Section 13.2 – Secondary storage management (Former Student’s Note)

Index 13.2 Disks 13.2.1 Mechanics of Disks 13.2.2 The Disk Controller 13.2.3 Disk Access Characteristics

A five-level memory hierarchy. Memory Hierarchies A five-level memory hierarchy. Tanenbaum, Structured Computer Organization, Fifth Edition, (c) 2006 Pearson Education, Inc. All rights reserved. 0-13-148521-0

Disks: The use of secondary storage is one of the important characteristics of a DBMS, and secondary storage is almost exclusively based on magnetic disks: Use Example 13.1

A disk with four platters. Magnetic Disks (2) A disk with four platters. Tanenbaum, Structured Computer Organization, Fifth Edition, (c) 2006 Pearson Education, Inc. All rights reserved. 0-13-148521-0

Structure of a Disk

Data in Disk 0’s and 1’s are represented by different patterns in the magnetic material. A common diameter for the disk platters is 3.5 inches.

Mechanics of Disks Two principal moving pieces of hard drive 1- Head Assembly 2- Disk Assembly Disk Assembly has 1 or more circular platters that rotate around a central spindle. Platters are covered with thin magnetic material

A disk with five zones. Each zone has many tracks. Magnetic Disks (3) A disk with five zones. Each zone has many tracks. Tanenbaum, Structured Computer Organization, Fifth Edition, (c) 2006 Pearson Education, Inc. All rights reserved. 0-13-148521-0

A portion of a disk track. Two sectors are illustrated. Magnetic Disks (1) A portion of a disk track. Two sectors are illustrated. Tanenbaum, Structured Computer Organization, Fifth Edition, (c) 2006 Pearson Education, Inc. All rights reserved. 0-13-148521-0

Top View of Disk Surface

Mechanics of Disks Tracks are concentric circles on a platter. Tracks are organized into sectors which are segments of circular platter. Sectors are indivisible as far as errors are concerned. Blocks are logical data transfer units.

Disk Controller Control the actuator to move head assembly Selecting the surface from which to read or write Transfer bits from desired sector to main memory

Simple Single Processor Computer

Disk Access characteristics Seek time: The disk controller positions the head assembly at the cylinder containing the track on which the block is located. The time to do so is the seek time. The force to move disk controller is about 5g (gravitational force) Rotational latency: The disk controller waits while the first sector of the block moves under the head. This time is called the rotational latency.

Disk Access characteristics Transfer time: All the sectors and the gaps between them pass under the head, while the disk controller reads or writes data in these sectors. This delay is called the transfer time. Latency of the disk: The sum of the seek time, rotational latency, transfer time is the latency of the time.

Disks: Example 13.1 Megatron 747 16 surfaces 216 tracks per surfaces 28 (average) sectors per track 212 bytes per sector 1 mega bytes per track

Disks: Example 13.1 Megatron 747 Disk If blocks are 214 bytes or 16384 bytes, then one block uses 4 consecutive sectors, and there are (on the average) 32 blocks per track.

Disks: Example 13.2 The disk rotates at 7200 rpm; i.e., it makes one rotation in 8.33 millisecond. To move the head assembly between cylinders take one millisecond to start and stop. Plus one millisecond for every 4000 cylinders traveled. Thus the head moves one track in 1.00025

Disks: Example 13.2 milliseconds and move from the innermost to outermost track, a distance of 65, 536 tracks, in about 17.38 millisecond Gaps occupy 10% of the space around a track. Take two rotations to read whole track

Quizz_9_8 16 surfaces 216 tracks per surfaces 28 (average) sectors per track 212 bytes per sector 1 mega bytes per track If blocks are 214 bytes or 16384 bytes, then one block uses 4 consecutive sectors, and there are (on the average) 32 blocks per track. The disk rotates at 7200 rpm; i.e., it makes one rotation in 8.33 millisecond. To move the head assembly between cylinders take one millisecond to start and stop. Plus one millisecond for every 4000 cylinders traveled. Thus the head moves one track in 1.00025 milliseconds and move from the innermost to outermost track, a distance of 65, 536 tracks, in about 17.38 millisecond Gaps occupy 10% of the space around a track. Take two rotations to read whole track