2Magnetic TapeWhat is magnetic tape?Thin layer of material capable of storing a magnetic signalUsually contains Iron OxideProtected by backing layer called the “Substrate”
3Examples of Magnetic Media Some you are probably familiar with:Cassette tapesVHS video tapeComputer Tape8-trackDAT
4Pros and Cons of Magnetic Tape Inexpensive to store large amounts of information.A reel of tape could store as much as 225 MB in the early 1980s.No other storage technology could compare to the price of tapeSequential access – Slow accessTapes are highly susceptible to magnetic fields and changes in temperature and humidityTapes are also susceptible to physical damageTape miss-feedsMagnetic particle instabilitiesSubstrate deformation
5Magnetic Tape Uses Today Today, tape based systems are usually used for backup purposes only.Tape still provides the most cost effective method of storing larger amounts of backup informationReliable as long as temperature is kept low and humidity conditions are kept right.Although hard disks are increasingly used as a backup mechanism, tapes are portable.Backups can be taken off-site for greater risk prevention
6Hard DisksToday, most people use Hard Disks for secondary storageThe basic technology used in hard disks is similar to that of magnetic tapeMagnetic material is layered onto a high-precision aluminum diskThe disk head can move to any point on the platter almost instantly compared to tapeWith tape, the head touches the tape. With disks, the head never touches the platterTape moves at approx 5 cm/s. Disk platters move at up to 7500 cm/s (272 km/h!)
7Hard DisksTo increase capacity, a hard disk will usually contain several platters
8Hard DisksThe heads never touch the platters, but they are very close. This makes hard disks susceptible to mechanical shock.
9Storing Information on a Hard Disk Each platter is broken up into tracks and sectorsTracks are concentric circles on the diskEach track is broken up into a series of sectorsTrack (yellow ring)Sector (yellow ringbetween the lines)
10Sectors and BlocksSectors are further broken up into blocksA block is a fixed size unit of storage512 bytes/block is most commonly used1024 bytes/block is common with SCSI disks2048 bytes/block is used with CDsIf the user stores onto the hard disk a file which is larger than the block size, then multiple blocks are used.
11Blocks and FilesIf a file takes up multiple blocks, it is necessary to keep track of which blocks comprise that fileEach block is assigned an addressThe 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 diskFATs are often used to keep track of the filename and directory as well.
12FilesystemsFiles are managed within a “filesystem”The filesystem defines how and where files are stored within a hard disk (or partition)Common filesystems include:FAT16 (MSDOS)VFAT (Windows 95)FAT32 (Windows 98)NTFS (Windows NT)UFS (UNIX)ext2/ext3 (Linux)ISO9660 (CD Roms)
13FilesystemsWhen a disk is formatted, a filesystem is placed on the diskThe filesystem reserves space for the FAT.The remaining space is available for filesWhen a file is to be saved into the filesystem, the system looks for the best location to save the fileIt is usually best to save the file in contiguous blocks.If the disk is nearly full, it may not have enough contiguous blocks to save the file. In that case, the filesystem will place the file wherever it can find space.
14Deleting FilesIn order to make the delete operation faster, when a file is deleted ONLY the FAT is updated.The actual data still remains on the diskThis is why it MAY be possible to recover a file which has been removed.The FAT is told that the blocks where the file was stored are now available for writingThe data will remain until another file is stored in the same blocks.As more files are written on the disk, it is less likely that a delete file can be recovered.
15Hard Disk Fragmentation When files are deleted, their blocks are open for writing.What if the next file which is written in the same blocks is larger than the file which was originally written there?The system has to break the file up into pieces throughout the filesystemThis is called fragmentation. It considerably slows the access of files.Most modern Filesystems contain automatic defragmentation utilities.
16Hey! Wait a minute!When you purchase a hard disk, the manufacturer indicates its capacity.When you format that hard disk, you’ll notice that its capacity is less than what the manufacturer told you.Formatting the disk requires space for the FATManufacturers use decimal numbers to represent number of bytes1GB = 1,000,000,000 bytesThere is a class action suit currently filed against the major disk manufacturers because of this
17Floppy DisksFloppy disks are similar to hard disksBecause the medium is “floppy”, the disks cannot operate at the same speeds as a hard disk.Floppy disks are older technology which haven’t received a lot of attention since the late 1980s.Their capacity hasn’t increased muchMany computers today do not even come with floppy disk drives anymore.
18Floppy Disk HistoryThe first floppy disks were 8 Inches in diameterThe held about 256 KThe next generation were 5.25 inches in diameterCould hold up to 1.44 MBThe next generation were 3.5 inches in diameterThese disks are held within a hard plastic caseA spring loaded flap protects the disk from dust and greasy fingers.Could hold up to 2.88 MB
19Zip / Jaz DisksSimilar to floppy disksThey are removableTheir platters are made of a hard materialCan operate at higher speeds than floppies.Newer standards hold more dataMost people have outgrown floppy disks
20Optical MediaOptical disks are very much like hard disksHard disks store information using magnetic materialBits are stored by changing the magnetic properties of the magnetic materialBits are read by picking up the tiny magnetic field with a read headOptical disks store information as pits in a physical mediumA laser is used to determine if a pit is present or not.
21CD Roms use the same technology as audio Compact Discs. A master disc is created.Copies of the disc are created through a pressing processThe discs are aluminum sandwiched between plasticCDs are single sided.LabelAcrylicAluminumPlastic
22CD Roms must be pressed. They are read only CDR discs can be written once and read many timesCDRs are made out of aluminum and plastic, but also contain a dye layerThis dye is modified by a laser when the disc is being writtenThe laser heats up the dye and it becomes non-reflectiveLabelAcrylicAluminumDyePlastic
23Storing Information on a CD Because CDs were originally intended for audio output, there is a single track of data which spirals out from the center of the disc
24CD-RW is similar to CD-R The main difference is that the dye can be made reflective again through an erase processIn this way, CD-RW discs can be written many timesToo much erasing, and the dye starts to fade.LabelAcrylicAluminumDyePlastic
25DVD – Digital Versatile Disk DVDs hold approx 7 times the information that CDs do in the same amount of storage spaceDVDs come in 3 typesSingle 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 pits are smallerMore pits can be stored on a DVDNarrower track
26DVD – Multiple LayersEach side of a DVD can contain 2 layers, doubling the amount of data that can be storedOne layer is semi-transparent. The laser can be focused “through” that layer onto the second layer.If you took the track off of a single layer of a DVD and stretched it out into a straight line, it would be 7.5 miles long!If you did the same with a double layer, double sided disc, the track would stretch to over 30 miles!
27DVD-R and DVD-RWThese two technologies are still relatively new and still fairly expensive.DVD-R and DVD-RW use similar techniques as CD-R and CD-RWA dye layer is used for data writingThe dye layer in DVD-RW is erasable