Understanding hard drive performance

16/08/2013 Understanding Hard Drive Performance

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Understanding Hard Drive Performance

By Patrick Schmid , MARCH 5, 2007 5:20 AM

1. A Guide To Hard Drive Selection

If you compare hard drive developments with the advances in graphics or CPUs you will notice one thing:

there are few must-haves and really no "big bangs". Hard drives seem to be boring, but that's only at first

glance. In fact, there has been considerable progress, as storage densities and performance have

increased continuously; there just isn't an obvious way to take notice of it except for the increasing

capacities. Even experts sometimes can't tell two similar drives apart if not for their product label, but

performance may vary quite a bit. Even if you compare drives with supposedly similar technical

specifications, let's say hard drives within a model family, there are measurable differences.

I intentionally wrote "measurable" differences, because there is no way anyone can tell a Hitachi drive

from a Western Digital drive when they are powering comparable systems (assuming that current products

are used). Noticeable differences exist between a 10,000 RPM Western Digital Raptor and a 7,200 RPM

drive, or between a RAID 0 setup and a single hard drive, but not within the gazillions of 7,200 RPM

mainstream drives, where variations are just nuances. Still, there are differences, as there are multiple

capacity points, varying cache sizes and Serial ATA or UltraATA versions available from all hard drive

makers.

This article will give you an overview of all parameters that are relevant to hard drive performance. These

are the drive form factor, platter diameter and platter count, recording technology and data density,

rotation speed and access time, interface and buffer memory. We then have a look at Seagate's Barracuda

7200.10 hard drive family, comparing technically similar hard drives with different capacity points, cache

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2. Hard Drive Internals

sizes and interfaces (Serial ATA vs. UltraATA). You might be surprised to learn that the largest model isn't

necessarily the fastest, and 16 MB drive cache doesn't seem to help much.

Join our discussion on this topic

This is a hard drive without the top cover that seals the inside of the drive to prevent intrusion of dust

particles that can damage the sensitive read/write heads. This particular model is broken, as you

might have already guessed from the circular impression, which is the result of a substantial head

crash.

Hard drives are based on one or more magnetic platters that hold concentric tracks. These are filled from

the outside to the inside, storing bits by magnetically aligning elements. Tracks located on top of each

other on different platters are collectively called a cylinder. A movable arm is used to position the

read/write heads on top of the platter. If there are several platters, the arm is more like a comb that fits

between the platters. The arm/comb moves in a manner similar to that of a record player, so the heads

can reach the inner and the outer areas of the platters. Both the top and the bottom of a platter are used

to store data.

Bits are organized in so-called sectors, which are combined into allocation units (clusters). A cluster is the

smallest chunk of data to store data. Depending on the file system (Windows uses NTFS or FAT32) the

cluster size may vary The larger a cluster, the better is the overall sequential throughput, but you will end

up wasting storage capacity if the average file size is much smaller than the cluster size.

Form Factors And Height

The most obvious difference between hard drives is their form factor, which is based on the platter

diameter. Hard drives for desktop computers use 3.5" platters, while mobile hard drives use a diameter of

2.5". Enterprise hard drives may look like 3.5" models, but they actually use smaller platter diameters to

enable higher rotation speeds. Hard drives for ultra-portable devices have platter diameters of only 1.8",

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3. Storage Density, Recording Technology

and there are various micro hard drives with only 1" and 0.8" platters as well.

Hard drives in the 3.5" form factor typically have a height of 1", which is sufficient to accommodate up to

five platters inside the drive. Notebook hard drives use single or twin platter designs and adhere to 9.5

mm or 12.5 mm height limits, though the latter isn't suitable for most notebook designs. If you look at 1"

and 0.8" hard drives you'll notice a tendency towards proprietary solutions and heights, because there

products are often optimized for certain customer requirements.

More platters certainly yields a higher storage capacity, because the total capacity is calculated by

multiplying the per-platter capacity with the number of platters. For example, a data density of 160 GB per

platter allows manufacturers to reach 640 GB capacity per drive with four platters. However, more platters

also mean more read/write heads, which increases the risk of hardware failure due to the larger number

of moving parts. Friction and energy requirements increase as well. In terms of price, one high capacity

drive is still less expensive than multiple smaller ones. The only exception is high performance RAID arrays

in servers, which intentionally run more hard drives to increase performance.

We already mentioned data density, but we should also talk about storage density, which is expressed in

gigabits per square inch. This cannot be directly compared to the data density in gigabytes per platter,

because manufactures do not always utilize the entire platter to store data. Also, the capacity per platter

usually refers to a 3.5" hard drive, while the storage density in gigabits per square inch can be compared

across different form factors. Storage density highly depends on the recording technology used.

