Hard Drives 101
Hugh Stroupe
December 12, 2013
What is a Hard Drive?
• A hard disk drive (HDD)is a data storage device used for
storing and retrieving digital information using rapidly
rotating disks (platters) coated with magnetic material
• An HDD retains its data even when powered off. Data is
read in a random-access manner, meaning individual
blocks of data can be stored or retrieved in any order
rather than sequentially.
• An HDD consists of one or more rigid ("hard") rapidly
rotating disks (platters) with magnetic heads arranged
on a moving actuator arm to read and write data to the
surfaces.
History
• The first IBM drive, the 350 RAMAC
developed in 1956, was approximately
the size of two refrigerators and stored 5
million 6-bit characters (the equivalent of
3.75 million 8-bit bytes or 3.75 MB or
megabytes) on a stack of Fifty 24 inch
disks.
• In 1961 IBM
introduced the model
1311 disk drive, which
was about the size of
a washing machine
and stored two million
characters on a
removable disk pack.
History
• Some high performance HDDs were manufactured with one
head per track, e.g., IBM 2305 so that no time was lost physically
moving the heads to a track. Known as Fixed-Head or Head-PerTrack disk drives they were very expensive and are no longer in
production.
• In 1973, IBM introduced a new type of HDD codenamed
"Winchester". Its primary distinguishing feature was that the disk
heads were not withdrawn completely from the stack of disk
platters when the drive was powered down. Instead, the heads
were allowed to "land" on a special area of the disk surface upon
spin-down, "taking off" again when the disk was later powered
on
Let’s Look at the Internals
Make up of the Platter
Technology
• A HDD records data by magnetizing a thin film of ferromagnetic
material on a disk. Sequential changes in the direction of
magnetization represent binary data bits. The data is read from
the disk by detecting the transitions in magnetization. User data
is encoded using an encoding scheme, such as run-length limited
encoding, which determines how the data is represented by the
magnetic transitions.
Technology
• Due to the extremely close spacing between the
heads and the disk surface, HDDs are vulnerable
to being damaged by a head crash.
• The HDD's spindle system relies on air density
inside the disk enclosure to support the heads at
their proper flying height while the disk rotates.
HDDs require a certain range of air densities in
order to operate properly.
• If the air density is too low, then there is not
enough lift for the flying head, and there is a risk
of head crashes and data loss.
Size Comparisons
Technology
Perpendicular recording can deliver more than three times
the storage density of traditional longitudinal recording.
Hard disk technology with longitudinal recording has an
estimated limit of 100 to 200 gigabit per square inch due to
the superparamagnetic effect. Perpendicular recording is
predicted to allow information densities of up to around 1
Tbit/sq. inch (1000 Gbit/sq. inch).
New Technology
• According to Seagate, its latest 1TB platter 3.5" drives
have shrunk read/write heads as small as they can
physically go. Similarly, tracks on those platters are
placed as close together as physically possible.
• Seagate's solution is something it calls Shingled
Magneting Recording (SMR). The process is pretty
simple. Track size is traditionally defined by the size of
the write heads, as they are larger than the read heads.
The track width is larger than necessary from the
perspective of reading data back in order to decrease the
chances of reading data from adjacent tracks. Seagate's
SMR exploits this reality.
New Technology
SMR shrinks the guard space between tracks and allows tracks to
overlap one another, like roofing shingles. Although data is written
to the entire width of the track, a smaller/trimmed portion of the
track (the width of the read head) is all that the drive cares about.
By allowing tracks to overlap, areal density can continue to scale
without further shrinking the size of the heads.
Hard Drive Interfaces
• Modified Frequency Modulation, commonly MFM, is a run-length limited
(RLL) coding scheme used to encode the actual data-bits on most floppy
disks. It was first introduced in disk drives with the IBM 3330 hard disk drive
in 1970. Floppy disk drive hardware examples include Amiga, most CP/M
machines as well as IBM PC compatibles.
In the ST-506 interface, the drive
was connected to a controller card
with two cables; a third cable
provided power. The drives were
"dumb", so-called because the
control card translated requests for
a particular track and sector from
the host system into a sequence of
head positioning commands, then
read the signal from the drive head
and recovered the data from it.
Enhanced Small Disk Interface
• Enhanced Small Disk Interface (ESDI) was a disk interface designed by Maxtor
Corporation in the early 1980s to be a follow-on to the ST-506 interface. ESDI
improved on ST-506 by moving certain parts that were traditionally kept on the
controller into the drives themselves, and also generalizing the control bus such
that more kinds of devices (such as removable disks and tape drives) could be
connected. ESDI used the same cabling as ST-506, and thus could easily be
retrofitted to ST-506 applications.
SCSI
• Since its standardization in 1986, SCSI has been commonly used in the
Amiga, Apple Macintosh and Sun Microsystems computer lines and PC
server systems.
• SCSI is available in a variety of interfaces. The first, still very common, was
parallel SCSI (now also called SPI), which uses a parallel bus design.
• Serial Storage Architecture (SSA), SCSI-over-Fibre Channel Protocol (FCP),
Serial Attached SCSI (SAS), Automation/Drive Interface − Transport Protocol
(ADT), and USB Attached SCSI (UAS) – break from the traditional parallel
SCSI standards and perform data transfer via serial communications.
• iSCSI preserves the basic SCSI paradigm, especially the command set,
almost unchanged, through embedding of SCSI-3 over TCP/IP.
SCSI Connectors
Parallel ATA
• Parallel ATA (PATA), originally AT Attachment, is
an interface standard for the connection of
storage devices such as hard disks, floppy drives,
and optical disc drives in computers. It uses the
underlying AT Attachment (ATA) and AT
Attachment Packet Interface (ATAPI) standards.
Fibre Channel
• Fibre Channel (FC) is a successor to parallel SCSI interface on
enterprise market. It is a serial protocol. In disk drives usually the
Fibre Channel Arbitrated Loop (FC-AL) connection topology is
used. FC has much broader usage than mere disk interfaces, and
it is the cornerstone of storage area networks (SANs). Recently
other protocols for this field, like iSCSI and ATA over Ethernet
have been developed as well. Confusingly, drives usually use
copper twisted-pair cables for Fibre Channel, not fibre optics. The
latter are traditionally reserved for larger devices, such as servers
or disk array controllers.
SATA
• Serial ATA (SATA). The SATA data cable has one data pair for
differential transmission of data to the device, and one pair for
differential receiving from the device, just like EIA-422. That
requires that data be transmitted serially. A similar differential
signaling system is used in RS485, LocalTalk, USB, Firewire, and
differential SCSI.
• SATA-1 Revision interface provides a 150 Mbps connection.
• SATA-2Revision interface provides a 300 Mbps connection.
• SATA-3 Revision interface provides a 600 Mbps connection. Also
can be externally connected as an eSATA.
SAS
• Serial Attached SCSI (SAS). The SAS is a new generation
serial communication protocol for devices designed to
allow for much higher speed data transfers and is
compatible with SATA. SAS uses a mechanically identical
data and power connector to standard 3.5-inch
SATA1/SATA2 HDDs, and many server-oriented SAS
RAID controllers are also capable of addressing SATA
hard drives. SAS uses serial communication instead of
the parallel method found in traditional SCSI devices but
still uses SCSI commands.
Any Questions?