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
E 6#

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",

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
Product
Barracuda
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
Platform
Asus NCL-DS (Socket 604)
Intel E7520 Chipset, BIOS 1005
RAM
Corsair CM72DD512AR-400 (DDR2-400 ECC, reg.)
2 x 512 MB, CL3-3-3-10 Timings
System Hard Drive
Western 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 Card
On-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

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
Series
Barracuda
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
Series
Barracuda
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
Series
Barracuda 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
Join our discussion on this topic

Understanding hard drive performance

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