Perpendicular Magnetic Recording (PMR) is the state-of-the-art recording technology today. Unlike

conventional, longitudinal recording along the track, in this technique the magnetic elements are aligned

vertically. This helps to reduce the risk of magnetic elements influencing each other, known as

superparamagnetism, and it allows the storage of more bits on the same area to increase areal density.

The hard drive industry is already hoping for a tenfold capacity increase in the long run, thanks to

perpendicular recording. The first terabyte hard drives using PMR will be available soon, offering record

storage capacities with high data integrity.

The future will bring Heat-Assisted Magnetic Recording (HAMR). In this technology, a laser heats up the

surface in order to reduce the intensity of the magnetic field required to influence magnetic particles on the

platters. This will allow further increases in data density, as the heat-assisted technology allows more

precise manipulation of magnetized elements.

High data density is desirable, as it has a positive impact on data transfer performance: the more bits the

drive can read concurrently, the faster it is. As a result, a new 3.5" 7,200 RPM hard drive always

outperforms an older model. However, access time doesn't benefit from higher storage densities, as the

head positioning cannot possibly be accelerated without putting substantial mechanical strain on the

components.

Spindle Speed

A drive's spindle speed in revolutions per minute (RPM) is by far the most important parameter in

assessing overall performance. A high spindle speed results in higher platter velocity, which means more

data passing the read/write heads. The faster a drive spins, the more data it can deliver or store in a

given time frame. But high spindle speeds also have a beneficial impact on access time: as soon as the

heads are aligned over a track it usually has to wait until the required sectors pass underneath. Higher

spindle speeds reduce this latency, although modern hard drives typically start caching data proactively

while waiting for the right sector(s) to pass the heads. Even then the drive might still have to wait for a

servo track, which is used to mark the beginning/end of a data track.

3.5" hard drives for servers and workstations spin at 10,000 or 15,000 RPM, while desktop drives normally



4. Platter Diameter

work at 7,200 RPM. Only Western Digital's Raptor brings 10,000 RPM into desktop PCs; it can still be

considered the perfect hard drive for enthusiasts. Still, it is very expensive from a cost per gigabyte

standpoint, because you'll have to fork out more money for the 150 GB Raptor than for a 500 GB 7,200

RPM drive.

Notebook drives typically rotate at slower speeds: 4,200 RPM drives are being replaced by 5,400 RPM

models even in some budget notebooks, but there are still only few 7,200 RPM notebook drives available.

One reason for this is the energy requirement, which increases at higher spindle speeds. Portable

computers usually depend on long battery run times, which is why system builders might hesitate to

deploy 7,200 RPM drives into mainstream notebooks. 1.8" and smaller hard drives run at 4,200 RPM, while

1" and 0.8" models operate at even slower speeds.

New 3.5" hard drives at 7,200 RPM provide up to 90 MB/s of transfer speed off the medium, while 2.5"

drives are considerably slower - in the area of 30-50 MB/s - and 1.8" models and smaller drives are much

slower still.

If you compare hard drives by only looking at their spindle speeds, you might believe that different

products should perform similarly, but this definitely is not true. High spindle speeds are imperative if you

want to get high performance, but the effective speed at which sectors pass the read/write heads varies

considerably.

All hard drives rotate at constant angular velocity, which means that they are designed to stay at one

rotation speed instead of adjusting the speed to the read/write heads as it is common in optical drives. As

a consequence, the distance covered by bits stored in the outer platter areas in a second is much higher

than on inner tracks. On the outside of a 3.5" platter, the track length is approximately ten inches, as

opposed to 2.5" close to the spindle motor. At 7,200 RPM this results in an absolute velocity of ~67 MPH

on the outside versus ~17 MPH on the inside of a platter. It is obvious why data transfer rates on the

outside of a rotating disk are far higher than on the inside.

It is for this reason that defragmentation tools, which realign files that are scattered and fragmented

across a hard drive, always place the Windows swap file at the very beginning of the storage medium,

which is where the swap file performs best. Another conclusion of the absolute speed comparison is that a

2.5" hard drive can never reach the high data transfer rates of a 3.5" hard drive, because the effective

data rotation speed isn't fast enough.

Platter Count

If you've already spent time dissecting a hard drive family and the properties of each drive, you'll know

that the available capacities do not always correspond to a manufacturer's statement on per-platter

capacity. For example, Seagate's Barracuda 7200.10 stores almost 200 GB per platter, yet there are

versions with 250 GB and 320 GB.

The explanation for this can be found in market requirements. Some customers might specifically ask for a

250 GB hard drive, even if it is only for the sake of comparing different quotes. Cost pressure is another

reason to offer certain capacity points: the average user might only be able to afford a 250 GB drive, and

she or he might not even need more storage capacity either. It is obvious that hard drive manufacturers

have to adjust to the so-called "sweet spots", and these have been at certain capacities. In the end,

manufacturing yields are an issue as well, and selling a large number of drives is more important than fully

utilizing the maximum storage capacity per platter for every product.

