L 6 - External Memory.pdf

TMC1214 Computer Architecture (Semester 2 2020/2021) External Memory Reference: Chapter 6 William Stallings Computer Organization and Architecture 8th Edition Types of External Memory 1. Magnetic Disk — RAID — Removable 2. Optical — CD-ROM — CD-Recorda...

TMC1214 Computer Architecture (Semester 2 2020/2021) External Memory Reference: Chapter 6 William Stallings Computer Organization and Architecture 8th Edition Types of External Memory 1. Magnetic Disk — RAID — Removable 2. Optical — CD-ROM — CD-Recordable (CD-R) — CD-R/W — DVD 3. Magnetic Tape Introduction: Hard disk Basics Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes. They were originally called "fixed disks" or "Winchesters" (a code name used for a popular IBM product). They later became known as "hard disks" to distinguish them from "floppy disks." Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies. Introduction: Hard disk Basics At the simplest level, a hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use the same magnetic recording techniques. Hard disks and cassette tapes also share the major benefits of magnetic storage -- the magnetic medium can be easily erased and rewritten, and it will "remember" the magnetic flux patterns stored onto the medium for many years.  HDD Evolution This chart shows the evolution of IBM hard disks over the past 15 years. Several different form factors are illustrated, showing the progress that they have made over the years in terms of capacity, along with projections for the future. 250 GB hard disks in laptops in five years? Based on past history, there's a good chance that it will in fact happen! Source: http://www.pcguide.com/ref/hdd/hist-c.html The Rate of Progression in PC Hard Disk Capacity Source: http://en.wikipedia.org/wiki/Moore's_law Capacity and Performance A typical desktop machine will have a hard disk with a capacity of between 10 and 40 gigabytes. There are two ways to measure the performance of a hard disk: — Data rate - The data rate is the number of bytes per second that the drive can deliver to the CPU. Rates between 5 and 40 megabytes per second are common. — Seek time - The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common. The other important parameter is the capacity of the drive, which is the number of bytes it can hold. Magnetic Disk Disk substrate coated with magnetizable material Substrate used to be aluminium Now glass —Improved surface uniformity – Increases reliability —Reduction in surface defects – Reduced read/write errors —Lower fly heights —Better stiffness —Better shock/damage resistance  Inside Hard disk: Electronics Board The best way to understand how a hard disk works is to take a look inside. It is a sealed aluminum box with controller electronics attached to one side. The electronics control the read/write mechanism and the motor that spins the platters. The electronics are all contained on a small board that detaches from the rest of the drive: Inside Hard Disk: Another example (SCSI) SCSI : Small Computer System Interface Photograph of a modern SCSI hard disk, with major components annotated. The logic board is underneath the unit and not visible from this angle. Terms: Platters: The platters are the actual disks inside the drive that store the magnetized data. Spindle and Spindle Motor: The platters in a drive are separated by disk spacers and are clamped to a rotating spindle that turns all the platters in unison. The spindle motor is built right into the spindle or mounted directly below it and spins the platters at a constant set rate ranging from 3,600 to 7,200 RPM. Terms: Read/Write Heads : — The read/write heads read and write data to the platters. — There is typically one head per platter side, and each head is attached to a single actuator shaft so that all the heads move in unison. — When one head is over a track, all the other heads are at the same location over their respective surfaces. Typically, only one of the heads is active at a time, i.e., reading or writing data. When not in use, the heads rest on the stationary platters, but when in motion the spinning of the platters create air pressure that lifts the heads off the platters. — The space between the platter and the head is so minute that even one dust particle or a fingerprint could disable the spin. This necessitates that hard drive assembly be done in a clean room. Terms: Head Actuator: —All the heads are attached to a single head actuator, or actuator arm, that moves the heads around the platters. Older hard drives used a stepper motor actuator, which moved the heads based on a motor reacting to stepper pulses. Each pulse moved the actuator over the platters in predefined steps. —Stepper motor actuators are not used in modern drives because they are prone to alignment problems and are highly sensitive to heat. Modern hard drives use a voice coil actuator, which controls the movement of a coil toward or away from a permanent magnet based on the amount of current flowing through it. This guidance system is called a servo.  