An Introduction to Disk Drive Modeling Chris Ruemmler & John Wilkes Hewlett-Packard Laboratories

1. An Introduction to Disk Drive ModelingChris Ruemmler & John WilkesHewlett-Packard Laboratories Presented by Hang Zhao

2. The earliest hard disks… • First Hard Disk (1956): IBM's RAMAC, capacity is 5 MB, stored on 50 24" disks • First Air Bearing Heads (1962): IBM's model 1301 increases both areal density and throughput by about 1000% • First Removable Disk Drive (1965): IBM's model 2310 • ……

3. A brief history of hard disk drive • Engineers over the last few decades have done at improving them in every respect: reliability, capacity, speed, power usage, and more.

4. Evolution of areal density of hard disk platters • The areal density of hard disk platters continues to increase at an amazing rate even exceeding some of the optimistic predictions of a few years ago. Modern disks are now packing as much as 20 GB of data onto a single 3.5" platter!

5. Why do we need to model disk drive behavior? • CPU technology is advancing rapidly; while the overall system behavior is restrict to disk system performance. • The behavior of disk drive itself is a dominant factor in overall I/O performance. • Existing hard disk models have limitations…

6. Mechanism recording component positioning component Controller microprocessor buffer memory interface to SCSI bus Characteristics of modern disk drives

7. Inner Track Arm Outer Track Sector Head Platter Platter Actuator Disk Drive Terminology • Several platters, with information recorded magnetically on both surfaces (usually) • Bits recorded in tracks, which in turn divided into sectors • Actuator moves head (end of arm,1/surface) over track (seek), select surface, wait for sector rotate under head, then read or write • Cylinder: all tracks under heads

8. The recording components • Modern disks range in size from 1.3 to 8 inches in diameter: • Smaller disks VS larger disks • less surface area/storage • consume less power • spin faster • smaller seek distance

9. The increased storage density The incremental trends result from • better linear recording density: a measure of how tightly the bits are packed within a length of track. (50,000 BPI 1994; 524,000 BPI 2000’) • packing separate tracks more closely together (20,000 TPI; 67,300 TPI around 2000’)

10. Platters and disk rotation • Platters rotates in lockstep on a central spindle at rates varying from 3,600 to 7,200 rpm • Higher spin rate increases transfer rates and shortens rotation latencies; on the other hand, power consumption increases • Each platter surface is associated with a disk head for writing and reading operating under a single read-write channel

11. The positioning components • Seeking: speed of head movement, limited by the power available for pivot and the arm’s stiffness. Seek time is composed of: • speedup • coast: for long seeks where arms move at maxυ • slowdown • settle: dominant factor of very short seeks

12. The demerit of “average” seek time • “Average” seek times are commonly used as a figure of merit for disk drives, but they can be misleading. • Independent seeks are rare in practice. • Shorter seeks are much more common. • The one-third-stroke calculation is only applicable for completely independent seeks. • N-1 weighted seek time calculation provides better approximation. Seek-time-versus-distance profile matters for modeling!

13. The track following system • Fine-tuning the head position at the end of a seek and keeping the head on the desired track. • Performing a head switch from one surface to the next in the same cylinder. • Aggressive and optimistic approach applied to head settling before a read operation.

14. Data layout in SCSI disk • Disk appears to client as a linear vector of addressable blocks, which are mapped to physical sectors on the disk. • Zoning: adjacent cylinders are grouped into zones • Track skewing: logical sector zero on each track is skewed for fast sequential access across track and cylinder boundaries. • Sparing: map flawed sectors to other locations

15. Mediates access to the mechanism Runs the track-following system Transfers data between disk drive and its client Manages the embedded cache The disk controller

16. Bus Interfaces

17. Caching requests • Read ahead • Write caching • Command queuing

18. Modeling disk drives • Disk drive cannot be modeled analytically with any accuracy due to its nonlinear, state-dependent behavior. • Limitations for current modeling strategies: • Seek times modeled as a linear function of seek distance • Rotational latency follows uniform distribution • Media transfer time ignored or as fixed constant • Bus contention often ignored

19. The simulator and traces • Event-based simulator in C++ • Disk drive is modeled as two tasks and some additional control structure • Representative samples from a longer trace series of HP-UX was selected • Two HP disk drives • HP C2200A for non-caching disk drive • HP 97560 for caching disk drive

20. Simulation model structure

21. Metric: the root mean square of the horizontal distance between real drive curve and model curve The demerit was presented in both absolute term and relative term Evaluation

22. Non-linear seek time profile is added to the 3rd model in Figure c. The cost of head and track switching was also included Rotational latency and spare –sector placement were added to the final model in Figure d. Evaluation cont.

23. Evaluation cont.

24. Factors not included in the final model: Soft-error reentries Individual spared sectors or tracks Summary of disk drive model

25. Impact of proposed disk drive model • 269 citations found with most recent reference “Including Models of Black-Box Storage Arrays-Terence Kelly Ira (2004)” • A Stochastic Disk I/O Simulation Technique, Winter Simulation Conference, 1997 • Modeling hard disk power consumption by Princeton, published in FAST ’03