Mass Storage – Disks
Mass Storage – Disks
We are concerned here with storing large amounts of data (and programs) permanently, hence another
name for these devices islong term storage devices. In relation to RAM, or primary memory, they are
also sometimes called secondary memory.
Many of the devices discussed here can also be attached externally, which is an important option for
Magnetic Media
Magnetic media for computers are digital. They save bits of data by altering the polarity of a section on
the surface of the medium. This change lasts indefinitely (like magnetizing a pin), so it is considered
permanent. Magnetic media includes hard disk drives, tapes, and a number of legacy devices. They also
work similarly to the magnetic strip on the back of a credit or ATM card.
Hard Disk Drives
A hard disk drive (HDD) is a non-volatile, random access
device for digital data. It features rotating rigid platters on a
motor-driven spindle within a protective enclosure. Data is
magnetically read from, and written to, the platter by
read/write heads that float on a film of air above the platters.
Introduced by IBM in 1956, hard disk drives have fallen in
cost and physical size over the years while dramatically
increasing in capacity. Hard disk drives have been the
dominant device for secondary storage of data in general
purpose computers since the early 1960s. They have
maintained this position because advances in their areal
recording density have kept pace with the requirements for
secondary storage. Today's HDDs operate on high-speed serial
interfaces; i.e., serial ATA (SATA) or serial attached SCSI
Hard Disk Drive with labeled parts
A hard-disk drive is normally located inside the computer case.
Additional drives may be added either inside the case or
attached externally via a port.
Hard disks come in various speeds. The key specifications to
be considered are averageaccess time and data transfer rate.
The access time includes both the seek time (time to move the
arm & head to the correct location) and latency (time for the
desired location to rotate under the read/write head). For a
particular size (0.85"-8") of disk platter, the faster the disk
Multi-platter Hard Disk Drive
spins (3,000-15,000 RPM), the lower the access (latency) time
will be and the faster the data transfer rate.
As shown in the image at right:
A) platters,
B) actuator arm,
C) read/write head,
D) segment – an arc on the track,
E) track,
F) sector.
The data organization is essentially the same as the floppy disk
shown below, except that all the tracks that line up one above
the other are call a cylinder. A cluster, not shown here, is two
or more contiguous segments that hold data for a single file. (A
file might have multiple clusters.)
Multiple Drives & RAID
Most PCs have a single hard disk installed. A system can be
purchased with a second hard disk or the user may install the
second as an upgrade. With two disks, one is designated the
master and the other the slave, although neither is in control of
the other. Both appear as separate hard disks to the system.
(Note that this is different from disk partitions which appear as
separate disks to the system, but are actually a single physical
hard drive.) RAID disks combine disks differently. In this
instance, multiple physical disks are present, but they appear to
the system to be a single device.
RAID is the abbreviated term for a Redundant Array of
Independent Drives. RAID is now used as an umbrella term
for computer data storage schemes that can divide and
replicate data among multiple physical disk drives. The
physical disks are said to be in a RAID array, which is
addressed by the operating system as one single disk. The
different schemes or architectures are named by the word
RAID followed by a number (e.g., RAID 0, RAID 1). Each
scheme provides a different balance between two key goals:
increase data reliability and increase input/output performance.
RAID 0 uses the concept of striping to evenly split data
between 2 or more drives. This allows the computer to access
Screenshot showing a single physical
disk with multiple partitions
Raid 0
By en:User:C burnett [GFDL or CC-BY-SA-3.0], via
multiple drives simultaneously to increase data transfer rate
and response time. The disadvantages of this setup is
reliability. If one of the drives fail, all your data is lost. This is
because fragments of each file are stored on different disks.
With two drives, you are "twice" as likely to experience a
RAID 1, is the setup in which two (or more) disks are used to
create a copy, assuring no data loss if one or the other drive
fails. Each disk has a full copy. Performance wise, there is no
Wikimedia Commons
and By Paul Mullins: constructed
RAID 2 uses striping and parity bits to correct errors or
recover all data when a single disk fails. Successively higher
levels use different schemes to increase performance and/or
recover from more failures.
