single-ended devices
should not be attached to the LVD bus, as doing so will force
all devices to run at the slower single-ended speed. Support for
single-ended interfaces has been deprecated in the SPI-5
standard (which describes Ultra-640), so future devices may not
be electrically backward compatible.
SCSI Caution note:
Modern Single Connector Attachment (SCA) devices may be
connected to older controller/drive chains by using SCA
adapters. Although these adapters often have auxiliary power
connectors, use caution: it is possible to quickly destroy the
drive by connecting external power. Always try the drive without
auxiliary power first. SCSI devices are generally
backward-compatible, i.e., it is possible to connect an ultra-3
SCSI hard disk to an ultra-2 SCSI controller and use it (though
with reduced speed and feature set). Each SCSI device (including
the computer's host adapter) must be configured to have a unique
SCSI ID on the bus. Also, the SCSI bus must be terminated with a
terminator. Both active and passive terminators are in common
use, with the active type much preferred (and required on LVD
buses). Improper termination is a common problem with SCSI
installations. It is possible to convert a wide bus to a narrow
one, with wide devices closer to the adapter. To do this
properly requires a cable which terminates the wide part of the
bus. This is sometimes referred to as a cable with high-9
termination. Specific commands allow the host to determine the
active width of the bus. This arrangement is discouraged.
Serial Attached SCSI (SAS) is another computer bus technology
primarily designed for transfer of data to and from devices like
hard drives, CD-ROM drives and so on. SAS is a serial
communication protocol for Direct Attached Storage (DAS)
devices. It is designed for the corporate and enterprise market
as a replacement for parallel SCSI, allowing for much higher
speed data transfers than previously available, and is
backwards-compatible with SATA drives. Though SAS uses serial
communication instead of the parallel method found in
traditional SCSI devices, it still uses SCSI commands for
interacting with SAS End devices.
FB or Fiber Channel:
Fibre Channel is a gigabit-speed network technology primarily
used for network storage. It is a very high speed bus that is
used for NAS(Network Attached Storage). Fibre Channel is
standardized in the T11 Technical Committee of the InterNational
Committee for Information Technology Standards (INCITS), an
American National Standards Institute–accredited standards
committee. It started for use primarily in the supercomputer
field, but has become the standard connection type for storage
area networks in enterprise storage. Despite its name, Fibre
Channel signaling can run on both twisted-pair copper wire and
fiber optic cables. Fibre Channel Protocol (FCP) is the
interface protocol of SCSI on the Fibre Channel.
OPTICAL DISC TYPES:
Read-only media (ROM):
DVD-ROM: These are pressed similarly to CDs. The reflective
surface is silver or gold colored. They can be
double-sided/single-layered, or double-sided/double-layered. As
of 2004, new double-sided discs have become increasingly rare.
DVD-D: a new self-destructing disposable DVD format. Like the EZ-D,
it is sold in an airtight package, and begins to destroy itself
by oxidation after several hours.
DVD Plus: combines both DVD and CD technologies by providing the
CD layer and a DVD layer. Not to be confused with the DVD+
DVD-R for Authoring: a special-purpose DVD-R used to record DVD
masters, which can then be duplicated to pressed DVDs by a
duplication plant. They require a special DVD-R recorder, and
are not often used nowadays since many duplicators can now
accept ordinary DVD-R masters.
DVD-R (strictly DVD-R for General): can record up to 4.5 GB in a
similar fashion to a CD-R disc. Once recorded and finalized it
can be played by most DVD-ROM players.
DVD-RW: can record up to 4.7 GB in a similar fashion to a CD-RW
DVD-R DL: a derivate of DVD-R that uses double-layer recordable
discs to store up to 8.5 GB of data.
DVD-RAM (current specification is version 2.1): requires a
special unit to play 4.7GB or 9.4GB recorded discs (DVD-RAM disc
are typically housed in a cartridge). 2.6GB discs can be removed
from their caddy and used in DVD-ROM drives. i Top capacity is
DVD+R: can record up to 4.7 GB single-layered/single-sided DVD+R
disc, at up to 16x speed. Like DVD-R you can record only once.
DVD+RW: can record up to 4.7 GB at up to 16x speed. Since it is
rewritable it can be overwritten several times. It does not need
special "pre-pits" or finalization to be played in a DVD player.
DVD+R DL: a derivate of DVD+R that uses dual-layer recordable
discs to store up to 8.5 GB of data.
Dual Layer recording allows DVD-R and DVD+R discs to store
significantly more data, up to 8.5 Gigabytes per disc, compared
with 4.7 Gigabytes for single-layer discs. DVD-R DL (dual layer
was developed for the DVD Forum by Pioneer Corporation, DVD+R DL
(double layer — see figure) was developed for the DVD+RW
Alliance by Philips and Mitsubishi Kagaku Media (MKM). A Dual
Layer disc differs from its usual DVD counterpart by employing a
second physical layer within the disc itself. The drive with
Dual Layer capability accesses the second layer by shining the
laser through the first semi-transparent layer. The layer change
mechanism in some DVD players can show a noticeable pause, as
long as two seconds by some accounts. More than a few viewers
have worried that their dual layer discs were damaged or
defective. DVD recordable discs supporting this technology are
backward compatible with some existing DVD players and DVD-ROM
drives. Many current DVD recorders support dual-layer
technology, and the price point is comparable to that of
single-layer drives, though the blank media remains
significantly more expensive.
HD DVD: High Density DVD, or High-Definition DVD is a
high-density optical disc format designed for the storage of
data and high-definition video. HD DVD has a single-layer
capacity of 15 GB and a dual-layer capacity of 30 GB. There is
also a double-sided hybrid format which contains standard
DVD-Video format video on one side, playable in regular DVD
players, and HD DVD video on the other side for playback in high
definition on HD DVD players. JVC has developed a similar hybrid
disc for the Blu-ray format. These hybrid discs make retail
marketing and shelf space management easier. This also removes
some confusion from DVD buyers since they can now buy a disc
compatible with any DVD/HD DVD player in their house. The HD DVD
format also can be applied to current red laser DVDs in 5, 9, 15
and 18 GB capacities which offers a lower-cost option for
Blu-ray: A Blu-ray Disc is a high-density optical disc format
for the storage of digital media, including high-definition
video. The name Blu-ray Disc is derived from the blue-violet
laser used to read and write this type of disc. Because of this
shorter wavelength (405 nm), substantially more data can be
stored on a Blu-ray Disc than
on the common DVD format, which uses a red, 650 nm laser. Blu-ray
Disc can store 25 GB on each layer, as opposed to a DVD's 4.7
GB. Several manufacturers have released single layer and dual
layer (50 GB) recordable BDs and rewritable discs. Blu-ray Disc
is similar to PDD, another optical disc format developed by Sony
(which has been available since 2004) but offering higher data
transfer speeds. PDD was not intended for home video use and was
aimed at business data archiving and backup. Blu-ray Disc is
currently in a format war with rival format HD DVD. About 9
hours of high-definition (HD) video can be stored on a 50 GB
disc. About 23 hours of standard-definition (SD) video can be
stored on a 50 GB disc.
On average, a single-layer disc can hold a High Definition
feature of 135 minutes using MPEG-2, with additional room for 2
hours of bonus material in standard definition quality. A
double-layer disc will extend this number up to 3 hours in HD
quality and 9 hours of SD bonus material.
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