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Chapter 6

Chapter 6. External Memory Disk and RAID (Redundant Arrays of Independent Disks) CS-147 Fall 2010 Jonathan Wang. Magnetic Disk. Coated with magnetizable material for read and write purpose. The substrat used to be aluminum. Recently use glass. Better stiffness

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Chapter 6

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  1. Chapter 6 External Memory Disk and RAID (Redundant Arrays of Independent Disks) CS-147 Fall 2010 Jonathan Wang

  2. Magnetic Disk • Coated with magnetizable material for read and write purpose. • The substrat used to be aluminum. • Recently use glass. • Better stiffness • Greater shock/damage resistance • Lower fly height • Improved uniformity of surface helps to reduced read-write errors

  3. Magnetic Write and Read Mechanism • Head: • Fixed head • One read-write head per track • Heads build into a fixed ridged arm • Movable head • One read-write head per surface • Build into a movable arm When the track passes under the head, it generates a current of the same polarity as the one already recorded.

  4. Disk Data Layout • Contains: • Tracks • Same with as the head. • Intertrack Gaps • Sectors • Fixed-length (512 bytes) is commonly used in industry. • InterSector Gaps Gaps are there to minimize errors due to misalignment of head or interference of magnetic field.

  5. Disk Layout Methods • CAV – Constant Angular Velocity • Multiple Zone Recording: to enhance density(capacity)

  6. Characteristics • Movable Head or not • Removability • Provides unlimited storage capacity • Easy data transfer between systems • Multiple Platter • Single or double sided.

  7. Disk Performance Parameters • Seek Time : time to position the head at the track • Rotational Delay : The time it takes for the begining of the sector to reach the head • Transfer Time : time required for the transfer T = Transfer time b = Number of bytes to be transfered N = Number of bytes on a track r = rotation speed in rev/sec • Units usually is in ms, and considered average case

  8. RAID • Stand for Redundant Arrays of Independent Disks • RAID is a set of physical disk drives viewed by the perating system as a single logical drive • Data are distributed across the physical drives of array in ascheme known as striping, describes subseuently. • Redundant disk capacity is used to store parity information, which quarantees data recoverability in case of a disk failure. • Uses Array Management Software • Level 0 ~ 6 and more, such as RAID 10 (a combination of RAID 0 and RAID 1)

  9. RAID Level 0 • Not a true member of RAID family • No redundancy or fault tolerance • High transfer capacity for large and small I/O data • It's there because it distrbites datas across mutiple disks • No parity coculation is needed • Easy to implement

  10. RAID Level 0 • In a transaction environment, there may be hundreds of I/O requests per second. A disk array can provide high I/O executtion rates by balancing the I/O load across mutiple disks. • Parallel processing • Any error is uncorrectable • One disk's failure will result in all data in an array being lost

  11. RAID Level 1 • Redundancy is achieved by having a mirror disk • Insufficient use of space • Read request is really efficiency (the one involves minimum seek time plus rotational latency) • Write request could be done parallelly (T = the larger one) • Recovery is really simple.Just replace the broken diskwith a new one

  12. RAID Comparison – see Table 6.4 p.204

  13. Reference • Text Book - Computer Organization and Architecture: Designing for Performance, 8th Edition By William Stallings, Prentice Hall • Wikipedia http://en.wikipedia.org/wiki/RAID

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