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An in-depth analysis of various disk algorithms and techniques, including scan, look, c-look, shortest positioning time first, and solid-state disks (ssd). It discusses the advantages and disadvantages of each algorithm, as well as modern disk scheduling concepts and command queuing in action.
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15-‐410, F’ 1
Dave Eckhardt & Todd Mowry Brian Railing & Steve Muckle Contribu=ons from n Eno Thereska, Rahul Iyer n 15-‐ n “How Stuff Works” web site L23_Disks
15-‐410, F’ 2
Anatomy of a Hard Drive Common Disk Scheduling Algorithms A brief men=on of SSD's
15-‐410, F’ 4 If we take the cover off, we see that there actually is a “hard disk” inside Taken from “How Hard Disks Work” hCp://computer.howstuffworks.com/hard-‐disk2.htm
15-‐410, F’ 5 These spin at thousands of RPM (5400, 7200, 10000, …) Slower disks use less power Taken from “How Hard Disks Work” hCp://computer.howstuffworks.com/hard-‐disk2.htm A hard drive usually contains mul=ple disks, called pla5ers
15-‐410, F’ 7 The arm is moved by a voice coil actuator Slow, as computers go n Accelera=on =me n Travel =me Taken from “Hard Disk Drives” hCp://www.pcguide.com/ref/hdd Oklobdzija, Comp. Eng. Handbook, 2002
15-‐410, F’ 8 Both sides of each plaXer store informa=on Each side of a plaXer is called a surface Each surface has its own read/write head Taken from “How Hard Disks Work” hCp://computer.howstuffworks.com/hard-‐disk2.htm
15-‐410, F’ 10 Each surface is divided by concentric circles, crea=ng tracks tracks
15-‐410, F’ 11 These tracks are further divided into sectors
15-‐410, F’ 13
Modern hard drives use zoned bit recording n Disk has tables to map track# to #sectors n Sectors are all roughly the same linear length n Some old low-‐level code s=ll thinks of “C/H/S addressing”, but that's not what actually happens (for more than a decade!) n “sector address” names a sector like “page” names a frame Taken from “Reference Guide – Hard Disk Drives” hCp://www.storagereview.com/map/lm.cgi/zone
15-‐410, F’ 14 Let's read in a sector from the disk read/write head desired sector disk rotates counter-‐clockwise
15-‐410, F’ 16 Let's read in a sector from the disk read/write head
15-‐410, F’ 17 Let's read in a sector from the disk read/write head
15-‐410, F’ 19 Let's read in a sector from the disk read/write head
15-‐410, F’ 20 Let's read in a sector from the disk read/write head
