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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk Performance Specifications | Transfer Performance Specifications ]

Internal Media Transfer Rate

The internal media transfer rate of a drive (often just called the media transfer rate or the media rate) refers to the actual speed that the drive can read bits from the surface of the platter, or write bits to the surface of the platter. It is normally quoted in units of megabits per second, abbreviated Mbit/sec or Mb/s. Typical values for today's drives are in the hundreds of Mb/s, with a maximum media rate of about 500 Mb/s being high-end at the time of this writing.

Note: Media transfer rates are not normally specified in megabytes per second, even though sustained transfer rates are; this is not 100% consistent, however. Watch out for this discrepancy when looking at the numbers.

Media transfer rate can be confusing to understand even for the serious hard disk enthusiast; it's equally difficult to describe. :^) For starters, let's explain what it is not. It is only related to what is going on inside the hard disk, and therefore has nothing directly to do with the interface transfer rate. It differs from the sustained transfer rate in that it refers only to the speed of reading or writing bits to a single track of one surface of the disk. Nothing else is included--no positioning, no track or head switching. A track holds a relatively small amount of data--under 0.25 MB with current technology. This means that almost no real-world reads or writes occur on a single track except for very short files, and the performance when reading those is primarily limited by positioning, not transfer. The end result of this is that the media transfer rate does not have much relevance to real-world use of a drive. It is primarily a "theoretical" specification that illustrates the state of the drive's technology. It is used almost exclusively for comparing drives against each other. It is also the basis for the calculation of the sustained transfer rate specification.

Media transfer rates are not constant across the entire surface of a platter. Let's recall for a moment the fact that modern disk drives use zoned bit recording. This is done because the length of the inner tracks on the disk is much shorter than that of the outer tracks. ZBR allows the outer tracks to have more sectors per track than the inner tracks. However, since every track is spinning at the same speed, this means that when reading the outer tracks, the disk is transferring more data per second when when reading the inner tracks. For this reason, the media transfer rate decreases as you move from the outer tracks of the disk to the inner ones.

The explanation above is the reason that there is no single "media transfer rate" figure for a modern hard disk.  They are typically stated as a range, from minimum to maximum (with the maximum figure given alone, of course, if only one number is provided). For example, the IBM Deskstar 34GXP (model DPTA-373420) has a media transfer rate of between approximately 171 Mb/s and 284 Mb/s depending where on the disk you are reading: that drive has 12 different zones. This drive has 272 sectors in its innermost zone, and 452 sectors on its outside tracks.

Another important thing to remember about the media transfer rate (and another reason why it is a theoretical measure only) is that it includes all bits read or written to the disk, not just user data. As discussed in detail here, some of the data storage space in a sector is reserved for overhead. This means that you cannot assume that the media rate represents the rate at which user data can be read from the disk. Taking the IBM drive above again as an example, its maximum media transfer rate is 284 Mb/s, but the maximum rate that the drive can read user data is about 222 Mb/s in the outside zone.

It's not really feasible to calculate the media transfer rate from other drive specifications, because manufacturers typically do not publish details of their sector format and other pertinent overhead characteristics. The best that you can do is approximate the value by looking at the rate at which user data can be streamed from a given part of the disk. To so do 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 of user data there are in a sector (512). 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):

User Data Transfer Rate = (Spindle Speed / 60 * Sectors Per Track * 512 * 8) / 1,000,000

This formula shows the derivation of the 222 Mb/s figure above: use 7200 for the 34GXP's spindle speed, and 452 sectors on its outside tracks. Note that 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, it's almost certain that you are looking at the logical BIOS geometry!) And again, remember that this is not the same as the media transfer rate; to get that figure you'd have to replace the "512" above with the total number of bits, including overhead, contained in each sectors of the disk.

The media transfer rate of the drive is primarily affected by all of the various data recording and encoding factors, as well as the size of the platters, and the drive's spindle speed. In addition, the drive's controller must be fast enough to be able to handle the fastest rate that the disk can read or write, but manufacturers ensure that this is never an issue by beefing up their controllers where necessary.

Next: Head Switch Time


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