Ultra ATA 66 Hard Drive Roundup - March 2000

Before we get too far along, let's make a brief recap of what each specification means:

Rotational Speed - The rotational speed, as the name suggests, is the speed at which the platters rotate inside the hard drive enclosure. All else being equal, a faster rotational speed translates into faster read/write performance as the data can be written to, or read from, the platters at a faster rate. This also has the benefit of reducing a drives latency, as the time it takes the requested block of data to arrive under the head is reduced with higher rotational speeds. A disk drives latency is the time it takes the platters to make ½ of a full revolution. Since latency is a direct product of rotational speed, a 5400-RPM drive will have a latency of 5.55 ms and a 7200-RPM drive will have a latency of 4.17 ms. This number is determined by first finding how long it takes the platters to make one complete revolution. First, lets covert the platter speed into rotations per second or 7200/60 (60 seconds per minute) to get 120 rotations per second. Now we determine the time it takes to make 1 revolution by taking the inverse of our rotations per second, in this case 120, so 1 / 120 is 0.008333 seconds for one revolution. Since we need ½ of this time, we divide 0.008333 by 2 to get our latency of 0.0041665 seconds or rounded up to 4.17 ms (1 ms = .001 or 1/1000 of a second).

Cache Memory – As in other parts of your computer, cache memory acts as a buffer to store temporary chucks of data that have been “pre-read” off the drive, or depending on the drive, to hold data that has yet to be written to the drive. The drives use a variety of software algorithms to help predict what data will be accessed next, in determining what to load into the on-board cache memory. If the assumptions are right and you request data that has been pre-loaded into the cache memory, this is also known as a cache hit, your system will save a little time in that it can instantly transfer the data at the maximum burst transfer rate, as it didn’t have to wait for the drive to access it off the platters first. However, this does not help with long sequential transfers of data or in cases where the next requested data was not loaded into the cache (a cache miss); in both cases the drive will have to read data off the disk at normal speeds and no performance increase will be realized. It should be noted that this is where the higher burst transfer rate of an Ultra ATA/66 drive would pay off. If you have a larger cache buffer and the data you requested happened to be all pre-loaded into the cache memory, your data would be transferred at or very near 66 MB/sec. This “data burst” then, is the main advantage to the higher transfer rates. Of course, even with a 2 MB buffer, in long sequential transfers greater than cache size, the cache would be emptied very quickly and you would once again revert to the physical platter read/write transfer rates.

Seek Time – The most common seek time quoted for hard drives is the Average Seek Time or the average amount of time it takes the data heads to perform a random seek of the drive. This is commonly determined by taking ½ of the Full Stroke Time or time it would take the data heads to move from the outermost track to the innermost track or vice versa. Seek times are measured in milliseconds (thousandths of a second) and are often in the range of 7 ms to 11 ms on modern hard drives. Once again, if all other parameters of a drive are equal, the drive with the lowest seek time will offer the better performance. Since seek times relate mainly to the data heads ability to move from one track to another in order to begin reading new data, this specification is important in cases where data is read out of sequence and is spread about on the hard drive.

Capacity – Capacity, which is one of the main things people look for when choosing a hard drive, is the amount of data that can be stored on a hard drive. Capacity is measured in megabytes (MB’s) or millions of bytes and, more recently, gigabytes (GB’s) or billions of bytes. One thing to watch out for when reading the specifications of a drive is whether the number quoted is formatted or unformatted capacity. Sometimes manufactures will quote the unformatted capacity of a drive, which is always higher, but usually it’s the formatted capacity you are looking at when buying a drive.

Another thing that should be pointed out while we are on the subject of capacity, is that when you install your drive into you computer and fire up your operating system, the amount of disk space showing as available is often less than what you were expecting. This is caused by the different methods at which the manufactures and your computer use to compute capacity. Manufactures use a rounding method, which artificially inflates the listed capacity.

An example of this is that 1 KB of data is actually 1024 bytes, 1 MB of data is 1024 KB’s, and 1 GB of data is equal to 1024 MB’s. So 1 gigabyte of data equals 1024 bytes x 1024 KB’s x 1024 MB’s or 1,073,741,824 bytes of data. This is how your computer arrives at its calculation of available storage space. Hard drive manufactures, round these figures down to use an even 1000 in their calculations. This practice, which has become standard, results in 1 gigabyte being equal to 1,000,000,000 bytes or 1000 bytes x 1000 KB’s x 1000 MB’s. So when a manufacturer lists a drive as having 25 GB’s of formatted storage capacity, they are saying you have 25,000,000,000 bytes of storage space and your computer shows you this by listing 23.28 GB’s of space available.

Drive Interface - Even though all of these drives feature the Ultra ATA/66 interface, this interface simply shows the maximum burst transfer rate that the drive can use to communicate with the rest of the system. You should think of the maximum speed of the interface as the theoretical maximum that could be attained and not what the current generation of drives are able to send and receive data at. With the exception of data that is already in the drives cache memory (see above), current hard drives will transfer data at a much slower rate. This rate is often referred to as the maximum sustained transfer rate. The maximum sustained data rate is determined by some of the above factors, rotational speed, on-board data cache and seek times, as well as other factors such as the number of platters and the density at which the data can be recorded. We just wanted to bring this up, as you shouldn’t expect to get double the performance out of an Ultra ATA/66 drive as you would an Ultra ATA/33 unit.

When choosing a hard drive, the three major criteria that you will consider before making your purchase decision are price, desired capacity, and performance. The second two are often dictated by the first, which is price. Since all manufactures are fairly consistent in terms of labeling their hard drives storage capacity, as we outlined above, the main criteria left then is performance. You could simply go by the listed specifications such as those above, but often times those don’t tell the whole story. So now that we have summarized the basic information that you often have access to when shopping for a new hard drive, let’s get on with the comparison and see if these advertised numbers really tell us anything about the drives actual performance.