Drivers' Ed: Hard Drive Performance

Q: Which hard drive specification is most important to overall system performance ?

Host Transfer Rate
Drive RPM (revolutions per minute)
Disk Transfer Rate (Media Rate)
Seek Time
Cache Size
PC Data Handling
All of the above

A: 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.

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).

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.

Factors Affecting Hard Drive Performance
(In their relative order of importance)

MECHANICAL LATENCIES
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.

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.
RPM
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.
DISK TRANSFER RATE
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.

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.
PC DATA HANDLING
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.
HOST TRANSFER RATE
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:
```
	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
```
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.

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.

Cache Buffer Size - Is Bigger Always Better ?

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.

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.

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.

Confusion Over Mode 4 (The Speed Trap)

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.

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.

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.

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.

Western Digital's "Integrated Performance"
AC31600 (1.6 GB) Hard Drive

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^TM buffer can exceed the capabilities of a Mode 3 Enhanced IDE interface.

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