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: 1.Tracks  Same with as the head. 2.Intertrack Gaps 3.Sectors  Fixed-length (512 bytes) is commonly used in industry. 4.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 disk with 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/RAIDhttp://en.wikipedia.org/wiki/RAID