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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance | Hard Disk Internal Performance Factors ]

Internal Data Transfer Rate

Since the obvious objective in using a hard disk is to transfer data to the hard drive and onto the disks, or off the disks and out of the drive, the rate of data transfer is of paramount importance. It is my personal opinion that this is an underrated performance metric, especially compared to the much more commonly stated metrics of seek time and interface transfer rate.

First, a word about terminology. Transfer rates are confusing in part because of the phrase "transfer rate" can mean so many different things. What we are talking about here is the rate at which the hard disk can physically read data from the surface of the platter and transfer it to the internal drive cache or read buffer, ready for sending over the interface to the system. This is the drive's internal data transfer rate. This is opposed to the speed that the data can then be sent from the buffer over the interface to the system, which is the external data transfer rate.

The internal data transfer rate, which is the real rate that data can be read from the disk, is often called the sustained transfer rate, while the external rate is called the peak or burst transfer rate. The reason for these terms is that the external rate is usually much higher than the internal rate. So the drive can burst data over the interface at the higher rate when it finds the data requested already in the buffer. But the buffer is quite small compared to the size of the disk (less than 1 MB in most cases), so for a sustained read of any reasonable size, the platters themselves must be accessed, and the overall data transfer rate will drop down to only whatever the drive can handle internally.

To make matters even more confusing (sorry!) even the internal data transfer rate has more than one value. No disk can maintain even its sustained transfer rate over a prolonged period of time, because this rate is only produced under ideal conditions: reading a small number of consecutive sectors over the fastest part of the disk. In the "real world", reading data involves seeking to different parts of the disk, using different heads on different platters, etc., so that a read of a 1 MB file from your disk will never proceed at the full stated maximum sustained transfer rate (although it will be much closer than to the burst transfer rate).

Calculating the data transfer rate is reasonably simple, provided you know the true specifications of the drive; figuring the transfer rate will show you what design parameters have an impact on this performance measure. The transfer rate is a measure of the amount of data that can be accessed over a period of time. So we need to know how much data is able to pass under the read/write heads in one second. This is dependent on the density of the data (how tightly packed the data is into each linear inch of disk track), and also how fast the disk is spinning. The density of the data can be calculated easily if we know how many sectors are on the track, since we know how many bytes there are in a sector. The speed of the disk is calculated in RPM, so we divide it by 60 to get revolutions per second. This gives us a calculation of the data transfer rate in megabits per second as follows (to get the result in megabytes per second, simply divide by 8):

Note: You need the true physical geometry here; the logical BIOS setup parameters will give incorrect results. If the geometry you are using says the disk has 63 sectors per track and 16 heads, chances are very high that you are looking at the logical BIOS geometry.

Recall that modern disk drives use zoned bit recording, which means that the inner tracks have fewer sectors per track than the outer ones. Since the data transfer rate is directly proportional to the number of sectors per track, this means the data transfer rate for the outside tracks of a disk can be as much as double the data transfer rate for the inside tracks! Let's take as an example the Quantum Fireball TM drive whose zones and quoted transfer rates we saw in the section on ZBR. The outermost zone uses 232 sectors per track. The drive is spinning at 5400 RPM, so this yields a total of 85.5 Mbits/s or 10.7 MB/s. This is equal to Quantum's stated maximum data transfer rate for the drive.

You may notice that in the table in the discussion of ZBR, there is a stated data transfer rate for the outermost zone of 92.9 Mbits/s, which is substantially higher than the 85.5 number we calculated. The reason is that we only looked at the "real" data that was being read from the drive, 512 bytes per sector. Each sector on this drive actually holds 540 bytes, because 28 bytes are used for ECC. If we replace the 512 in the formula with 540, we get 90.2 Mbits/s. The remaining difference is probably from other overhead associated with the control structures of the drive.

When looking at quoted or measured data transfer rates, it's important to take several things into account. There are various assumptions and tricks that various manufacturers use in different ways to help paint their products in the best possible light. There are also problems with how some benchmarks that claim to measure transfer rates really do it:

Next: Spindle Speed

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