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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 ]

External (Interface) Transfer Rate

The internal transfer rate of the drive represents the speed with which bits can be moved to (from) the hard disk platters from (to) the hard disk's integrated controller. The external or interface transfer rate represents the speed which which those bits are moved between the hard disk and the rest of the PC. This is usually faster than the internal rate because it is a purely electronic operation, which is typically much faster than the mechanical operations involved in accessing the physical disk platters themselves. This is in fact a major reason why modern disks have an internal buffer.

The external transfer rate is unique among hard disk specifications in that it has almost nothing to do with the hard disk itself. The integrated controller must have the right chip to support the interface, but that's about it. The external transfer rate is dictated primarily by the type of interface used, and the mode that the interface operates in. Support for a given mode has two requirements: the drive itself must support it, and the system--usually meaning the system BIOS and chipset, or add-in controller card--must support it as well. Only one or the other does absolutely no good. External transfer rate is affected by a variety of interface issues, discussed in much more detail in the section on external interface performance factors.

External transfer rate is a perennial candidate for "most overrated hard disk specification". The reason is that external transfer rate specs are usually very high and impressive; manufacturers print them in big bold letters on their retail boxes, and system makers highlight them on their spec sheets. Unfortunately, they usually have very little to do with real-world performance, because the drive's internal characteristics limit transfer performance.

As I've mentioned before, transfer consists of two steps, internal and external. For typical transfers, the net speed of the transfer cannot be any higher than the slower of these two components. Since the external transfer rate of a drive is usually much higher than its internal sustained transfer rate, that means that the STR will be the bottleneck, and thus the factor that limits performance; the high transfer rate of the interface is mostly wasted. As an analogy, suppose you have a 1/2" garden hose connected to a 3/4" pipe. The 1/2" segment will be what limits the flow of water; increasing the 3/4" pipe to 1" or even higher won't make a difference in how much water you get at the end of the pipe.

There is one occasion where the external transfer rate does come into play: if the data requested by the system is already in the disk's internal cache or buffer. In that case, the data can be sent from the buffer to the rest of the system at the full speed of the interface, whatever that happens to be. Unfortunately, these situations represent such a small percentage of total requests that the net effect of the higher interface speed on overall performance is small. Today's IDE/ATA hard disks are designed to operate with an interface speed of 100 MB/s, but their sustained transfer rates are barely pushing 40 MB/s. This means the 100 MB/s speed only applies for the occasional transfer that does not require actual access to the hard disk platters.

There is one area where the interface speed is very important to pay attention to: you do not want it to be too low or performance will suffer. If you take the 3/4" pipe mentioned above and reduce its diameter to 1/4", suddenly it becomes the bottleneck, not the 1/2" diameter hose. If the interface does not have enough speed to allow the hard disk to run at its full STR, then performance can be substantially degraded. Since interfaces are relatively inexpensive this is a situation you generally want to avoid: instead, upgrade the interface. This issue occurs only when putting a new, fast drive into a rather old, slow system.

A graphical representation of why interface transfer rates are over-rated. In the
diagram above, which is "drawn" to scale, each pixel represent 500,000 bytes.
The blue box is a 45 GB hard disk (the IBM 75GXP.) The green box (see it?)
is the drive's internal 2 MB cache. The red box is the average sustained transfer
rate from the platters to the cache, and the magenta box is the 100 MB/s theoretical
interface transfer rate. As you can see, the cache is dwarfed by the disk, and the
interface transfer rate is limited by the sustained transfer rate. STR is what matters
when streaming data from the big blue box, instead of just the tiny green one.

Hard disk manufacturers always provide lots of "head room" by upping the interface standards in anticipation of advances in sustained transfer rates. In 2000 they moved from Ultra ATA/66, which was already sufficiently fast for modern drives, to the 100 MB/s Ultra ATA/100 interface. This despite there being no IDE/ATA drive available that has an STR of even half that figure. It's good to plan for the future; certainly a motherboard supporting a 100 MB/s interface will give you more "room for expansion". Just don't think it will be noticeably faster than one that "only" supports 66 MB/s, with today's drives. And also don't forget that by the time drives need that throughput, you may be using a different motherboard or PC altogether.

Note: I want to explicitly qualify my statements on this page by saying that they apply primarily to the IDE/ATA interface, as well as single-disk environments on SCSI. If you are running many drives on a SCSI channel--such as you would with a SCSI RAID array--the speed of the interface does become important very quickly, since the drives share the bandwidth of the interface. See this discussion on RAID bus bandwidth for more on this issue.

Next: Other Performance Specifications

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