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Group 6 Presentation

Group 6 Presentation. Andres Reyes Jesus Amundarain Alberto Lacaci Shawn Gunness Andres Marcial Stephen Creaville Jimmy Jean-Paul. Magnetic Disk. The primary computer storage device It consists of magnetically coated disks Glass is the used as the substrate. Advantages:

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Group 6 Presentation

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  1. Group 6 Presentation Andres ReyesJesus Amundarain Alberto Lacaci Shawn Gunness Andres Marcial Stephen Creaville Jimmy Jean-Paul

  2. Magnetic Disk

  3. The primary computer storage device • It consists of magnetically coated disks • Glass is the used as the substrate. • Advantages: • Increases reliability • Reduces surface defects • More resistance to damage • Better stiffness Magnetic Disk

  4. Read and Write Mechanisms • For writing, the surface of the disk or tape is moved past the read/write head. By discharging electrical impulses at the appropriate times, bits are recorded as tiny, magnetized spots of positive or negative polarity. • For reading, the surface is moved past the read/write head, and the bits that are present induce an electrical current across the gap.

  5. Read (contemporary) • Most modern disk drives use magneto resistive sensor for reading, but use inductive coils for writing. • High frequency operation • Higher storage and speed

  6. Tracks and SpotsThe disk surface is divided into concentric tracks (circles within circles). The thinner the tracks, the more storage. The data bits are recorded as tiny magnetic spots on the tracks. The smaller the spot, the more bits per inch and the greater the storage. • SectorsTracks are further divided into sectors, which hold a block of data that is read or written at one time; for example, READ SECTOR 782, WRITE SECTOR 5448. In order to update the disk, one or more sectors are read into the computer, changed and written back to disk. Data Organization and Formatting

  7. Track and Sectors

  8. The bit located near centre of rotating disk passes fixed point slower than the bit on the outside of the disk • Constant angular velocity (CAV) keeps the disk spinning at a fixed rate. This means the heads cover more distance per unit of time on the outer tracks than on the inner tracks. • Individual tracks and sectors addressable • Lower data density • Can use zones to increase capacity • Each zone has fixed bits per track Disk Velocity

  9. Finding Sectors • Find start of track and sector • Format disk • Marks tracks and sectors

  10. An early removable disk drive from IBM that put the heads and platters (disks) in a sealed unit for greater speed. • The drive had one permanent and one removable spindle, each holding 30MB. Winchester Disk Format

  11. 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 • 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 Characteristics

  12. 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 Platter

  13. A floppy disk is a data storage medium that is composed of a disk of thin, flexible ("floppy") magnetic storage medium encased in a square or rectangular plastic shell. • Floppy disks in 8 inch, 5¼ inch and 3½ inch. • Small capacity • Up to 1.44Mbyte (2.88M never popular) Floppy Disk

  14. Seek time • Moving head to correct track • (Rotational) latency • Waiting for data to rotate under head • Access time = Seek + Latency • Transfer rate Speed

  15. Raid system • Drive arrays are ways to share resources • The most common drive arrays are RAID • RAID stands for Redundant Arrays of Inexpensive Disks

  16. RAID 0 Stripped Disk Array without Fault Tolerance (Nonredundant) Does not include redundancy to improve performance Increase probability that requests of data being issued of different blocks could be issued on parallel, reducing the I/O queuing time. RAID level 0 can be implemented for two situations: High Data Transfer Capacity and High I/O Request Rate

  17. RAID 1 Mirroring and Duplexing (Mirrored) Redundancy is achieved by duplicating all the data A read request can be serviced by either of the two disks. The controller reads alternate sectors from each drive. Time is effectively reduced the data read time. Write time is dictated by the slower of the two writes Recovery from failure is achieved accessing the second drive Main disadvantage is cost.

  18. RAID 2 Data Striping with Error Recovery Uses multiple disks for error- detection and correction functions The number of redundant disks is proportional to the log of the number of data disks. A complex error-detection and correction algorithm is employed. Big hardware overhead, thus is not normally used with microcomputer systems

  19. RAID 3Parallel Transfer with Parity Striping (Bit-interleaved parity) Same organization as RAID 2 but only a single redundant disk is required, independently of the disk array size A redundancy is added just in case of a disk failure so data lost can be reconstructed using the parity disk Because data are striped in very small strips, RAID 3 can achieve very high data transfer rates Only one I/O request can be executed at a time. Thus, in a transaction-oriented environment, performance suffers If an error occurs, the controller reads the array again to verify the error. This is a time-consuming low-efficiency method of error correction

  20. RAID 4 Independent Data Disks with Shared Parity Disk (Block-level parity) Controller interleaves sectors across the drives in the array Independent access technique for high I/O request rates I/O requests satisfied in parallel Only one parity-checking drive is allotted for error control

  21. XOR properties: • A xor A = 0 • 1 xor 1 = 0 • 1 xor 0 = 1 • 0 xor 0 = 0 • Example: 1110 xor 0111 = 1001 • In general, A xor B xor C xor D xor E is TRUE if an ODD number of variables are TRUE • XOR is used in RAID 3-6 for creating parity information. • It is possible to bake up information like two bytes, 10011100 and 01101100 coming from different hard drives if we xor them and store the result in another hard drive Bitwise operation

  22. Suppose the following hard drives in a RAID 3-6 scheme: • X3(i) = parity drive; X2(i) = 10011100, and X1(i) = 01101100, data drives, xored together • X3(i) = X2(i) xor X1(i) • 11110000 = 10011100 xor 01101100 • In the event of a failure, for example X1(i) fails, then is possible to recover the information adding • X1(i) xor X3(i) to the equation as follows • X3(i) xor X1(i) xor X3(i) = X2(i) xor X1(i) xor X1(i) xor X3(i) • Because A xor A =0 we can eliminate X3(i) in the left member and X1(i) in the right member, leaving the equation as: • X1(i) = X2(i) xor X3(i) • Substituting values • X1(i) = 10011100 xor 11110000 • X1(i) = 01101100, so the data has been recovered. Data Recovery

  23. RAID 5 Independent Data Disks with Distributed Parity Blocks Similar organization as RAID 4 Alters the RAID 4 specification by allowing the parity function to rotate through the different drives Error checking an correction are the function of all the drives The distribution of parity strips across all drives avoids the potential I/O bottleneck found in RAID 4 Very popular because it can be used on small arrays, and it has a high level of error recovery built in.

