Secondary Storage Devices
Lecture # 3
Secondary Storage Devices
Classification of Physical Storage Media
• Can differentiate storage into:
– volatile storage:
• loses contents when power is switched off
– non-volatile storage:
• Contents persist even when power is switched
off.
Physical Storage Media
• Cache
– Close to CPU
– fastest and most costly form of storage; volatile; managed by the computer system hardware.
• Main memory:
– Fast access (10 to 100 of nanoseconds; 1 nanosecond = 10–9 seconds)
– Generally too small (or too expensive) to store the entire database
• Flash memory
– Data can be written at a location only once, but location can be erased and written to again
• Magnetic-disk
– Data is stored on spinning disk, and read/written magnetically
– Primary medium for the long-term storage of data
• Optical storage
– non-volatile, data is read optically from a spinning disk using a laser
– CD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular forms
• Tape storage
– non-volatile, much slower than disk, tape can be removed from drive  storage costs much
cheaper than disk, but drives are expensive
–
Storage Hierarchy
• Typical storage hierarchy:
– Main memory (RAM) for currently used data.
– Disk for the main database (secondary
storage).
– Tapes for archiving older versions of the data
(tertiary storage).
Secondary Storage Devices
• Magnetic media
• Tape
• Disks
• Optical Media
• Compact Discs
• CD-R, WORM (Write Once, Read Many)
• CD-RW
• DVD
• DVD-R
• DVD-RW
Examples of Magnetic Media
• Some you are probably familiar with:
• Cassette tapes
• VHS video tape
• Computer Tape
• 8-track
• DAT
Hard Disks
• Today, most people use Hard Disks for
secondary storage
• The basic technology used in hard disks is
similar to that of magnetic tape
•The disk head can move to any point on the
platter but in the tap can not move.
•In tap: The head touches the tape. In disks:
the head never touches the platter
•Data is stored and retrieved in units called disk
blocks or pages.
Hard Disks
Storing Information on a Hard Disk
• Each platter is broken up into tracks and sectors
• Tracks are concentric circles on the disk
• Each track is broken up into a series of sectors
Track
Sector (ring between
the lines)
Sectors and Blocks
• Sectors are further broken up into blocks
• A block is a fixed size unit of storage
• 512 bytes/block is most commonly used
• 1024 bytes/block is common with SCSI disks
• 2048 bytes/block is used with CDs
• If the user stores onto the hard disk a file which
is larger than the block size, then multiple blocks
are used.
Blocks and Files
• If a file takes up multiple blocks, it is necessary to keep
track of which blocks comprise that file
• Each block is assigned an address
• The location of a “file” is stored in what is called a “File
Allocation Table” (or FAT)
• When the hard disk is formatted, several blocks are
reserved so that the Operating System can manage
where files are stored on the disk
• FATs are often used to keep track of the filename and
directory as well.
Filesystems
• Files are managed within a “filesystem”
• The filesystem defines how and where files are stored
within a hard disk (or partition)
• When a disk is formatted, a filesystem is placed on the
disk
• Common filesystems include:
• FAT16 (MSDOS)
• VFAT (Windows 95)
• FAT32 (Windows 98)
• NTFS (Windows NT)
• UFS (UNIX)
• ext2/ext3 (Linux)
• ISO9660 (CD Roms)
Optical Media
• Optical disks are very much like
hard disks
•Optical disks store information as
bits in a physical medium
• A laser is used to determine if a
bit is present or not.
CD ROM
• CD Roms use the same technology as audio
Compact Discs.
•Copies of the disc are created through a
copies process
• The discs are aluminum sandwiched
between plastic
• CDs are single sided.
Label
Acrylic
Aluminum
Plastic
CD-R
•CDR discs can be written once and read many times
• CDRs are made out of aluminum and plastic, but
also contain a dye layer
• This dye is modified by a laser when the disc is
being written
• The laser heats up the dye and it becomes nonreflective
Label
Acrylic
Aluminum
Dye
Plastic
CD-RW
• CD-RW is similar to CD-R
• The main difference is that the dye can be
made reflective again through an erase
process
• In this way, CD-RW discs can be written
many times
• Too much erasing, the dye starts to fade.
Label
Acrylic
Aluminum
Dye
Plastic
DVD – Digital Versatile Disk
• DVDs hold approximate 7 times the information
that CDs do in the same amount of storage space
• DVDs come in 3 types
• Single Sided/Single Layer (4.7 GB)
• Single Sided/Double Layer (8.5 GB)
• Double Sided/Double Layer (17 GB)
• DVD uses a laser with a shorter wavelength so the
bits are smaller
• More bits can be stored on a DVD
• Narrower track
Fragment
What is file fragment?
• Files may be fragmented in several sectors of
HD
• Fragmentation can slow down the system
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Defragment
• Each time file is opened, the O/S has to reassemble
the file from different areas of the hard drive
• Disk Defragmentation :
– Analyzes the disk
– Rearrange the disk by placing files in contiguous
blocks
– To reduce disk arm movement during sequential
accesses
– Does not change the linked structure of the file
system!
• Utility should be run at least monthly. Can vastly
improve system performance.
Defragmenting Volumes
• OS attempts to save files in locations on the
hard disk that are large enough to
accommodate the entire file.
• If there is no suitable location, OS saves
fragments of the file in several locations.
• This fragmentation of files on the hard disk
decreases system performance because the
computer must read file data from various
locations on the hard disk.
Defragmenting Example (Windows OS)
• Windows
provides
defragmenting:
two
methods
of
– Disk Defragmenter, which is a snap-in tool.
– The defrag command-line tool.
• Both tools enable you to defragment files or
volumes of any cluster size
• Windows provides the Disk Defragmenter utility
to reorganize clusters contiguously
– Improves performance by minimizing movement of
the read/write heads
– Should be used regularly to ensure system runs at peak
performance
Using Disk Defragmenter
21
Disk Defragmentation Options in Windows
Description
Analyse
Click this button to analyze the disk for
fragmentation. After the analysis, there is graphic
representation of how fragmented the partition
is, and a dialog box appears informing you if the
disk should be defragmented or not.
Defragment
Click this button to defragment the disk. During
defragmentation,
there
is
a
graphic
representation of the defragmented partition.
Pause
Click this button to temporarily stop analyzing or
defragmenting a volume.
Stop
Click this button to interrupt and stop analyzing
or defragmenting a volume.
View Report
Click this button to view additional information
about the files and folders that were analyzed.
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Before and after Defragmentation
• Files become fragmented as they are stored in
noncontiguous clusters;
• a defragmenting utility moves files to contiguous
clusters and improves disk performance
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