11-08-2012, 03:19 PM
ABCs of Disk Drives
ABCs of Disk Drives.ppt (Size: 1.35 MB / Downloads: 303)
HDD Organization
Typical configurations seen in disks today
Platter diameters: 3.7”, 3.3”, 2.6”
RPMs: 5400, 7200, 10000, 15000
0.5-1% variation in the RPM during operation
Number of platters: 1-5
Mobile disks can be as small as 0.75”
Power proportional to: (# Platters)*(RPM)2.8(Diameter)4.6
Tradeoff in the drive-design
Read/write head
Reading – Faraday’s Law
Writing – Magnetic Induction
Data-channel
Encoding/decoding of data to/from magnetic phase changes
Disk Medium Materials
Aluminum with a deposit of magnetic material
Some disks also use glass platters
Eg. Newer IBM/Hitachi products
Better surface uniformity and stiffness but harder to deposit magnetic material
Anti-Ferromagnetically Coupled media
Uses two magnetic layers of opposite polarity to reinforce the orientation.
Can provide higher densities but at higher manufacturing complexity
Storage Density
Determines both capacity and performance
Density Metrics
Linear density (Bits/inch or BPI)
Track density (Tracks/inch or TPI)
Areal Density = BPIxTPI
Head Switch
Process of switching the data channel from one surface to the next in the same cylinder
Vertical alignment of cylinders difficult at high TPI
Head might need to be repositioned during the switch
Can be one-third to a half of the settle-time
Sparing
There can be defective sectors during the manufacture of disks
References to them are remapped to other sectors
Slip sparing
References to flawed sectors are slipped by a sector/track
Stroke efficiency
Fraction of the overall disk capacity that is not used for sparing, recalibration tracks, head landing-zones etc.
Around 2/3 for modern disks