  24. RAID 6 Independent Data Disks with Two Independent Distributed Parity (Dual Redundancy) Parity is calculated and stored in separate blocks and different disks Requires at least two additional drives to operate Example: if a data user needs N disks, RAID 6 array will be N+2 disks Relatively expensive, but it provides an extremely high fault tolerance level for critical applications

  25. Raid comparison

  26. Raid comparison (continued)

  27. CD-ROM • CD-Recordable (CD-R) • CD-R/W • DVD Optical Memory

  28. Originally used audio recording • 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 Optical Storage CD-ROM

  29. CD Operation

  30. Audio is single speed • Constant linier velocity • Other speeds are quoted as multiples • e.g. 48x • Quoted figure is maximum drive can achieve CD-ROM Drive Speeds

  31. CD-ROM Format • Mode 0=blank data field • Mode 1=2048 byte data+error correction • Mode 2=2336 byte data

  32. Random Access on CD-ROM • Difficult • Does not go to exact position desired • Read Random address is difficult With the use of Constant Linear Velocity

  33. FOR: • Relatively Large capacity • Easy to mass produce • Removable • Robust/Durable • AGAINST: • Slow • Read only (cannot be updated) • Access time is longer than Mag. Disk Drive CD-ROM

  34. CD-Recordable (CD-R) • WORM (WRITE ONCE READ MULTIPLE) • Now affordable • Compatible with CD-ROM drives • CD-RW • Erasable • Getting cheaper • Mostly CD-ROM drive compatible • Phase change • Material has two different reflectivities in different phase states Other Optical StorageAdvantages

  35. Digital Video Disk • Used to indicate a player for movies • Only plays video disks (however could be used for any storage application) • Digital Versatile Disk • Used to indicate a computer drive • Will read computer disks and play video disks DVD

  36. Multi-layer • Very high capacity (4.7G per layer) • Full length movie on single disk • Using MPEG compression • Movies carry regional coding DVD - technology

  37. Magnetic Tape

  38. Magnetic Tape The first kind of secondary memory was invented for recording sound by Fritz Pfleumer in 1928 in Germany. Magnetic recording is used for audio, video, and computer data storage. Tape systems use the same reading and recording techniques as disk systems. Data on the tape are structured as a number of parallel tracks running length wise. Early tape systems used nine tracks, which made it possible to store data one byte at a time, with an additional parity bit as the ninth track. Later tape systems used 18 or 36 tracks. Parallel recording is recording data in digital word or double word. Serial recording is data laid out as sequence of bits along each track. The recording technique used in serial tapes is referred to as serpentine recording. In this technique, when data are being recorded, the first set of bits is recorded along the whole length of the tape. When the end of the tape is reached, the heads are repositioned to record a new track, and the tape is again recorded on its whole length, this time in the opposite direction. Magnetic tape have been used in different formats such as open reels, UNIVAC, DEC, Cartridges, IBM formats and cassettes. Linear Tape-Open (LTO) Tape Drives Developed late 1990s Open source alternative to proprietary tape systems As of 2008, the highest capacity tape cartridges can store 1 TB of uncompressed data. (Sun Storage Tek T1000B & IBM TS 1130)

  39. Main advantages Relatively cheap Can hold in excess of 1 TB of data (it is possible to get devices that will back up over 5 TB of data) Backup capacity is easily expanded by simply using more tape. Main disadvantages Serial access - this means that in order to get to something on the tape, you have to go through everything that comes before it.  Think of it like a video tape.  The program that you want to watch might have been recorded half way through the tape, but you have to wind forward right from the start to get to it. Slow - Because of serial access, it is relatively slow to find the data that you need (however in an exam question, don't just say 'slow', compare it to a storage device with faster access. Specialist hardware is needed to read the tapes.  Most standard PCs do not come with the hardware required to use them. Not suitable for heavy use - As you may realize from video tapes at home - they are a bit fragile with a tendency to stretch and tangle!  Tape is best suited for back-up purposes. Advantages & Disadvantages

  40. LTO-1 LTO-2 LTO-3 LTO-4 LTO-5 LTO-6 Release date 2000 2003 2005 2007 TBA TBA Compressed capacity 200 GB 400 GB 800 GB 1600 GB 3.2 TB 6.4 TB Compressed transfer rate (MB/s) 40 80 160 240 360 540 Linear density (bits/mm) 4880 7398 9638 13300 Tape tracks 384 512 704 896 Tape length 609 m 609 m 680 m 820 m Tape width (cm) 1.27 1.27 1.27 1.27 Write elements 8 8 16 16 Linear Tape-Open (LTO) Tape Drives

  41. Magnetic Disk: • - What are the advantages of using glass as a substrate? • What is the differences between latency and seek time? • RAID: • - What is RAID and how many types are there? • What is the importance of parity bit? • Optical Memory: • -Name four types of optical memory? • -With the use of Constant Linear Velocity is it easy or difficult to access random data on CD-ROM? • Magnetic Tape: • - What are the main advantages of magnetic tape? • What are magnetic recording used for? Review Questions

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