1. 13.2 Disks 13.2.1 Mechanics of Disks Presented by Chao-Hsin Shih Feb 21, 2011

2. Mechanics of Disks Two principles moving piece of a disk driven – Disk assembly – consists of one or more circular platters 0’s and 1’s are represented by different patterns in the magnetic material Diameter: 3.5 inches – Head assembly

3. Components of disk Tracks - organize the disk. ( concentric circle on a single platter) Sectors – organize the track. Indivisible unit. Gaps - represent about 10% of the total track and are used to identify the beginnings of sectors. Blocks – logical units of data that are transferred between disk and main memory. Consists of one ore more sectors. (13.1.2)

4. Head Assembly Holds the disk heads. One head for each surface. Head crash Read/Alter the magnetism Attached to an arm. Move in and out together.

5. Megatron 747 disks Characteristics: – 8 platters providing 16 surfaces – 2 16 tracks per surface – 2 8 sectors per track – 2 12 bytes per sector Capacity: – 16(surface) * 2 16 (track) * 2 8 (sector) * 2 12 (byte) = 2 40 (byte) = terabyte Disk access: – 1 track holds 2 8 * 2 12 bytes (megabyte) – If blocks are 2 14 bytes, 2 8 * 2 12 / 2 14 = 32 blocks on a track

6. Thank you!

7. The Disk Controller A small processor capable of: – Control mechanical actuator: move head assembly – Select a sector in the cylinder at which the heads are positioned – Transfer bits – Buffer track in local memory

8. Disk Accessing Characteristics Three steps to access a block: – Disk controller positions the head assembly Seek Time – Disk controller wait while the first sector of the block moves under the head Rotational latency – All the sectors and the gaps pass under the head while the disk controller read/write data Transfer time Latency : seek time + rotational latency + transfer time (avg: 10ms, maximum: 20ms)

9. Data reading time The time takes to read a 16384 byte from Megatron 747 disk. Time properties of the disk: – Disk rotate: 7200rpm ( 8.33 ms/rotation) – start/stop: 1ms, 4000 cylinders/ms The head move 1 track in 1.00025ms, innermost to outermost track: