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Computer Files & Data Storage

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Computer Files & Data Storage STORAGE, FILE CONCEPTS, FILE ORGANIZATION, UTILITIES - Part 2 - (Pages 16-19, 194-199, OFF25-OFF37 - Discovering Computers & Microsoft Office 2016) Continued from previous outline II. One of the earliest forms of PC storage is Magnetic storage. Magnetic storage introduced us to hard drives, floppy diskettes, and magnetic tape. B. How does magnetic storage work? The surface of magnetic media (plastic or metal) is covered with a special finish called magnetic oxide coating. Utilizing the magnetic oxide coating, the surface of the plastic or metal basically becomes magnetized. The data we create is converted into magnetic particles. The magnetic oxide coating functions as a magnet to draw and retain the magnetic particles (our data) to the disc surface. Mystery revealed. Life expectancy for data recorded on magnetic media is approximately 3 years. Why such a short time? The magnetic oxide coating diminishes over time through simple wear and tear. When the magnetic oxide coating weakens it can no longer retain the magnetic particles (our data) thus causing the particles to be released and lost. Bummer. Examples of magnetic storage media: 1. Magnetic tape – tape is comprised of strands of plastic (much like a cassette tape) Magnetic tape is used primarily as a backup system because the access to information is very slow. Why? There is no way to pinpoint the exact location of a file recorded on tape (when needing to retrieve it), you must read all the preceding content of the tape until you locate the file you wish to retrieve. Magnetic tape is not formatted like all other storage media. Purpose of formatting? Formatting establishes addresses and a File Allocation Table (FAT). The FAT is like a directory that keeps track of every file’s location. Again, tape is not formatted so there are no addresses identifying the location of files. This is the equivalent of having to deliver the mail and the letters or correspondence you wish to deliver contains no addresses. To deliver the mail you’re forced to go door by door inquiring who lives in a residence until you find the recipient of the mail. A mailman’s nightmare. Characteristics of tape include the following: Fall 2016 1 a. Inexpensive, portable, large volume storage capacity b. Data is stored & retrieved sequentially (one record after another – no addressing system) / slow access to information because you have to go file by file through the tape until you locate the file you need c. Requires a tape unit or tape drive – not standard or common on PCs d. Most PC tape units are external (USB), not typically built into the system unit e. Assumes whatever drive letter is available (only A,B, and C are assigned) f. Ideal for making backups where the primary concern is just to have extra copies of files 2. Hard disk (drive) – By far the most common magnetic storage device for personal computers, primarily because of the capacity, most hard drives store a computer’s software. A hard drive (disk) is comprised of one or more metallic platters varying in size and capacity. A hard disk may be fixed (internal) or removable (external). Other hard disk characteristics Components include the following: of a hard drive a. The hard disk surface is more durable (metal) than tape or floppies (plastic), therefore data is considered more secure. b. The Hard Drive is a Direct access storage device (DASD) – files may be accessed randomly, unlike tape where access is sequential. This is why access to information on the hard disk is much faster. The operating system uses the "FAT" to identify a file's location. Before a hard disk is utilized, it is first formatted. Formatting creates storage units on disk containing addresses, every time a file is saved, the file name and its storage address is recorded in the FAT. The operating system uses the FAT to pinpoint the exact location of a file, no need to check other locations like when using magnetic tape. Fall 2016 2 Magnetic tape has no FAT, that’s why every preceding record has to be checked, the operating system doesn’t have an address to identify the location of a file. c. A critical component of the hard drive is the “read and write” head (check diagram on previous page). The read/write head is used to record and read information from the disk surface. It functions much like a needle on a record player. If the needle is not functioning, you can’t play records. The faster the drive's disk platter spins the faster the computer can access and record data. The speed of the drive is measured in “revolutions per minute” (rpm). d. Although