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Interface Speed and the External Data Transfer Rate

The objective of using a hard disk is to transfer data to and from the disks. This involves two steps: the internal part is the process of actually reading or writing the disk platters, and the external part is moving the data from the inside of the drive out to the system, or vice-versa. The internal transfer rate of the drive is discussed in this section, while the external data transfer rate is discussed here.

Transfer rates are confusing in part because of the many different types that are often discussed. The external transfer rate is also sometimes called the interface transfer rate or host transfer rate, because it is the speed of transfer over the interface between the hard disk and the rest of the PC. The external and internal transfer rates are often called the burst and sustained transfer rates, respectively. The external rate is called burst because it can be much higher than the sustained rate, for short transfers. The sustained rate is so named because when you are dealing with a large transfer, the limiting factor is how fast the disk's internal mechanisms can operate.

The external transfer rate is the speed at which data can be exchanged between the system memory and the internal buffer or cache built into the drive. This is usually faster than the internal rate because it is a purely electronic operation, which is 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 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) must support it as well. Only one or the other does absolutely no good. Support for the higher transfer modes also usually requires that the interface be over a high-speed system bus such as VLB or PCI, which most today are. See also the section on system issues.

The two most popular hard disk interfaces used today are SCSI and IDE/ATA (and enhancements of each). IDE uses two types of transfer modes: PIO and DMA. I'm not going to get into all the details on interfaces and transfer modes here, because I have another large section devoted entirely to them. I did want to compare the theoretical maximum transfer rates of the different interface options, so I've "mashed" them together in the table below to show you a comparative summary:

There are a boatload of caveats that go with these numbers, and you really need to understand the interfaces themselves to know what they are. Obviously these are theoretical maximums that don't take into account command processing overhead etc. Also see the comparison of SCSI and IDE for more issues of relevance to interface performance.

For high performance, it is important that the external (burst) transfer rate be higher than the internal (sustained) transfer rate of the drive. If it isn't, then the drive is not being used to its maximum potential and the interface should really be upgraded. The most popular hard disk interfaces used in modern PCs today are EIDE or ATA-2 drives running with PIO mode 4 or multiword DMA mode 2, meaning a theoretical maximum of 16.6 MB/s. This exceeds the internal transfer rate of any hard disk on the market today.

Once you get above the internal transfer rate, the extra "room" is only of value if whatever you are trying to read from the disk is already in the buffer. If the hard disk itself can only do sustained reads of 7 MB/s, then over time, the extra 9.6 MB/s of the 16.6 MB/s interface speed does not do a lot for you. However, having that extra slack is of use in making sure peak transfers have the capacity they need. Many hard disk manufacturers try to trick their buyers by putting the 16.6 MB/s speed of the interface in big letters in their ads, but as you know now, this is not the most important number. The newest drives support Ultra ATA and its 33.3 Mb/s interface speed, which makes the confusion even worse because virtually no systems are going to exploit even half of that capacity right now.

There is also another consideration: the number of devices that are sharing the interface. This is particularly a concern with SCSI, which allows for many devices on a bus (IDE/ATA and enhancements allow just two per channel). If you are using four hard disks on a SCSI bus in a server that is handling many simultaneous requests, and each drive has an internal transfer rate of 8 MB/s, that 40 MB/s for Ultra Wide SCSI will probably, at many points in time, be in full use.

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