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Selecting storage media

Introduction
When comparing and contrasting different storage devices, we could use a number of criteria:

• • The type of technology they used (solid state, magnetic or optical).
  • How fast the media can be accessed.
  • Whether data can be accessed directly or serially.
  • How much data can be stored on the media.
  • What the media might typically be used for.
  • How commonly used the media is.
  • The cost of the media and the cost of the actual device used to read from or write to it.
  • Whether the media is read-only or read-write.
  • Whether the storage medium is 'virtual' and requires an Internet connection or physical.
  • How portable the device is.

How the capacity of secondary storage media is measured
Data storage is measured in 'bytes'. When you talk about storage media, however, you quickly end up talking about thousands of bytes, millions of bytes, millions and millions of bytes and so on. There are lots of different ways of talking about these large numbers. We have met the following summary before:

• • 1 Kilobyte (1 Kbyte) is 1024 bytes exactly, or 2¹⁰ bytes exactly, or about 1000 bytes, or about a thousand bytes.
  • 1 Megabyte (1 Mbyte) is 1048576 bytes exactly, or 2²⁰ bytes exactly, or about 1000 000 bytes, or about a million bytes.
  • 1 Gigabyte (1 Gbyte) is 1073741824 bytes exactly, 2³⁰ bytes exactly, or about 1000 000 000 bytes, or about a thousand million bytes.

So 15 Kbytes is about 15 thousand bytes. 128 Mbytes is about 128 million bytes. 20 Gbytes is about 20 thousand million bytes. More often than not, you don't need to know the exact number of bytes, just an approximation!

Storage devices
There are a wide range of storage devices that can be used.

Floppy disks
For many years, floppy disks used to be the main storage medium after the hard drive, although these days, you would be unlikely to see modern computer systems with one. Drives typically cost less than ten pounds while the disks themselves cost pennies. A disk typically stores 1.44 Mbytes, or about one and a half million bytes of data. Because they were widely used by PCs and were cheap and were ideal for backing up and transporting small personal files. They were of limited use, however, if files contain multimedia because these types of files can be large. Floppy disks are read-write devices because you can both read from them and write to them. It is possible to make them temporarily read-only devices by write-protecting them. You can do this by sliding the write-protect tab on the floppy disk into the write-protect position.

Floppy disks are magnetic devices. They are flexible, plastic disks that have been coated in a magnetic substance, either on one side (single-sided) or both sides (double-sided) of the disk. When a floppy disk is placed into a drive, the disk starts to rotate hundreds of times a second and a 'head' moves over the disk to read from or to write to the disk. When files are 'saved', areas of the disk are magnetised by the head. When files are 'read', the magnetic areas are picked up and decoded back into data by the head. The speed that a floppy disk rotates might sound fast but is much slower than a hard drive, which rotates at thousands of times a second. Consequently, it is much slower to read and write to a floppy disk compared to a hard disk.

Floppy disks are known as direct access storage devices. This is because the disk drive head can go straight to a particular place on a disk, find the data and retrieve it. It doesn't have to go through all the other data files first. With some data media, you have to do just that, as you will see later.

Zip disks and Jaz disks
These magnetic disks are similar to floppy disks but have a much higher capacity. Zip disks hold typically 250 Mb whilst Jaz disks hold 2 Gbytes typically. Data on Jaz disks can be accessed very quickly, at the same kind of speed as hard disks. Zip disks are a little slower but still a big improvement on floppy disk data access.

Hard disks
A hard disk is a little like having lots of floppy disks stacked one on top of each other and then sealed in a unit! The disks are in fact not floppy at all but hard magnetic disks, each with their own read/write head that moves over the disk that they are responsible for. We already know that they spin very quickly when compared to floppy disks but hard disks constantly spin, as opposed to floppy disks, which start spinning only when they are needed for a read or a write operation. No time is wasted, therefore, waiting for a hard disk to get up to speed before you can use it! Both of these reasons explain why floppy disks work much slower than hard disks. Hard disks, like floppy disks, are direct access devices - you can go straight to a file on an area of disk without having to go through all the other files first. Hard disks are used to store applications and data files. They can hold huge amounts of data compared to a floppy disk. A typical hard disk today might hold 80 Gbytes of data but bigger disks seem to come onto the market every few months! All personal computers have hard disks, although it is possible for workstations on a network to exist without a hard disk - they make use of the server's hard disk to store applications and data. These types of computers can be referred to as 'thin clients'. Hard disks can also be used as back-up devices. A computer can be fitted with a second hard disk known as a mirror hard disk or a 'raid' data storage system. As the name suggests, this second hard disk is used to keep an identical copy of the main hard disk. Then, if the main hard disk fails (and every hard disk, indeed every storage device, will fail sooner or later), you can use the mirror disk to recover your applications and data with the minimum of effort.