For these reasons, many hard drives do not utilize their full storage capability, which means that the

slower inner tracks of products with an odd ratio of capacity and platter count (per-platter capacity) aren't

used. While this crops the capacity, it also ensures better minimum transfer rates.



5. 8 Or 16 MB Buffer

Although there are still many drives using a 2 MB buffer, 8 MB can be considered standard today for

mainstream desktop hard drives, and there are more and more drives with 16 MB cache as well. Larger

hard drive caches not only make sense because of low DRAM prices, but from a technical standpoint as

well. Hard drives utilize algorithms to pre-cache data, or to leave data in the cache memory in case it is

requested again. Serial ATA drives also require a certain amount of memory to store incoming commands,

because most products are capable of reorganizing these in order to process them as efficiently as

possible, requiring little physical head movement. This feature is called Native Command Queuing (NCQ),

and it also requires some memory to work, although the memory requirements are minor.

We wanted to check if there are differences between drives that sport 8 MB or 16 MB of buffer memory.

Since we had received almost the complete Seagate Barracuda 7200.10 family, we picked four different

500 GB models to answer the question. All of them use three platters and only vary in terms of their

interfaces - SATA/300 or UltraATA/100 - and cache size.

Seagate Barracuda 7200.10 500 GB Comparison Table

Manufacturer Seagate Seagate Seagate Seagate

ProductBarracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Model ST3500630A ST3500630AS ST3500830A ST3500830AS

Capacity 500 GB 500 GB 500 GB 500 GB

Spindle Speed 7,200 RPM 7,200 RPM 7,200 RPM 7,200 RPM

Platter 3 3 3 3

Cache 16 MB 16 MB 8 MB 8 MB

Native Command Queuing

(NCQ)No yes no yes



6. Test Setup

Interface UltraATA/100 SATA/300 UltraATA/100 SATA/300

System Hardware

Processor(s)2 x Intel Xeon Processor (Nocona core)

3.6 GHz, FSB800, 1 MB L2 Cache

PlatformAsus NCL-DS (Socket 604)

Intel E7520 Chipset, BIOS 1005

RAMCorsair CM72DD512AR-400 (DDR2-400 ECC, reg.)

2 x 512 MB, CL3-3-3-10 Timings

System Hard DriveWestern Digital Caviar WD1200JB

120 GB, 7,200 RPM, 8 MB Cache, UltraATA/100

Mass Storage Controller(s)Intel 82801EB UltraATA/100 Controller (ICH5)

Silicon Image Sil3124, PCI-X

Networking Broadcom BCM5721 On-Board Gigabit Ethernet NIC

Graphics CardOn-Board Graphics

ATI RageXL, 8 MB

System Hardware

Performance Measurementsc’t h2benchw 3.6

I/O Performance

IOMeter 2003.05.10

Fileserver-Benchmark

Webserver-Benchmark

Database-Benchmark

Workstation-Benchmark

System Software & Drivers

OS Microsoft Windows Server 2003 Enterprise Edition, Service Pack 1

Platform Driver Intel Chipset Installation Utility 7.0.0.1025

Graphics Driver Default Windows Graphics Driver

Benchmark Results : 8 MB Vs. 16 MB Buffer

Access Time

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7. Read Transfer Rates

Interface Performance

Write Transfer Rates



I/O Performance









8. Application Performance: Windows XP Start



Application Performance: File Writing

Welcome The Barracudas: 250, 320, 400, 500, 750 GB



We already talked about manufacturers cropping storage capacity for the sake of providing products for

common "sweet spot" capacity points. Now let's check what the difference is between the slowest and the

fastest Barracuda 7200.10 drives. As we'll see, the rule of thumb of the top model being the fastest clearly

isn't valid here.

Manufacturer Seagate Seagate Seagate Seagate Seagate

SeriesBarracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Model ST3250820A ST3250820AS ST3320820A ST3320820AS ST3400620AS

