Disk storage is a general category of storage mechanisms where data are
recorded by various electronic, magnetic, optical, or mechanical changes
to a surface layer of one or more rotating disks. A disk drive is a device
implementing such a storage mechanism. Notable types are the hard disk drive
containing a non-removable disk, the floppy disk drive and its removable
floppy disk, and various optical disc drives and associated optical disc
media. (The spelling disk and disc are used
interchangeably except where trademarks preclude one usage, e.g. the Compact
Disc logo. The choice of a particular form is frequently historical, as in
IBM's usage of the disk form beginning in 1956 with the "IBM 350 disk storage
unit"). Background
Audio information was originally recorded by analog methods. Similarly
the first video disc used an analog recording method. In the music industry,
analog recording has been mostly replaced by digital optical technology
where the data are recorded in a digital format with optical information.
The first commercial digital disk storage device was the IBM 350 which
shipped in 1956 as a part of the IBM 305 RAMAC computing system. The
random-access, low-density storage of disks was developed to complement the
already used sequential-access, high-density storage provided by tape
drives using magnetic tape. Vigorous innovation in disk storage technology,
coupled with less vigorous innovation in tape storage, has reduced the difference
in acquisition cost per terabyte between disk storage and tape storage; however,
the total cost of ownership of data on disk including power and management
remains larger than that of tape. Disk storage is now used in both
computer storage and consumer electronic storage, e.g., audio CDs and video
discs. Access methods
Digital disk drives are block storage devices. Each disk is divided into
logical blocks. Blocks are addressed using their logical block addresses.
Read from or writing to disk happens at the granularity of blocks.
Originally the disk capacity was quite low and has been improved in one of
several ways. Improvements in mechanical design and manufacture allowed smaller
and more precise heads, meaning that more tracks could be stored on each of
the disks. Advancements in data compression methods permitted more
information to be stored in each of the individual sectors.
The drive stores data onto cylinders, heads, and sectors. The sectors unit is
the smallest size of data to be stored in a hard disk drive and each file will
have many sectors units assigned to it. The smallest entity in a CD is called a
frame, which consists of 33 bytes and contains six complete 16-bit stereo
samples. The other nine bytes consist of eight CIRC error-correction bytes and
one subcode byte used for control and display.
The information is sent from the computer processor to the BIOS into a
chip controlling the data transfer. This is then sent out to the hard drive via a
multi-wire connector. Once the data are received onto the circuit board of the
drive, they are translated and compressed into a format that the
individual drive can use to store onto the disk itself. The data are then
passed to a chip on the circuit board that controls the access to the drive.
The drive is divided into sectors of data stored onto one of the sides of one
of the internal disks. An HDD with two disks internally will typically store
data on all four surfaces. The hardware on the drive tells the
actuator arm where it is to go for the relevant track and the compressed
information is then sent down to the head which changes the physical
properties, optically or magnetically for example, of each byte on the drive,
thus storing the information. A file is not stored in a linear manner, rather,
it is held in the best way for quickest retrieval.
Rotation speed and track layout Mechanically there are two different
motions occurring inside the drive. One is the rotation of the disks inside the
device. The other is the side-to-side motion of the head across the disk as it
moves between tracks. There are two types of disk rotation
methods: constant linear velocity varies the
rotational speed of the optical disc depending upon the position of the head,
and constant angular velocity spins the
media at one constant speed regardless of where the head is positioned.
Track positioning also follows two different methods across disk storage
devices. Storage devices focused on holding computer data, e.g., HDDs, FDDs,
Iomega zip drives, use concentric tracks to store data. During a sequential read
or write operation, after the drive accesses all the sectors in a track it
repositions the head(s) to the next track. This will cause a momentary delay
in the flow of data between the device and the computer. In contrast, optical
audio and video discs use a single spiral track that starts at the inner
most point on the disc and flows continuously to the outer edge. When
reading or writing data there is no need to stop the flow of data to switch
tracks. This is similar to vinyl records except vinyl records started at the
outer edge and spiraled in toward the center.
Interfaces The disk drive interface is the
mechanism/protocol of communication between the rest of the system and the
disk drive itself. Storage devices intended for desktop and mobile
computers typically use ATA and SATA interfaces. Enterprise systems and
high-end storage devices will typically use SCSI, SAS, and FC interfaces in
addition to some use of SATA. Basic terminology
Disk - Generally refers to magnetic media and devices.
Disc - Required by trademarks for certain optical media and devices.
Platter – An individual recording disk. A hard disk drive contains a set of
platters. Developments in optical technology have led to multiple
recording layers on DVDs. Spindle – the spinning axle on which the
platters are mounted. Rotation – Platters rotate; two
techniques are common: Constant angular velocity keeps the disk
spinning at a fixed rate, measured in revolutions per minute. This means the
heads cover more distance per unit of time on the outer tracks than on the
inner tracks. This method is typical with computer hard drives.
Constant linear velocity keeps the distance covered by the heads per unit
time fixed. Thus the disk has to slow down as the arm moves to the outer
tracks. This method is typical for CD drives.
Track – The circle of recorded data on a single recording surface of a platter.
Sector – A segment of a track Low level formatting – Establishing the
tracks and sectors. Head – The device that reads and writes
the information—magnetic or optical—on the disk surface.
Arm – The mechanical assembly that supports the head as it moves in and
out. Seek time – Time needed to move the head
to a new position. Rotational latency – Average time, once
the arm is on the right track, before a head is over a desired sector.
Data transfer rate - The rate at which user data bits are transferred from or
to the medium. Technically, this would more accurately be entitled the "gross"
data transfer rate. See also
Disk drive performance characteristics RAID
USB flash drive Disk array
References