most hard drives are internal, external hard drives are now very popular. e. A major negative concerning the hard drive is the susceptibility of a head crash. A head crash is often fatal because it can result in loss of access to all data. Most disk drives are guaranteed for no more than three years. Disk drives contain mechanical parts that over time simply fail (through wear & tear). The drive’s parts must all be operational to store and retrieve data. When the parts fail, there is no way to retrieve data, in essence you lose everything on your hard drive, files are no longer accessible. A head crash can also occur when contaminants (smoke, dust, hair, etc.) enter your disk drive and come in contact with the “read and write” head. The contaminants can cause a scratch or damaging mark to be placed on an area of disk where information is recorded. Because of the damage to the disk, the data in that area can no longer be read which results in the loss of data. Much like when you have a scratch on a record (LP) or music CD, a scratch can result in loss of the access to recorded music. If the contaminant damages the read/write head, major problem, data is no longer accessible. Fluctuations in electrical power can also contribute to a head crash if it results in scratches made to the disk while in the process of writing to the disk. A head crash does not delete data, a head crash means that the mechanical components of the disk drive are no longer functioning thus making it impossible to retrieve data. f. Hard drives are typically identified as drive C: g. Hard drives provide the largest capacity of all PC storage devices, 4-10 TB 3. Floppy diskettes (legacy device) – Floppies consist of a round piece of plastic protected by an outer covering. Floppies vary in size and storage capacity. Before CDs & flash drives, everybody used floppies. Early PCs had no hard drive, most PCs functioned with 2 floppy drives (A: & B:). Fall 2016 3 a. 5.25 Double sided double density (DS/DD) can store up to 360 KB (wow) b. 5.25 Double sided high density (DS/HD) can store up to 1.2 MB c. 3.5 Double sided double density (DS/DD) can store up to 720 KB d. 3.5 Double sided high density (DS/HD) can store up to 1.44 MB e. Very inexpensive but provide very low storage capacity f. Provide direct access to data (DASD) g. Media is portable or mobile h. Typically identified as drive A: and drive B: C. Optical storage – Information recorded on an optical disc is written with lasers, not the magnetic technology employed by magnetic media. Lasers in essence "burn" information onto the storage disc Data recorded on optical media is considered less susceptible to environmental damage (dust, hair, smoke, etc.). Optical storage has far greater storage capacity than removable magnetic media (floppies). A typical CD ROM, CD-R, or CD-RW can store approximately 700 MB, a DVD has gigabyte capacity. Data on optical storage can be retained for over 30 years. Advances in technology provide even greater capacity for Fall 2016 4 optical media, as in the case of Blu-Ray DVDs. The different types of optical discs are identified below: 1. CD-ROM – “Compact Disc Read Only Memory” - contains permanent, unalterable information – comes with prerecorded information that cannot be altered 2. CD-R – “Compact Disc Recordable” - data can be written onto the disc, once initial recording is completed, cannot be deleted or modified– If you want to record on a CDR disc you need a recordable CD drive 3. CD-RW – “Compact Disc Rewriteable” allows the user to record, erase, and modify the disc content several times as desired, in that sense, functions like magnetic media – on the contrary, CD-R discs can be written to only once. 4. DVD – “Digital Video Disc” - contains gigabyte capacity. A DVD drive can read CDs but a CD drive cannot read DVDs. DVDs can be either “read only” or “recordable”. Many new technologies exist providing greater capacities (HD-DVD, Blu-Ray DVD) DVD drives can recognize any type of optical disc, be it CD or DVD. This makes DVD drives the most versatile storage device. 5. Disadvantage: Slow read and write time. The read and write speed of optical drives is measured at 150 KB per second. Read and write speeds for optical drives are identified by a number, 6X, 10X, 20X, etc. What do the numbers represent? The read/write speed of the drive. Again, the original drive had a read/write speed of 150 KB per second. The number to the left of the (X) represents the improvement over the original speed. If a drive is identified as 6X, it means the speed is 900 KB per second (150 KB x 6) – Read and write time is slow when compared to most magnetic media & solid state storage.
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