Capacity 250 GB 250 GB 320 GB 320 GB 400 GB

Rotation Speed 7200 U/Min 7200 U/Min 7200 U/Min 7200 U/Min 7200 U/Min

Platter 2 2 2 2 3

Cache 8 8 8 8 16


(NCQ)no yes no yes yes

Interface Ultra ATA/100 SATA/300 Ultra ATA/100 SATA/300 SATA/300

Manufacturer Seagate Seagate Seagate Seagate Seagate

SeriesBarracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Barracuda

7200.10

Model ST3400820A ST3400820AS ST3500630A ST3500630AS ST3500830A

Capacity 400 GB 400 GB 500 GB 500 GB 500 GB

Rotation Speed 7200 U/Min 7200 U/Min 7200 U/Min 7200 U/Min 7200 U/Min

Platter 3 3 3 3 3

Cache 8 8 16 16 8


(NCQ)no yes no yes no

Interface Ultra ATA/100 SATA/300 Ultra ATA/100 SATA/300 Ultra ATA/100

Manufacturer Seagate Seagate Seagate Seagate

SeriesBarracuda Barracuda Barracuda Barracuda



9. Benchmark Results

7200.10 7200.10 7200.10 7200.10

Model ST3500830AS ST3750640A ST3750840A ST3750640AS

Capacity 500 GB 750 GB 750 GB 750 GB

Rotation Speed 7200 U/Min 7200 U/Min 7200 U/Min 7200 U/Min

Platter 3 4 4 4

Cache 8 16 8 16


(NCQ)yes no no yes

Interface SATA/300 Ultra ATA/100 Ultra ATA/100 SATA/300

The versions with 250 and 320 GB utilize two platters, the 400 and 500 GB capacity points are based on

three platters, and the 750 GB top model uses four platters. Only the 750 GB top model actually utilizes

the maximum per-platter capacity of 190+ GB. The following table lists what the waste capacity is for each

model.

Barracuda 7200.10Theoretical Capacity MaximumWaste in GBWaste in %

250 GB, 2 Platter 332 GB 82 GB 25%

320 GB, 2 Platter 332 GB 12 GB 3%

400 GB, 3 Platter 570 GB 170 GB 30%

500 GB, 3 Platter 570 GB 70 GB 12%

750 GB, 4 Platter 750+ GB - 0%

We took a look both at Serial ATA and UltraATA drives for this project; hence you will find one diagram for

each on the following pages.

Data Transfer Diagram

We created these transfer diagrams using the performance data of all hard drives using the same

interface. Thus, they not only show the performance of a single drive, but the absolute maximum and

minimum values that we found across the lineup between 250 and 750 GB.





10. Data Transfer Rates

With the results so close, it is impossible to make a simple statement: there are differences between

Serial ATA and UltraATA, but they are not significant. However, you might notice that SATA drives based on

a high yield of the available per-platter capacity tend to provide faster overall transfer rates.









11. Interface Bandwidth

As expected, Serial ATA has a huge bandwidth advantage over UltraATA. Yet the differences are irrelevant

in everyday life, as the drives do not hit the bandwidth bottleneck of UltraATA/100, which clearly is 85 MB/s

in case of the 7200.10. SATA/300 may send almost 200 MB/s over the wires, but this only reflects the

performance at which data is retrieved from a drive's cache memory, not the platters themselves.



Access Time

These are obvious results: the drives with the most capacity waste had the quickest access times, as the

read/write heads have a shorter operating range due to the unused inner storage areas. However, we



did not observe this tendency within the UltraATA drives, where access times seem to be slightly longer

across the product line.



12. Application Performance

I/O-Performance









13. I/O-Performance, Continued









14. Conclusion

The I/O benchmarks prove that drive models that do not utilize the full capacity potential, indeed provide

more I/O operations per second than the high capacity models.

We looked at several Seagate Barracuda 7200.10 drives to get down to the performance nitty-gritty. In

doing so, we found that there is hardly any difference between two drives that only differ in their cache

sizes: 16 MB cache has no significant advantage over 8 MB across our benchmark suite, and this applies

both to Serial ATA and to UltraATA drives. We would have expected that at least the SATA drives would



show some degree of benefit, but in the case of the 7200.10 family, 16 MB cache is a waste of money if

you have a cheaper 8 MB alternative. At the same time, 16 MB cache doesn't hurt either if the price is

about the same...

Knowing that there are differences between members of a hard drive family you should now be able to

make a more confident buying decision. Hard drives whose platter configurations don't utilize the maximum

per-platter capacity show slightly quicker access times, because the operating range of the drive is

somewhat reduced, while units that fully utilize the maximum capacities offer slightly better data transfer

rates.

That said, we have to make clear that the differences between the quickest and the slowest hard drive

model within a product family are clearly smaller than differences between product generations. In our

experience, a new product will always outperform an older one.

We have inserted the 15 Seagate hard drives into our Interactive 3.5" Hard Drive Charts, so you can

compare them to the rest of the market. Happy hard drive hunting!

Related Articles:

2007 HDD Rundown: Can High Capacities Meet High Performance?

15 Years Of Hard Drive History: Capacities Outran Performance

Conventional Hard Drive Obsoletism? Samsung's 32 GB Flash Drive Previewed

Quo Vadis, Hard Drive? The 50th Anniversary of the HDD

Find Your Notebook Hard Drive: 2.5" Performance Charts

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http://www.tomshardware.com/2006/09/20/conventional_hard_drive_obsoletism/index.html

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