Inside Hard disk: Beneath the Board Underneath the board are the connections for the motor that spins the platters, as well as a highly-filtered vent hole that lets internal and external air pressures equalize: The arm - This holds the The platters - These read/write heads and is controlled typically spin at 3,600 or by the mechanism in the upper-left 7,200 rpm when the drive is corner. The arm is able to move the operating. These platters heads from the hub to the edge of are manufactured to the drive. The arm on a typical amazing tolerances and are hard-disk drive can move from hub mirror-smooth to edge and back up to 50 times per second Inside Hard disk: Platters and Heads In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three platters and six read/write heads: Read and Write Mechanisms Recording and retrieval via conductive coil called a head May be single read/write head or separate ones During read/write, head is stationary, platter rotates Write — Current through coil produces magnetic field — Pulses sent to head — Magnetic pattern recorded on surface below Read (traditional) — Magnetic field moving relative to coil produces current — Coil is the same for read and write Read (contemporary) — Separate read head, close to write head — Partially shielded magneto resistive (MR) sensor — Electrical resistance depends on direction of magnetic field — High frequency operation – Higher storage density and speed Inductive Write MR Read Data Organization and Formatting Concentric rings or tracks —Gaps between tracks —Reduce gap to increase capacity —Same number of bits per track (variable packing density) —Constant angular velocity Tracks divided into sectors Minimum block size is one sector May have more than one sector per block Disk Data Layout Disk Data Layout Data is stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a track. A typical track is shown in yellow; a typical sector is shown in blue. A sector contains a fixed number of bytes -- for example, 256 or 512. Either at the drive or the operating system level, sectors are often grouped together into clusters. The process of low-level formatting a drive establishes the tracks and sectors on the platter. The starting and ending points of each sector are written onto the platter. This process prepares the drive to hold blocks of bytes. High-level formatting then writes the file-storage structures, like the file- allocation table, into the sectors. This process prepares the drive to hold files. Disk Velocity Bit near centre of rotating disk passes fixed point slower than bit on outside of disk Increase spacing between bits in different tracks Rotate disk at constant angular velocity (CAV) —Gives pie shaped sectors and concentric tracks —Individual tracks and sectors addressable —Move head to given track and wait for given sector —Waste of space on outer tracks – Lower data density Can use zones to increase capacity —Each zone has fixed bits per track —More complex circuitry Disk Layout Methods Diagram Finding Sectors Must be able to identify start of track and sector Format disk —Additional information not available to user —Marks tracks and sectors Physical Characteristics of Disk Systems Head motion —Fixed (rare) or movable head Disk portability —Removable or fixed Sides —Single or double (usually) sided Platter —Single or multiple platter Head mechanism —Contact (Floppy) —Fixed gap —Flying (Winchester) Head Motion Fixed head —One read write head per track —Heads mounted on fixed ridged arm Movable head —One read write head per side —Mounted on a movable arm Disk Portability Removable disk —Can be removed from drive and replaced with another disk —Provides unlimited storage capacity —Easy data transfer between systems Nonremovable disk —Permanently mounted in the drive Multiple Platter One head per side Heads are joined and aligned Aligned tracks on each platter form cylinders Data is striped by cylinder —reduces head movement —Increases speed (transfer rate) Multiple Platters Floppy Disk 8”, 5.25”, 3.5” Small capacity — Up to 1.44Mbyte (2.88M never popular) Slow Universal Cheap Obsolete? Detail Info: Read article “How Floppy Disk Drives Work “ Floppy Disk History of the Floppy Disk Drive The floppy disk drive (FDD) was invented at IBM by Alan Shugart in 1967. The first floppy drives used an 8-inch disk (later called a "diskette" as it got smaller), which evolved into the 5.25-inch disk that was used on the first IBM Personal Computer in August 1981. The 