An Introduction to Disk Drive An Introduction to Disk Drive ModelingModeling

Chris Ruemmler & John WilkesChris Ruemmler & John WilkesHewlett-Packard LaboratoriesHewlett-Packard Laboratories

Presented by Hang ZhaoPresented by Hang Zhao

The earliest hard disks…The earliest hard disks…

First Hard Disk (1956): First Hard Disk (1956): IBM's RAMAC, capacity is 5 IBM's RAMAC, capacity is 5 MB, stored on 50 24" disks MB, stored on 50 24" disks

First Air Bearing Heads (1962): First Air Bearing Heads (1962): IBM's model 1301 IBM's model 1301 increases both areal density and throughput by about 1000% increases both areal density and throughput by about 1000%

First Removable Disk Drive (1965): First Removable Disk Drive (1965): IBM's model IBM's model 2310 2310

…… ……

A brief history of hard disk driveA brief history of hard disk drive

Engineers over the last few decades have done at improving them in every Engineers over the last few decades have done at improving them in every respect: reliability, capacity, speed, power usage, and more. respect: reliability, capacity, speed, power usage, and more.

Evolution of areal density of hard disk Evolution of areal density of hard disk

plattersplatters

The areal density of hard disk platters continues to increase at an amazing rThe 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. ate 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" Modern disks are now packing as much as 20 GB of data onto a single 3.5" platter! platter!

Why do we need to model disk drive Why do we need to model disk drive behavior?behavior?

CPU technology is advancing rapidly; while thCPU technology is advancing rapidly; while the overall system behavior is restrict to disk syse overall system behavior is restrict to disk system performance. tem performance.

The behavior of disk drive itself is a dominant The behavior of disk drive itself is a dominant factor in overall I/O performance.factor in overall I/O performance.

Existing hard disk models have limitations…Existing hard disk models have limitations…

Characteristics of modern disk drivesCharacteristics of modern disk drives

MechanismMechanism recording componentrecording component positioning componentpositioning component

ControllerController microprocessormicroprocessor buffer memorybuffer memory interface to SCSI businterface to SCSI bus

Disk Drive TerminologyDisk Drive Terminology

Several Several plattersplatters, with information recorded magnetically on both , with information recorded magnetically on both surfacessurfaces (usually) (usually)

ActuatorActuator moves moves headhead (end of (end of armarm,1/surface) over track (,1/surface) over track (seekseek)), select , select surfacesurface, wait for , wait for sectorsector rotate under rotate under headhead, then read or write, then read or write

CylinderCylinder: all tracks under heads : all tracks under heads

Bits recorded in Bits recorded in trackstracks, which in turn divided into , which in turn divided into sectorssectors

Platter

OuterTrack

InnerTrack

Sector

Actuator

HeadArm

Platter

The recording componentsThe recording components

Modern disks range in size from 1.3 to 8 Modern disks range in size from 1.3 to 8 inches in diameter:inches in diameter:

Smaller disks VS larger disksSmaller disks VS larger disks less surface area/storageless surface area/storage consume less powerconsume less power spin fasterspin faster smaller seek distancesmaller seek distance

The increased storage densityThe increased storage density

The incremental trends result fromThe incremental trends result from better linear recording density: a measure of better linear recording density: a measure of

how tightly the bits are packed within a length how tightly the bits are packed within a length of track. of track. (50,000 BPI 1994; 524,000 BPI 2000’)(50,000 BPI 1994; 524,000 BPI 2000’)

packing separate tracks more closely together packing separate tracks more closely together (20,000 TPI; 67,300 TPI around 2000’)(20,000 TPI; 67,300 TPI around 2000’)

Platters and disk rotationPlatters and disk rotation

Platters rotates in lockstep on a central spindle Platters rotates in lockstep on a central spindle at rates varying from 3,600 to 7,200 rpmat rates varying from 3,600 to 7,200 rpm

Higher spin rate increases transfer rates and Higher spin rate increases transfer rates and shortens rotation latencies; on the other hand, shortens rotation latencies; on the other hand, power consumption increases power consumption increases

Each platter surface is associated with a disk Each platter surface is associated with a disk head for writing and reading operating under a head for writing and reading operating under a single read-write channelsingle read-write channel

The positioning componentsThe positioning components

Seeking: speed of head movement, limited by tSeeking: speed of head movement, limited by the power available for pivot and the arm’s stiffhe power available for pivot and the arm’s stiffness. Seek time is composed of:ness. Seek time is composed of: speedupspeedup coastcoast: for long seeks where arms move at max: for long seeks where arms move at maxυυ slowdownslowdown settlesettle: dominant factor of very short seeks: dominant factor of very short seeks