Tape Drives
A tape drive is a data storage device that reads and writes data
on a magnetic tape. It is typically used for offline, archival
data storage. Tape media generally has a favorable unit cost
and long archival stability.
A tape drive provides sequential access storage, unlike a disk
drive, which provides random access storage. A disk drive can
move its read/write head(s) to any random part of the disk in a
DDS tape drive with 4 types of
very short amount of time, but a tape drive must spend a
data storage media on top,
considerable amount of time winding tape between reels to
read any one particular piece of data. As a result, tape drives from right hand side: 3.5" floppy disk
have very slow average seek times. Despite the slow seek time, (1.44 MB), 310m QIC DC tape (250
MB, ~6 mm wide, 1020 ft length),
tape drives can stream data to and from tape very quickly. For
Data8 tape (5 GB, 8 mm, 367
example, popular Linear Tape-Open drives can reach, as of
DDS-4 tape (20 GB, 4 mm,
2010, continuous data transfer rates of up to 140 MB/s, which
492 ft)
is comparable to hard disk drives.
Optical Media
Blu-ray Disc (BD) is an optical disc storage medium designed
to supersede the DVD format. The standard physical medium
is a 12cm plastic optical disc, the same size as DVDs and CDs.
Blu-ray Discs contain 25 GB per layer, with dual layer discs
(50 GB) the norm for feature-length video discs and additional
layers possible in the future.
Optical media operates by creating a single spiral track that
records data bits (zeroes & ones) as sections of different
reflectivity. The sections (or the transition from one to another)
represent bits permanently, although the media itself (the disk)
is easily damaged. The single spiral is sequential in nature, but
the device can easily move radially (unlike a tape), making it a
random-access medium. The laser follows a track as the disk
spins, sensing differences in reflectivity.
Discs are available in read only (BD), record once (BD-R) and
rewritable (BD-RE) formats. Recording to a disc involves
using the laser in high power mode to alter the reflectivity of a
die on the disk. For rewritable disks, the die can be changed
back to its initial state and reused. As indicated above, these
dies may be layered with the laser focused on a particular layer
to store more data.
Most BD devices are backward compatibles with DVDs and
CDs, although you should check the specifications to be sure
before purchase.
Underside of a DVD-R disc
Schematic representation of a crosssection through a 3D optical storage
disc (yellow) along a data track (orange
The concepts for storing data on a DVD, or Digital Video
Disc, are similar to those of BD. However, the laser used is not
capable of focusing as finely, so the data area for each bit is
larger (which means less capacity) and DVD players cannot
read or write BDs. DVD devices are generally backward
compatible with CDs.
The capacity for a DVD-R or DVD-RW is 4.5 GB per layer.
They are available as double-sided (two layers) for 9 GB
DVD-RAM is a rewritable 4.7 GB random access disc used
primarily for camcorders. It is not related to Random Access
Memory (RAM).
Inside a DVD player, tray opened —
#15 How optical storage works
The CD or Compact Disc, which might be considered a legacy device, is still widely used. It is capable
of hold 700 MB (0.7 GB) of data. CD devices cannot read or write either DVD or BDs.
Solid State Drives A solid-state drive (SSD) is a data
storage device that uses solid-state memory to store persistent
data with the intention of providing access in the same manner
of a traditional hard disk drive. SSDs are distinguished from
traditional hard disk drives (HDDs), which are
electromechanical devices containing spinning disks and
movable read/write heads. SSDs, in contrast, use microchips
which retain data in non-volatile memory chips and contain no
moving parts. Compared to electromechanical HDDs, SSDs
are typically less susceptible to physical shock, are silent, and
have lower access time and latency. SSDs use the same bus
interface as hard disk drives, thus easily replacing them in
most applications.
The disassembled components of a
hard disk drive (left) and of the PCB
and components of a solid-state drive
SSDs gain their performance edge over hard disks because
they do not have the lengthy access time of the mechanical
device (and no "spin up" time). Speeds for SSD are roughly
equivalent to traditional RAM. The lack of moving parts also
provides the edge in terms of physical damage and failure
The primary disadvantages are that there are a limited number
of writes to a flash device and they are not yet as big, in terms
of data storage capacity, as hard disks. There is no limit
(except for eventual device failure) to the number of writes on
a hard disk.
