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This presentation delves into the characteristics and components of modern disk drives, exploring recording and positioning mechanisms, disk controllers, and simulation models for performance improvement. It discusses factors influencing disk drive performance and highlights the importance of creating accurate disk drive models. The presentation covers key concepts such as recording components, disk positioning, seek time optimization, track following, data layout strategies, and disk controller functions like caching and command queuing. By examining the intricacies of disk drive technology, this presentation provides valuable insights for understanding and improving disk drive performance.
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Paper by: Chris Ruemmler and John Wikes Presentation by: Timothy Goldberg, Daniel Sink, Erin Collins, and Tony Luaders
Introduction • Disk Drive performance improvements at 7-10% • Compared to microprocessors at 40-60% or disk storage capacities at 60-80% (annually) • Simulation models to compare alternative approaches • High quality disk drive model • Error factor 14 times smaller
Outline • Introduction • Characteristics of Modern Disk Drives • Recording Components • Positioning Components • Disk Controller • Modeling Disk Drives
Characteristics of Modern Disk • Non-removable magnetic disk drives • Contain a mechanism and controller • Recording Components: rotation disks and heads • Positioning Components: moves heads into correct position with track-following system • Emphasis on features that could be important when creating a disk drive model
Recording Components • Smaller disks: • Less surface area for data • Less power consumption • Can spin faster • Smaller seek distances • Increased storage density: • Better linear recording density, maximum rate of flux changes • Packing separate tracks of data more closely together • May contain from 1 to 12 platters • Stack rotates in lockstep
Recording Components • Spindle rotation speed: • Higher spin speed increases transfer rates, shortens rotation latencies • Higher power consumption, requires better bearings • Each platter surface has a disk head • Responsible for recording (writing) • And sensing (reading) magnetic flux variation • Single Read-Write data channel • Can be switched between the heads • Responsible for encoding and decoding data stream into or from a series of magnetic phase changes stored on the disk
Positioning Components • Data surfaces are set up to store data in tracks • Modern disks have about 2,000 cylinders and are 3.5 inches. • Cylinder is a single stack of tracks at a common distance from the spindle • To access the data stored on a track, the disk arms must rotate all the disks to get the desired track to the disk head. • This system ensures that the track is reached even with interruptions • External vibrations, shocks, and disk flaws (non circular tracks)
Seeking • The speed of head movement • Faster seeking requires more power • Half the seek time requires 4x power • Seek is composed of: • Speedup (arm moves until at half seek distance) • Coast (for long seeks, max velocity) • Slowdown (rest close to desired track) • Settle (puts disk head on desired location)
Track Following • Fine-tuning the head position at the end of the seek and keeping the head on the desired track • Determines if head is correctly aligned by using positioning information on the disk at manufacturing time • Performs head switches • When the controller switches its data channel from one surface to the next in the same cylinder
Data layout • A disk appears to its client computer as a linear vector of addressable blocks which are mapped to physical sectors on the disk. • Using this method, the disk can hide bad sectors and do low-level performance optimizations. • Zoning: tracks are longer at the outside of a platter than at the inside. • Maximize storage capacity • Track skewing: faster sequential access across track boundaries • Allows data to be read or written at nearly full media speed • Sparing: stores a list of flaws in the desk surface to be skipped
The Disk Controller • Mediates access to the mechanism • Runs the track-following system • Transfers data between the disk drive and the client • Manages an embedded cache
caching of requests • Speed-matching buffer can be extended to include some form of caching for both reads and writes. • Caches in disk drives are relatively small because of space limitations. • Read-ahead: faster than seeking if the cache gets a hit • Write caching: saves cache information • Cache is volatile, losing its contents if power to the drive is lost • Command queuing: allows for multiple outstanding requests at the same time • Disk controller determines the best execution order, subject to additional host constraints.