The demerit of “average” seek timeThe demerit of “average” seek time

““Average” seek times are commonly used as a figure Average” seek times are commonly used as a figure of merit for disk drives, but they can be misleading.of merit for disk drives, but they can be misleading. Independent seeks are rare in practice.Independent seeks are rare in practice. Shorter seeks are much more common.Shorter seeks are much more common. The one-third-stroke calculation is only applicable for The one-third-stroke calculation is only applicable for

completely independent seeks.completely independent seeks. N-1 weighted seek time calculation provides better N-1 weighted seek time calculation provides better

approximation.approximation.

Seek-time-versus-distance profile matters for modeling! Seek-time-versus-distance profile matters for modeling!

The track following systemThe track following system

Fine-tuning the head position at the end of a Fine-tuning the head position at the end of a seek and keeping the head on the desired track.seek and keeping the head on the desired track.

Performing a head switch from one surface to Performing a head switch from one surface to the next in the same cylinder.the next in the same cylinder.

Aggressive and optimistic approach applied to Aggressive and optimistic approach applied to head settling before a read operation.head settling before a read operation.

Data layout in SCSI diskData layout in SCSI disk

Disk appears to client as a linear vector of Disk appears to client as a linear vector of addressable blocks, which are mapped to addressable blocks, which are mapped to physical sectors on the disk.physical sectors on the disk. Zoning: adjacent cylinders are grouped into zonesZoning: adjacent cylinders are grouped into zones Track skewing: logical sector zero on each track is Track skewing: logical sector zero on each track is

skewed for fast sequential access across track and skewed for fast sequential access across track and cylinder boundaries.cylinder boundaries.

Sparing: map flawed sectors to other locations Sparing: map flawed sectors to other locations

The disk controllerThe disk controller

Mediates access to the Mediates access to the mechanismmechanism

Runs the track-following Runs the track-following systemsystem

Transfers data between Transfers data between disk drive and its clientdisk drive and its client

Manages the embedded Manages the embedded cachecache

Bus InterfacesBus Interfaces

Caching requestsCaching requests

Read aheadRead ahead Write cachingWrite caching Command queuingCommand queuing

Modeling disk drivesModeling disk drives

Disk drive cannot be modeled analytically Disk drive cannot be modeled analytically with any accuracy due to its nonlinear, state-with any accuracy due to its nonlinear, state-dependent behavior.dependent behavior.

Limitations for current modeling strategies:Limitations for current modeling strategies: Seek times modeled as a linear function of seek Seek times modeled as a linear function of seek

distancedistance Rotational latency follows uniform distributionRotational latency follows uniform distribution Media transfer time ignored or as fixed constantMedia transfer time ignored or as fixed constant Bus contention often ignoredBus contention often ignored

The simulator and tracesThe simulator and traces

Event-based simulator in C++Event-based simulator in C++ Disk drive is modeled as two tasks and some Disk drive is modeled as two tasks and some

additional control structureadditional control structure Representative samples from a longer trace Representative samples from a longer trace

series of HP-UX was selectedseries of HP-UX was selected Two HP disk drivesTwo HP disk drives

HP C2200A for non-caching disk driveHP C2200A for non-caching disk drive HP 97560 for caching disk driveHP 97560 for caching disk drive

Simulation model structureSimulation model structure

EvaluationEvaluation

Metric: the root mean square of the horizontal distance between real Metric: the root mean square of the horizontal distance between real drive curve and model curvedrive curve and model curve

The demerit was presented in both absolute term and relative termThe demerit was presented in both absolute term and relative term

Evaluation cont.Evaluation cont.

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

model in Figure d. model in Figure d.

Evaluation cont.Evaluation cont.

Summary of disk drive modelSummary of disk drive model

Factors not included in Factors not included in the final model: the final model: Soft-error reentriesSoft-error reentries Individual spared sectors Individual spared sectors

or tracksor tracks

Impact of proposed disk drive modelImpact of proposed disk drive model

269 citations found with most recent reference 269 citations found with most recent reference “Including Models of Black-Box Storage Arrays-“Including Models of Black-Box Storage Arrays-Terence Kelly Ira (2004)”Terence Kelly Ira (2004)”

A A Stochastic Disk I/O Simulation Technique, Winter Simulation Conference, 1997

Modeling hard disk power Modeling hard disk power consumption by Princeton, consumption by Princeton, published in FAST ’03published in FAST ’03