5.25-inch disk held 360 kilobytes compared to the 1.44 megabyte capacity of today's 3.5-inch diskette. The 5.25-inch disks were dubbed "floppy" because the diskette packaging was a very flexible plastic envelope, unlike the rigid case used to hold today's 3.5-inch diskettes. By the mid-1980s, the improved designs of the read/write heads, along with improvements in the magnetic recording media, led to the less-flexible, 3.5-inch, 1.44-megabyte (MB) capacity FDD in use today. For a few years, computers had both FDD sizes (3.5-inch and 5.25-inch). But by the mid-1990s, the 5.25-inch version had fallen out of popularity, partly because the diskette's recording surface could easily become contaminated by fingerprints through the open access area. Winchester Hard Disk (1) Hard disk drives are sometimes called Winchester drives, Winchester being the name of one of the first popular hard disk drive technologies developed by IBM in 1973. Developed by IBM in Winchester (USA) Sealed unit One or more platters (disks) Heads fly on boundary layer of air as disk spins Very small head to disk gap Getting more robust Winchester Hard Disk (2) Universal Cheap Fastest external storage Getting larger all the time —Multiple Gigabyte now usual Source: http://www.pcmag.com/encyclopedia_term/0,1237,t=Winchester+disk&i=54590,00.asp Removable Hard Disk (Storage) Removable Hard Disk (Storage) There are several reasons why removable storage is useful: — Commercial software — Making back-up copies of important information — Transporting data between two computers — Storing software and information that you don't need to access constantly — Copying information to give to someone else — Securing information that you don't want anyone else to access Removable Hard Disk (Storage) Magnetic: Zip — The main thing that separates a Zip disk from a floppy disk is the magnetic coating used. — On a Zip disk, the coating is of a much higher quality. The higher-quality coating means that the read/write head on a Zip disk can be significantly smaller than on a floppy disk (by a factor of 10 or so). The smaller head, in conjunction with a head-positioning mechanism that is similar to the one used in a hard disk, means that a Zip drive can pack thousands of tracks per inch on the disk surface. — All of these features combine to create a floppy disk that holds a huge amount of data -- up to 750 MB at the moment. Removable Hard Disk (Storage) Magnetic: Cartridges (JAZ) — Another method of using magnetic technology for removable storage is essentially taking a hard disk and putting it in a self-contained case. — One of products using this method is the Iomega Jaz. Each Jaz cartridge is basically a hard disk, with several platters, contained in a hard, plastic case. The cartridge contains neither the heads nor the motor for spinning the disk; both of these items are in the drive unit. Removable Hard Disk (Storage) Magnetic: Portable Drives — Completely external, portable hard drives are quickly becoming popular, due in a great part to USB technology. — These units, like the ones inside a typical PC, have the drive mechanism and the media all in one sealed case. The drive connects to the PC via USB cable and, after the driver software is installed the first time, is automatically listed by Windows as an available drive. All obsoleted by CD-R and CD-R/W? Speed Seek time —Moving head to correct track (Rotational) latency —Waiting for data to rotate under head Access time = Seek + Latency Transfer rate Timing of Disk I/O Transfer RAID There are many applications, particularly in a business environment, where there are needs beyond what can be fulfilled by a single hard disk, regardless of its size, performance or quality level. Many businesses can't afford to have their systems go down for even an hour in the event of a disk failure; they need large storage subsystems with capacities in the terabytes; and they want to be able to insulate themselves from hardware failures to any extent possible. RAID These situations require that the traditional "one hard disk per system" model be set aside and a new system employed. This technique is called Redundant Arrays of Inexpensive Disks or RAID. ("Inexpensive" is sometimes replaced with "Independent", but the former term is the one that was used when the term "RAID" was first coined by the researchers at the University of California at Berkeley, who first investigated the use of multiple-drive arrays in 1987.) RAID The fundamental principle behind RAID is the use of multiple hard disk drives in an array that behaves in most respects like a single large, fast one. There are a number of ways that this can be done, depending on the needs of the application, but in every case the use of multiple drives allows the resulting