Modern Disk Drives: Components and Performance Analysis

1. Paper by: Chris Ruemmler and John Wikes Presentation by: Timothy Goldberg, Daniel Sink, Erin Collins, and Tony Luaders

2. 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

3. Outline • Introduction • Characteristics of Modern Disk Drives • Recording Components • Positioning Components • Disk Controller • Modeling Disk Drives

4. 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

5. 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

6. 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

7. Disk Drive

8. 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)

9. 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)

10. 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

11. 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

12. 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

13. 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.