Computer Notes- Storage

Introduction -Storage is a media and methods, which is used to keep information available for later use.

 Primary Storage
 Secondary Storage

 It is the main memory of computer.
 It keeps track of current processing
 It is volatile, i.e., turning the power off erases all of the data.
 For main memory, computers use RAM or Random Access Memory.
 These memory chips are the fastest, but most expensive type of storage.

Main Memory = Primary Storage

 It is the auxiliary storage of computer.
 It keeps track of what is not currently being processed.
 It is the stuff that is 'Filed away', but is ready to be pulled out when needed.
 It is non-volatile, i.e., turning the power off does not erase the data.
 Auxiliary storage is used for input data and programs as well as saving the result of processing.
Auxiliary Storage = Secondary Storage Magnetic Disks

These are commonly used type of auxiliary storage. They come in numerous sizes and materials and by the method of magnetism data are stored on the magnetic surface. The advantages of using magnetic disk is high storage capacity, reliable and providing the direct access to data. There is a drive that spins the disk very quickly underneath a read/write head. It reads data from a disk and writes data to a disk.

These disks come in two different sizes that are
(i) 5*(1/4)
(ii) 3*(1/2)
Both sizes are made of mylar with an oxide coating. This coating provides the magnetic quality for the disk.

All the magnetic disks have the similar format. They are divided into the following areas
(i) Tracks A track is a circular ring on one side of the disk and each track has a number. The diagram given here shows three tracks of a disk.
(ii) Sectors A disk sector is a wedge shaped piece of the disk as shown in the given figure.
A track sector is the area of intersection of a track and a sector.
(iii) Clusters A cluster is a set of track sectors, ranging from 2 to 32. 1 cluster is the minimum space used by any read or write
(iv) Cylinders A cylinder is a set of matched tracks.

 All data is erased.
 All the surfaces are checked for physical and magnetic defects.
 A root directory is created to list where things are on the disk.

The capacity of a magnetic disk depends on the following factors
 The number of sides used (either single-sided or double sided).
 The recording density (i.e., closeness of the bits on a track sector of the innermost track).
 The number of tracks on the disk.

 5*(1/4) floppy -360 KB or 1.2 MB
 3*(1/2) floppy -720 KB or 1.44 MB

 Early ones-20 MB
 Currently-TB(Tera byte 1TB=1024 GB)
 Where 1 GB = 1 gigabyte = 1024 MB

In order to access the data, the various steps are required that are as follows:
(i) Seek: moves the head to proper track , which measured as seek time in terms of milliseconds.
(ii) Rotate: It rotates the disk under the head to the correct sector that is measured as rotational delay in terms of milliseconds.
(iii) Settle It lowers the head to disk and waits for vibrations from moving to stop. It is measured as settling time in terms of milliseconds.
(iv) Data Transfer It copies data to main memory and is measured as data transfer rate in terms of Kbps.

Magnetic tapes are same as VCR tape in the way of storing data. The speed of data access can be quite slow, however, when the tape is long and whatever desired is not near the start. Therefore, this is used for mainly major backups of large amounts of data.

There are mainly two types of magnetic tape as Tape Reel and Tape Cassette. Each of the type has its own requirements. The older systems designed for networks use reel-to-reel tapes. Newer systems use cassettes holding more data than that of the huge reels. Tape Formats The magnetic tape is divided into the following format Density Higher density means more data on shorter tape. It is measured as bpi = bits per inch. It ranges from 800 bpi to 6250 bpi. Blocks Just as a floppy is divided into tracks and sectors, in the same way, a tape is divided into logical blocks. One file can take up a number of logical blocks, but must take up one whole block at least.

The gaps set on the tape are of two types
(i) Interblock gap – It separates logical blocks.
(ii) Interrecord gap–It is wider and separates the records.

For the optical disks, an entirely different method is used to record data. They include the various kinds of CD and DVD disks.

 They are formed of layers.
 Data are arranged in a spiral groove on starting from the centre of the disk.
 Digital data are in form of 1's and 0's.
 The different types of optical disks use different materials and methods to absorb and reflect the light.
 The absorption and reflection of laser light is read as a ‘1’ or a '0' by the computer.

 CD-ROM (Compact Disc-Read Only Memory) is the most common type of optical disk.
 It looks like an audio CD but the recording format is quite different.
 CD-ROM disks are used for computer software.
 DVD stands for Digital Video Device and are used for recording movies.
 The CDs and DVDs are of the Write Once Read Many (WORM) variety.
 They can't be changed once they are created.
 The data layer of this disk is physically molded into the polycarbonate.

 The optical disks (CD-R, DVD-R and DVD + R) that we can record on our own computer are called writable or recordable.
 A writable disk is useful as a backup medium when we need long term storage of our data.
 The data layer for this disk is an organic dye that the writing laser changes.

 A rewritable disk is an option for backup storage of changing data.
For example CD-RW, DVD-RW, DVD + RW, DVD + RAM.
 We can erase and write on these disks as many as 1000 times, for CD-RW and even 100000 times for the DVD-RW types.
 The data layer for this disk uses a phase changing metal alloy film.

 An optical disk is physically harder to break or melt or warp.
 It is not sensitive to being touched as it can be cleaned.
 It does not get affected by the effect of magnetic fields.
 They can hold much more data than floppy disks.
 An optical disk is a great resource to store the software and data.

 They are solid state devices that read and write data electrically, instead of magnetically.
 Devices like digital cameras, digital camcorders and cell phones use compact flash, smart media or another flash memory card.

 They are also known as flash drive, flash pen, thumb drive, key drive and mini - USB drive.
 They are small in size and can plug into a USB port on the computer.
 They have storage capacities from 8 MB to 1 GB.
 Some of them include password protection and the ability to run software right off the USB drive.

 They are special drives that compress the data.
 They are regular external hard drive that can be used for back up.

 A chip on this type of card holds information like health records and auto repair records.
 They can hold more data than the smart cards as they don't need to do any processing.



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