storage subsystem to exceed the capacity, data security, and performance of the drives that make up the system, to one extent or another. The tradeoffs--remember, there's no free lunch--are usually in cost and complexity. Why Use RAID? RAID Benefits 1. Higher Data Security: Through the use of redundancy, most RAID levels provide protection for the data stored on the array. Data on the array can withstand even the complete failure of one hard disk without any data loss. 2. Fault Tolerance: RAID implementations that include redundancy provide a much more reliable overall storage subsystem than can be achieved by a single disk 3. Improved Availability: Availability refers to access to data. Good RAID systems improve availability both by providing fault tolerance and by providing special features that allow for recovery from hardware faults without disruption 4. Improved Performance: RAID systems improve performance by allowing the controller to exploit the capabilities of multiple hard disks to get around performance-limiting mechanical issues on individual hard disks. RAID (Summary) Redundant Array of Independent Disks 7 levels (0 – 6) Not a hierarchy Set of physical disks viewed as single logical drive by O/S Data distributed across physical drives Can use redundant capacity to store parity information General RAID Concepts: Mirroring Mirroring — Mirroring is one of the two data redundancy techniques used in RAID (the other being parity). — In a RAID system using mirroring, all data in the system is written simultaneously to two hard disks instead of one; thus the "mirror" concept. — The principle behind mirroring is that this 100% data redundancy provides full protection against the failure of either of the disks containing the duplicated data. Mirroring setups always require an even number of drives for obvious reasons. — Mirroring is used in RAID 1 General RAID Concepts: Mirroring The chief advantage of mirroring is that it provides not only complete redundancy of data, but also reasonably fast recovery from a disk failure. — Since all the data is on the second drive, it is ready to use if the first one fails. Mirroring also improves some forms of read performance (though it actually hurts write performance.) The chief disadvantage of RAID 1 is expense: that data duplication means half the space in the RAID is "wasted" so you must buy twice the capacity that you want to end up with in the array. General RAID Concepts: Striping The main performance-limiting issues with disk storage relate to the slow mechanical components that are used for positioning and transferring data. Since a RAID array has many drives in it, an opportunity presents itself to improve performance by using the hardware in all these drives in parallel. For example, if we need to read a large file, instead of pulling it all from a single hard disk, it is much faster to chop it up into pieces, store some of the pieces on each of the drives in an array, and then use all the disks to read back the file when needed. This technique is called striping, after the pattern that might be visible if you could see these "chopped up pieces" on the various drives with a different color used for each file. General RAID Concepts: Striping Striping can be done at the byte level, or in blocks. Byte-level striping means that the file is broken into "byte-sized pieces" The first byte of the file is sent to the first drive, then the second to the second drive, and so on. Block-level striping means that each file is split into blocks of a certain size and those are distributed to the various drives. The size of the blocks used is also called the stripe size (or block size, or several other names), and can be selected from a variety of choices when the array is set up  General RAID Concepts: Striping Block diagram of a RAID striping configuration. One controller (which again can be hardware or software) splits files into blocks or bytes and distributes them across several hard disks. The block size determines how many "pieces" files will be split into. In this example, the first block of file 1 is sent to disk #1, then the second block to disk #2, etc. When all four disks have one block of file 1, the fifth block goes back to disk #1, and this continues until the file is completed. General RAID Concepts: Striping Striping is used in the implementation of most of the basic, single RAID levels (and by extension, any multiple RAID levels that use those single RAID levels). However, the actual way striping is set up, and how it is used, varies greatly from level to level: — RAID 0 uses block-level striping without parity; — RAID 3 and RAID 7 use byte-level striping with parity; — RAID 4, RAID 5 and RAID 6 use block-level striping with parity. Note the