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<title>Western Digital Corporation -- (Products) Drives/Drivers' Ed: Performance</title>
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<h1 align=center>Drivers' Ed: Hard Drive Performance</h1>
<p></p>
<center><p><b>Q: Which hard drive specification is most important to overall
system performance ?</b> </p></center>
<ul TYPE="SQUARE">
<li>Host Transfer Rate </li>
<li>Drive RPM (revolutions per minute) </li>
<li>Disk Transfer Rate (Media Rate) </li>
<li>Seek Time </li>
<li>Cache Size </li>
<li>PC Data Handling </li>
<li>All of the above </li>
</ul>
<p><b>A:</b> The correct answer is actually a combination of "all
of the above," keeping in mind most of the above specifications are
interrelated when it comes to optimizing system performance. </p>
<p>The pie chart illustrates the relative influence of factors affecting
drive performance during a typical random I/O operation (reading and writing
to a hard drive). </p>
<p><b>The major determinate of hard drive performance is mechanical factors
which are one hundred times slower than the high-speed electronics contained
in a drive. </b></p>
<center><p></p></center>
<h3>Factors Affecting Hard Drive Performance
<br>(In their relative order of importance) </h3>
<ol>
<li><b>MECHANICAL LATENCIES</b>
<br>Mechanical Latencies include both Seek Time and Rotational Latency.
The seek time is a measure (in milliseconds) of how fast the hard drive
can move its read/write heads to a desired location. Rotational latency
is a measure of the average time (also in milliseconds) the read/write
heads must wait for the target sector on the disk to pass under them once
the read/write heads are moved to the desired target track.
<br>
<br>Mechanical latencies are the main hindrance to higher performance in
modern Enhanced IDE (EIDE) hard drives. The time delays of mechanical latencies
are one hundred times higher than electronic (non-mechanical) latencies
associated with the transferring of data. Therefore, reducing mechanical
latencies (a lowering of seek time and rotational latency) should be the
top consideration in improving hard drive performance.
<br>
<br></li>
<li><b>RPM
<br></b>This is the rotational speed of the media (disk), also referred
to as the spindle speed. Hard drives only spin at one constant speed. Typical
speeds are 3600 to 3880, 4500, and 5200 to 5400 revolutions per minute.
The slower the RPM, the higher the Mechanical Latencies. Disk RPM is a
critical component of hard drive performance because it directly impacts
the rotational latency and the Disk Transfer Rate explained below.
<br>
<br></li>
<li><b>DISK TRANSFER RATE
<br></b>The Disk Transfer Rate (sometimes called media rate) is the speed
at which data is transferred to and from the disk media (actual disk platter)
and is a function of the recording frequency. Typical units are bits per
second (BPS), or bytes per second. Modern hard disks have an increasing
range of Disk Transfer Rates from the inner diameter to the outer diameter
of the disk. This is called a "zoned" recording technique.
<br>
<br>The key media recording parameters relating to density per platter
are Tracks Per Inch (TPI) and Bits Per Inch (BPI). A track is a circular
ring around the disk. TPI is the number of these tracks that can fit in
a given area (inch). BPI defines how many bits can be written onto one
inch of a track on a disk surface. To greatly simplify, the Disk Transfer
Rate (the rate at which data is read and written to the disk) is dependent
upon the speed of the disk (RPM) and the density of the data on the disk
(BPI). Even most modern, high-speed, 5000 RPM hard drives are generally
limited to a maximum Disk Transfer Rate of approximately 9 to 10 MB per
second. This specification is critical to performance and must be weighed
carefully against such electronic latencies as Mode 3 PIO and Mode 4 PIO
host transfer rates explained below.
<br>
<br></li>
<li><b>PC DATA HANDLING
<br></b>After the data moves down the IDE cable from the drive to the host
interface, there are several factors that can affect drive performance
over which the hard drive has no control. PC Data Handling is independent
from the hard drive and very dependent upon the CPU type and speed, the
BIOS overhead (how the system issues commands to the hard drive), speed
and size of the system RAM and RAM cache, CPU-to-memory speed, and storage
subsystem performance. PC Data Handling is also affected by the caching
methods of such software applications as SMARTDRIVE, 32-bit disk access
operating system drivers, etc.
