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 Greater shock/damage resistance Lower fly height Improved uniformity of surface helps to

reduced read-write errors

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 armWhen the track passes under the head, it generates a current of the same polarity as the one already recorded.

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.

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

Characteristics Movable Head or not Removability

Provides unlimited storage capacity

Easy data transfer between systems

Multiple Platter Single or double sided.

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

T=b /rN

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)

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

RAID Level 0In 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

RAID Level 1Redundancy 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

RAID

Comparison – see Table 6.4 p.204

Reference

Text Book - Computer Organization and Architecture: Designing for Performance, 8th Edition By William Stallings, Prentice Hall

Wikipedia http://en.wikipedia.org/wiki/RAID