distinction between striping with and without parity: striping by itself involves no redundancy, and therefore, provides no data protection. General RAID Concepts: Parity Mirroring is a data redundancy technique used by some RAID levels, in particular RAID level 1, to provide data protection on a RAID array. While mirroring has some advantages and is well-suited for certain RAID implementations, it also has some limitations. It has a high overhead cost, because fully 50% of the drives in the array are reserved for duplicate data; and it doesn't improve performance as much as data striping does for many applications. For this reason, a different way of protecting data is provided as an alternate to mirroring. It involves the use of parity information, which is redundancy information calculated from the actual data values. General RAID Concepts: Parity You may have heard the term "parity" before, used in the context of system memory error detection; in fact, the parity used in RAID is very similar in concept to parity RAM. The principle behind parity is simple: take "N" pieces of data, and from them, compute an extra piece of data. Take the "N+1" pieces of data and store them on "N+1" drives. If you lose any one of the "N+1" pieces of data, you can recreate it from the "N" that remain, regardless of which piece is lost. Parity protection is used with striping, and the "N" pieces of data are typically the blocks or bytes distributed across the drives in the array. The parity information can either be stored on a separate, dedicated drive, or be mixed with the data across all the drives in the array. All of the RAID levels from RAID 3 to RAID 7 use parity; the most popular of these today is RAID 5.  RAID Level 0 Common Name(s): RAID 0. Technique(s) Used: Striping (without parity) Description: — The simplest RAID level, RAID 0 should This illustration shows how files really be called "AID", since it involves of different sizes are no redundancy. distributed between the drives — Files are broken into stripes of a size on a four-disk, 16 kiB stripe size dictated by the user-defined stripe size RAID 0 array. The red file is 4 of the array, and stripes are sent to kiB in size; the blue is 20 kiB; each disk in the array. the green is 100 kiB; and the — Giving up redundancy allows this RAID magenta is 500 kiB. level the best overall performance characteristics of the single RAID levels, especially for its cost. RAID 0 (Summary) No redundancy Data striped across all disks Round Robin striping Increase speed —Multiple data requests probably not on same disk —Disks seek in parallel —A set of data is likely to be striped across multiple disks RAID Level 1 Common Name(s): RAID 1; RAID 1 with Duplexing. Technique(s) Used: Mirroring or Duplexing Description: RAID 1 is usually implemented as mirroring; a drive has its data duplicated on two different drives using either a hardware RAID Illustration of a pair of controller or software (generally via the mirrored hard disks, operating system). If either drive fails, showing how the files are the other continues to function as a duplicated on both drives. single drive until the failed drive is replaced. RAID 1 (Summary) Mirrored Disks Data is striped across disks 2 copies of each stripe on separate disks Read from either Write to both Recovery is simple —Swap faulty disk & re-mirror —No down time Expensive RAID Level 2 Common Name(s): RAID 2. Technique(s) Used: Bit-level striping with Hamming code ECC. Description: — It is implemented by splitting data at the bit level and spreading it over a number of data disks and a number of redundancy disks. — The redundant bits are calculated using Hamming codes, a form of error correcting code (ECC). Each time something is to be written to the array these codes are calculated and written along side the data to dedicated ECC disks; when the data is read back these ECC codes are read as well to confirm that no errors have occurred since the data was written. If a single-bit error occurs, it can be corrected "on the fly". RAID 2 (Summary) Disks are synchronized Very small stripes —Often single byte/word Error correction calculated across corresponding bits on disks Multiple parity disks store Hamming code error correction in corresponding positions Lots of redundancy —Expensive —Not used  RAID Level 3 Common Name(s): RAID 3 Technique(s) Used: Byte-level striping with dedicated parity. Description: — Under RAID 3, data is striped across This illustration shows how files of multiple disks at a byte level; the exact different sizes are distributed number of bytes sent in each stripe between the drives on a four-disk, byte-striped RAID 3 array. As with varies the RAID 0 illustration, the red file — The parity information is sent to a is 4 kiB