<br>
<br></li>
<li><b>HOST TRANSFER RATE
<br></b>The speed at which the host computer can transfer data across the
IDE or EIDE interface. Processor Input/Output (PIO) modes and Direct Memory
Access (DMA) modes are defined in the ATA-2 industry specification as follows:
</li>
<pre> Mode 3 PIO 11.1 MB/sec
Mode 4 PIO 16.6 MB/sec
Mode 1 DMA 13.3 MB/sec
Mode 2 DMA 16.6 MB/sec
</pre>
<p>Modern host computer systems usually support Mode 3 or Mode 4 PIO. Faster
Host Transfer Rates in the future will use multi-word DMA modes as the
industry will not support any future PIO mode standards beyond mode 4.
<br>
<br>The computer system manufacturer is responsible for implementing a
Host Transfer Rate that is high enough to ensure that the host computer
is not the performance bottleneck. Implementing increasingly higher Host
Transfer Rates without corresponding increases in Disk Transfer Rates on
the hard drive will not result in increased drive performance. </p>
</ol>
<h3>Cache Buffer Size - Is Bigger Always Better ?</h3>
<p>A Cache Buffer is similar to a water glass. When you are writing to
a hard drive, the host computer fills the glass and the disk media empties
it. If you are reading data from a hard drive, the disk media fills the
glass and the host computer empties it. </p>
<center><p>
<br>
<br></p></center>
<p>The reason that a bigger cache buffer is not always better (or faster)
is because the host computer (with Mode 3 PIO or Mode 4 PIO capabilities)
can empty or fill the glass much faster than the hard drive can empty or
fill it. When the host system can transfer data in or out of the cache
buffer faster than the media rate, a larger buffer size becomes irrelevant
because the host system is always "waiting" for the hard drive.
</p>
<p>Western Digital hard drives are designed with cache buffer sizes that
are matched to the Disk Transfer Rate capabilities of the drive and the
Host Transfer Rates of modern computer systems. All of our drives are benchmarked
with various cache buffer sizes to verify that the most cost-effective
and performance-effective cache size is implemented. </p>
<h3>Confusion Over Mode 4 (The Speed Trap)</h3>
<p>The Enhanced IDE program created the long-range road map for performance
enhancements which included faster disk and host transfers, Mode 3, Mode
4, etc. Currently, computer systems and hard drive controller silicon have
most of the elements needed to implement Mode 4 (a 16.6 MB/sec Host Transfer
Rate). However, to take advantage of Mode 4 performance, physical drive
architecture must also make some performance improvements in the area of
Mechanical Latencies and Disk Transfer Rate (media rate) as defined earlier.
</p>
<p>Some competitors, in their eagerness to supply a new feature, are prematurely
marketing Mode 4. While their drive controller silicon supports Mode 4
(which is very easy and inexpensive to implement), spindle speeds (RPM),
rotational latency, bit density, and other factors have not yet been improved
(these being very difficult and costly). The result is hard drives which
have the electronic capability to do Mode 4 transfer rates, but can't take
advantage of Mode 4 due to the slower Disk Transfer Rate of the drive.
</p>
<p>Western Digital will not be implementing Mode 4 on older drive products
as the host systems into which these drives are designed are not electrically
capable of Mode 4 data transfers, nor are the Disk Transfer Rates on these
drives beyond current Mode 3 capabilities. As next generation systems are
introduced, they will be paired with next generation drives. Those drives
will require and offer true Mode 4 capability from a total drive architecture
standpoint. </p>
<p>The bottom line is that most of today's current high-speed hard drives
don't require Mode 4 because they do not have the Disk Transfer Rate capabilities
to take advantage of this feature. Mode 4 is being misused as a "marketing
buzzword" for a feature that most hard drives can't yet use. When
looking for a hard drive, compare drive RPM, seek times, and average latency.
If your primary focus is getting a "Mode 4" drive, you may end
up with the equivalent of a turtle with racing stripes. </p>
<h3>Western Digital's "Integrated Performance"
<br>AC31600 (1.6 GB) Hard Drive </h3>
<p>The Caviar ® AC31600 is Western Digital's first drive to offer and
support true Mode 4 capabilities due to its 5200 RPM spindle speed, sub-10
ms seek time, 5.76 ms mechanical latency, and 77 Mbit/sec (9.625 MB/sec)
disk transfer rate. These specifications in conjunction with our advanced
128 KB CacheFlow4<sup><font SIZE=-2>TM</font></sup> buffer can exceed the
capabilities of a Mode 3 Enhanced IDE interface.
<br>
<br></p>
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