Fall 2010 MacBook Air
The latest version (Fall 2010) MacBook Air has neither a hard
disk drive nor an optical drive.
RAM Disks
RAM-disks are a kind of solid state device, based on volatile memory, such as DRAM. They are
characterized by ultrafast data access and are used primarily to accelerate applications that would
otherwise be held back by the latency of flash SSDs or traditional HDDs. RAM-disks (DRAM-based
SSDs) usually incorporate either an internal battery or an external AC/DC adapter and backup storage
systems to ensure data persistence while no power is being supplied to the drive from external sources.
If power is lost, the battery provides power while all information is copied from the RAM-disk to backup storage (HDD or SSD). When the power is restored, the information is copied back to the RAM-disk
from the back-up storage, and the RAM-disk resumes normal operation (similar to the hibernate
function used in modern operating systems).
RAM-disks of this type are usually fitted with DRAM modules of the same type used in regular PCs
and servers, which can be swapped out and replaced by larger modules. Like RAM (primary memory),
a RAM-disk does not suffer the limited number of writes problem associated with flash SSD devices.
Legacy Devices ]
A legacy system is an old method, technology, computer
system, or application program that continues to be used,
typically because it still functions for the users' needs, even
though newer technology is now available. Reasons for not
changing to a newer system may include cost, planned
retirement (that includes change), and resistance to change
from users.
A legacy system may include procedures or terminology which
are no longer relevant in the current context, and may hinder or
confuse understanding of the methods or technologies used.
The term "legacy" may have little to do with the size or age of
the system itself – current mainframes run 64-bit Linux and
Java alongside 1960s vintage code.
(Legacy) Commodore 64 Game
Floppy Disks
A "floppy disk" (or diskette) is a form of magnetic data
storage; thin, flexible, soft, flat piece of mylar plastic,
packaged in a 3.5 inch plastic case. The reason it was called a
floppy is because obsolete (8 and 5.25 inch formats) types
would "flop" as you waved them. Floppy disks were invented
by IBM and were a popular form of data storage from the
1970's to the 1990's. The key to their widespread use was their
inexpensive cost and ease of portability. Information could be
transferred to a floppy disk, stored, disk removed, then inserted
into another system to then be accessed.
3.5" Floppy Disk
5.25" Floppy Disk
Although there was a variety of different sizes of floppy disks
produced by various manufacturers, the most widely used was
the 3.5 inch. It is a form of secondary "permanent" storage and
can hold approximately 1.44 MB. The disk is inserted into a
floppy disk drive, a device that holds, spins, reads data from,
and writes data to a floppy disk.
With the new systems now available, the demise of the floppy
disk was inevitable. It has been replaced by CD, CD-ROM and
(USB) flash drives.
The data organization shown to the right is the same as a single
platter in a hard disk drive with: a track (A), a sector (B),
8" Floppy Disk
a segment or sector of a track (C), and a clusterof segments
The floppy disk was renowned, when in common use, as a
primary means of spreading viruses from computer to
computer. Even when networked computers became more
common, many users shared data and programs on floppy
disks. A virus attached to a specific program would only be
activated when that program was run by the user, however,
a boot sector virus required only that the PC be rebooted with
an infected disk in place. At that time, it was common for the
computer to attempt to boot first from the floppy disk drive (a
recovery feature when the hard disk unbootable). Current
viruses tend to spread through network enabled programs, such
as email.
Disk structure
public domain image
Zip Disks
A zip drive is a secondary storage device (internal or external)
that uses zip disks. Zip disks are cartridges similar to floppy
disks but capable of storing 70-500 times more memory. Zip
disks are disks with a special high quality magnetic coating
that have a capacity of 100, 250, or 750 MB.
Zip drives are almost obsolete amongst today's students as
CDs or flash memory sticks are much more convenient since
they can be read by almost all computers (zip disks need
special zip drives).
Zip Disk
Disk storage comparison
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