in size; the blue is 20 kiB; dedicated parity disk, but the failure of the green is 100 kiB; and the magenta is 500 kiB, with each any disk in the array can be tolerated vertical pixel representing 1 kiB of (i.e., the dedicated parity disk doesn't space. Notice that the files are represent a single point of failure in the evenly spread between three array.) drives, with the fourth containing parity information (shown in dark gray). RAID 3 (Summary) Similar to RAID 2 Only one redundant disk, no matter how large the array Simple parity bit for each set of corresponding bits Data on failed drive can be reconstructed from surviving data and parity info Very high transfer rates  RAID Level 4 Common Name(s): RAID 4 Technique(s) Used: Block-level striping with dedicated parity. Description: — RAID 4 improves performance by striping data across many disks in blocks, and provides fault tolerance through a dedicated parity disk. — It is like RAID 3 except that it uses blocks This illustration shows how files of instead of bytes for striping, and like RAID different sizes are distributed 5 except that it uses dedicated parity between the drives on a four-disk instead of distributed parity. Going from RAID 4 array using a 16 kiB stripe byte to block striping improves random size. Notice that as with RAID 3, the access performance compared to RAID 3, files are evenly spread between but the dedicated parity disk remains a three drives, with the fourth bottleneck, especially for random write containing parity information performance. RAID 4 (Summary) Each disk operates independently Good for high I/O request rate Large stripes Bit by bit parity calculated across stripes on each disk Parity stored on parity disk RAID Level 5 Common Name(s): RAID 5. Technique(s) Used: Block-level striping with distributed parity. Description: — RAID 5 stripes both data and parity information across three or more drives. It is similar to RAID 4 except that it exchanges the dedicated parity drive for a distributed parity algorithm, writing data and parity blocks across all the drives in the array. This illustration shows how files of — This removes the "bottleneck" different sizes are distributed that the dedicated parity drive between the drives on a four-disk represents RAID 5 array using a 16 kiB stripe — Fault tolerance is maintained by size. Contrast this diagram to the ensuring that the parity one for RAID 4, which is identical information for any given block of data is placed on a drive except that the data is only on separate from those used to three drives and the parity (shown store the data itself. in gray) is exclusively on the fourth.drive. RAID 5 (Summary) Like RAID 4 Parity striped across all disks Round robin allocation for parity stripe Avoids RAID 4 bottleneck at parity disk Commonly used in network servers RAID Level 6 Common Name(s): RAID 6 Technique(s) Used: Block- level striping with dual distributed parity. Description: — It stripes blocks of data and parity across an array of drives like RAID 5, except that it calculates two sets of This illustration shows how files of parity information for each different sizes are distributed between parcel of data. the drives on a four-disk RAID 6 array using a 16 kiB stripe size. This diagram is — The goal of this duplication is the same as the RAID 5 one, except that solely to improve fault you'll notice that there is now twice as tolerance; RAID 6 can handle much gray parity information, and as a the failure of any two drives result, more space taken up on the four in the array while other drives to contain the same data than the single RAID levels can handle other levels that use striping. at most one fault. RAID 6 (Summary) Two parity calculations Stored in separate blocks on different disks User requirement of N disks needs N+2 High data availability —Three disks need to fail for data loss —Significant write penalty RAID 0, 1, 2 RAID 3 & 4 RAID 5 & 6 Optical Storage CD-ROM/DVD Introduction CDs and DVDs are everywhere these days. Whether they are used to hold music, data or computer software, they have become the standard medium for distributing large quantities of information in a reliable package. Compact discs are so easy and cheap to produce that America Online sends out millions of them every year to entice new users. And if you have a computer and CD-R drive, you can create your own CDs, including any information you want. Understanding the CD: Material A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic. A cross section of a complete CD (not to scale) looks like this: Understanding the CD: Material  Understanding the CD: The Spiral & Bumps A CD has a single spiral track of data, circling from the inside of the disc to the outside. The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.)  