1. Disks and Files DBMS stores information on (“hard”) disks.
This has major implications for DBMS design!
READ: transfer data from disk to main memory (RAM).
WRITE: transfer data from RAM to disk.
Both are high-cost operations, relative to in-memory operations, so must be planned carefully!

2. Why Not Store Everything in Main Memory?
Costs too much. $100 will buy you either 512MB of RAM or 50GB of disk today --- that is disk storage 100 times cheaper (but it is approx times slower).
Main memory is volatile. We want data to be saved between runs. (Obviously!)
Typical storage hierarchy:
Main memory (RAM) for currently used data.
Disk for the main database (secondary storage).
Tapes for archiving older versions of the data (tertiary storage). 19

3. Components of a Disk The platters spin (say, 90rps).
Spindle
Tracks
Disk head
The platters spin (say, 90rps).
The arm assembly is moved in or out to position a head on a desired track. Tracks under heads make a cylinder (imaginary!).
Sector
Arm movement
Platters
Only one head reads/writes at any one time.
Arm assembly
Block size is a multiple of sector size (which is fixed). 21

4. Accessing a Disk Page Time to access (read/write) a disk block:
seek time (moving arms to position disk head on track)
rotational delay (waiting for block to rotate under head)
transfer time (actually moving data to/from disk surface)
Seek time and rotational delay dominate.
Seek time varies from about 1 to 20msec
Rotational delay varies from 0 to 10msec
Transfer rate is about 1msec per 32KB page 22

5. State of the Art: Barracuda 180
181.6 GB, 3.5 inch disk
12 platters, 24 surfaces
24,247 cylinders
7,200 RPM; (4.2 ms avg. latency)
7.4/8.2 ms avg. seek (r/w)
64 to 35 MB/s (internal)
0.1 ms controller time
10.3 watts (idle)
Track
Sector
Cylinder
Track Buffer
Arm
Platter
Head
Latency =
Queuing Time +
Controller time +
Seek Time + Rotation Time +
Size / Bandwidth
per access
per byte { +
47 to 26 MB/s (external)
source:

6. Disk Performance
Calculate time to read 64 KB (128 sectors) for Barracuda 180 X using advertised performance; sector is on outer track
Disk latency = average seek time + average rotational delay + transfer time + controller overhead
= 7.4 ms * 1/(7200 RPM) KB / (65 MB/s) ms
= 7.4 ms /(7200 RPM/(60000ms/M)) KB / (65 KB/ms) ms
= ms = 12.7 ms

7. 1 inch disk drive! 2000 IBM MicroDrive: 2006 MicroDrive?
1.7” x 1.4” x 0.2”
1 GB, 3600 RPM, 5 MB/s, 15 ms seek
Digital camera, PalmPC?
2006 MicroDrive?
9 GB, 50 MB/s!
Assuming it finds a niche in a successful product
Assuming past trends continue

8. Disk Characteristics in 2000
$828
$447
$435

9. Disk Characteristics in 2000

10. Disk Characteristics in 2000

11. Disk Characteristics in 2000