Storage devices
Introduction
Storage devices are known as 'non-volatile' devices. This is because when the power is removed from the device, the data files remain. These devices are long-term storage. They are perfect for backing-up files and applications, transporting them and sharing them. If you didn't have storage devices, you would have to reload applications and re-enter files each time you switched your computer back on! There are lots of different types of storage device to choose from.
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!
SIM cards (Subscriber Identity Module card)
Mobile phones have SIM cards. These have a very small amount of storage on them, to hold phone number information, contacts and so on. The storage capacity is very limited, however.
Floppy disks and Zip 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 as they can hold so little data compared to alternatives. 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 - you would struggle to store one good quality digital song on one disk! Zip disks are similar to floppy disks but have a much higher capacity. Zip disks hold typically 250 Mb and are still used to back up files.
Selecting a storage device
When selecting a storage device to use, you should consider a number of issues. These include:
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- How fast the media can be accessed (their 'read / write access times').
- Whether data can be accessed directly or serially, because this affects the time it takes to access data. Magnetic tape, for example, is serial access and very slow whereas flash drives are direct access and very fast.
- How much data can be stored on the media.
- What the media might typically be used for.
- How commonly used the media is and whether other computers are likely to be able to use that media. Most computers don't use floppy disks or Zip drives anymore, for example, so even if you had these on your computer and wanted to share files, it wouldn't be a good idea to use these.
- 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. some devices can only be written to once and then read many times (WORM) like a CD-ROM but other devices can be written to many times, like a pen drive.
- Whether the storage medium is 'virtual' and requires an Internet connection or whether it is physical.
- How portable and convenient the device is.