What the CD Player Does: The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the "lands" and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes. Optical Storage CD-ROM (Summary) Originally for audio 650Mbytes giving over 70 minutes audio Polycarbonate coated with highly reflective coat, usually aluminium Data stored as pits Read by reflecting laser Constant packing density Constant linear velocity CD-ROM Drive Speeds Audio is single speed —Constant linear velocity —1.2 ms-1 —Track (spiral) is 5.27km long —Gives 4391 seconds = 73.2 minutes Other speeds are quoted as multiples e.g. 24x Quoted figure is maximum drive can achieve Random Access on CD-ROM Difficult Move head to rough position Set correct speed Read address Adjust to required location CD-ROM for & against Advantage —Large capacity (?) —Easy to mass produce —Removable —Robust Disadvantage —Expensive for small runs —Slow —Read only Other Optical Storage CD-Recordable (CD-R) —WORM (write once read many) —Now affordable —Compatible with CD-ROM drives CD-RW —Erasable —Getting cheaper —Mostly CD-ROM drive compatible —Phase change – Material has two different reflectivity in different phase states. + i) amorphous state – which reflects light poorly. + ii) crystalline state – has a smooth surface which reflects light well. DVD - what’s in a name? A DVD is very similar to a CD, but it has a much larger data capacity. A standard DVD holds about seven times more data than a CD does. Digital Video Disk —Used to indicate a player for movies – Only plays video disks Digital Versatile Disk —Used to indicate a computer drive – Will read computer disks and play video disks Reference: How DVDs Work (http://electronics.howstuffworks.com/dvd.htm) DVD - technology Multi-layer Very high capacity (4.7G per layer) Full length movie on single disk —Using MPEG compression Finally standardized (honest!) Movies carry regional coding Players only play correct region films Can be “fixed” DVD – Writable Loads of trouble with standards First generation DVD drives may not read first generation DVD-W disks First generation DVD drives may not read CD- RW disks Wait for it to settle down before buying! CD and DVD Data Storage: DVD vs. CD DVDs can store more data than CDs for a few reasons: — Higher-density data storage – Single-sided, single-layer DVDs can store about seven times more data than CDs. A large part of this increase comes from the pits and tracks being smaller on DVDs. — Less overhead, more area – On a CD, there is a lot of extra information encoded on the disc to allow for error correction -- this information is really just a repetition of information that is already on the disc. — Multi-layer storage – To increase the storage capacity even more, a DVD can have up to four layers, two on each side. The laser that reads the disc can actually focus on the second layer through the first layer. Multi-layer storage Magnetic Tape Magnetic Tape Tape drives allow large companies as well as end users to backup large amounts of data. — Tape drives are capable of backing up a couple hundred megabytes to several gigabytes of information without having to spend large sums of money on disks. While Tape Drives are cost efficient and easy to use one major disadvantage Tape Drives have is the speed which they backup and recover information. — Tape drives are Sequential access devices, which means to read any data on the Tape Drive, the Tape Drive must read all preceding data. Magnetic Tape : TAPE DRIVE STANDARDS 8mm Tape Drive - Manufactured and available through Exabyte. 8mm tapes are similar to what are used in camcorder. 8mm tapes are a faster solution then the DAT and transfer up to 6M/Sec. While the tapes are similar to camcorder tapes it is recommended that to backup information you use 8mm tapes designed for your drive. DAT (Digital Audio Tape) - Please see next lecture slide DLT (Digital Linear Tape) - DLT drives are a robust and durable medium. The DLT segments the tape into parallel horizontal tracks and records data by streaming the tape across a single stationary head. Released in 1991 DLT drives are very reliable, high- speed, and high-capacity making the DLT drives an excellent use for Network backups. Magnetic Tape (Summary) Serial access Slow Very cheap Backup and archive Digital Audio Tape (DAT) Short for Digital Audio Tape, DAT was originally developed by Hewlett Packard and Sony as a solution for backing up information on a computer and/or network. Uses rotating head (like video) High capacity on small tape — 4Gbyte uncompressed — 8Gbyte compressed Backup of PC/network servers Reading Homework http://www.pcguide.com/ref/hdd/index.htm How Hard Disks Work How Removable Storage Works How Floppy Disk Drives Work How SCSI Works