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ch11 - Chapter11:StorageandFile Structure n n HardDisks(why...

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Chapter 11:  Storage and File  Structure n Overview of Physical Storage Media n Hard Disks (why?) n How it works n How to measure the performance n RAID
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Motivation n Suppose you are a DBA. Your database  applications are running very slow. What will  you do given a fixed budget? n Buy more CPU n Buy a CPU with more cache n Buy more memory n Buy larger disks n Buy faster disks n Buy more disks n Buy nothing, but do some reorganization
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Types of Storage Media CPU Cache Memory Hard disks  Where is database stored?     Why?   For the following SQL Select avg(salary) from emp What data need to be brought  into memory, cache, and CPU? The hope is frequently used data & program are kept in higher level  storage Price goes down  from top to bottom Capacity goes up Speed goes down Only hard disk is  non volatile  (meaning it  survives power off)
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Magnetic Hard Disk  Mechanism NOTE: Diagram is schematic, and simplifies the structure of actual disk drives
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Magnetic Disks n Data stored on the surface of a number of platters n Surface of platter divided into circular  tracks n Each track is divided into  sectors .    n A sector is the smallest unit of data that can be read or written.  n Sector size typically 512 bytes n A cylinder consists of the same tracks in all platter Platter Track 1 Track 2 Sector
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Magnetic Disks n Devices for read/write n One read head and one write head per platter, positioned  very close to the platter surface n Disk arm: read/write heads are attached to disk arm. One  arm per head.  n Arm assembly: all arms are mounted on the assembly n All arms move together, in left-right Arm Assembly Arm Arm Read  head Write head Platter
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Magnetic Disks n To read/write a sector of data n Disk arms move to the correct tracks  (since all arms move together, actually the same tracks from  all platter are reached) n Platter spins to the correct sector n Read/write whole sector Track 1 Track 2 Sector Arm at track 1
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Magnetic Disks n
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