Magnetic tape
Magnetic tape can hold lots of data, of the order of Gigabytes. They are used typically to back-up data files on networks. Network servers often have a back-up device fitted in the server that can hold 7 tapes, for example. These rotate automatically each day. The back-up process is automated, so that in the middle of the night, data is backed-up on a new tape and the tape gets ejected from the device. The Network Administrator then removes the tape in the morning and puts it in a fire safe. Magnetic tape is not a direct access device. It is a serial access device. If you lost one of your files and asked the Network Manager to recover it from a back-up tape, they would have to search through every file until they found the right one. They couldn't go straight to the file in question. For this reason, magnetic tapes are very slow devices, not suitable for fast access applications but ideal for applications where you won't probably need the data - like back-up applications! They are also cheap to store data on compared to other types of media. You can store more bytes per penny!

CD-ROM
This media is ideal for distributing software because they hold a lot of data (650 Mbytes or more) compared to floppy disks (1.44 Mbytes) and nowadays, most computers have a CD-ROM drive. It is much more convenient having software on one CD than having lots of floppy disks because you don't have to keep changing the media when installing the software. One CD is also less bulky than lots of floppy disks. CD-ROMs are direct access media but they do not use magnetic technology! They are optical storage media. They store information on pits in the surface of the CD and then use a laser to scan over the pits. CD-ROMs are read-only devices. The basic CD unit is very cheap. However, you would need to use a CD-R/W device and a special type of CD if you want to write to as well as read from a CD, in much the same way as you would read and write to a floppy disk.

CD-R/W
This kind of optical, direct access media is ideal for backing up personal files, especially those involving multimedia. It is suitable for this type of application because of the high storage capacity of CDs. You need to have a special CD-R/W device to write to CDs. The CDs themselves could be of the WORM type (Write Once Read Many times). This means that you can only write to the CD once (sometimes called 'burning a CD') but can read from it many times. This might be suitable if you wanted to make a back-up copy of some software you have bought or wanted to make and distribute some music you had recorded. CDs can also be Read-Write, which means they can be written to many times in the same way a floppy disk can be. A CD that has been created using a CD R/W device can be read from a standard CD-ROM device, usually after the installation of a small utility program. CD-R/W devices are now cheap to buy, of the order of twenty to thirty pounds.

Digital Versatile Disc (DVD)
DVDs are rapidly replacing CDs (and video tapes)! This optical, direct access, very fast media can hold approximately 17 Gbytes of data compared to the 650 Mbytes of a standard CD. A DVD player can read CDs as well as DVDs with the addition of extra software. They are typically used for distributing multimedia, especially high quality video. A DVD can store about 8 hours of high quality video! DVD recorders are also available. Although they cost hundreds of pounds their price will inevitably drop.

USB pen drive (flash drive)
These plug into a computer's USB port and provide a convenient way of transferring large amounts of data. Although they are relatively strong, you can corrupt the contents (so it is unreadable) by pulling out the pen drive from the PC before properly 'disconnecting ' it using the tool that comes with your operating system.

SD cards and micro SD cards
These small cards can hold very large amounts of data and are ideal for cameras and mobile phones, to hold pictures, videos and music.

Compressing data
Data compression refers to the process of 'squashing' data so that you can store more of it in the same space on a storage device. This can be done by using a utility program from within your operating system or using applications such as WinZip. A lot of data sent over the Internet is also compressed because this reduces the amount of time (and cost) of data transfer. It is compressed by the sending computer and de-compressed by the receiving computer. This happens automatically, without the knowledge of the users.

Cloud storage
Many companies these days do not back up their work onto a physical medium such as a tape or DVD but use internet storage instead. When they want to back something up, the data is first compressed to make it smaller. It is then sent over the Internet to a company, who stores it on their computers. This has quite a few advantages. You can set up the back-ups to automatically happen so no one will forget to do them. You don't need to buy expensive equipment to back-up work and you can't lose back-ups. Of course, you need an Internet connection and some companies aren't happy about sending their valuable data to another company to look after for security reasons. However, this way of storing data, known as 'cloud storage' is becoming increasingly popular. Many cloud storage companies offer a certain amount of free storage to individuals. You could do a search and start using cloud storage yourself!

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