Lots of Ways to Measure Disk Speed
There are so many ways to measure hard drive speed; it could make your head spin faster than the disk itself. The measurements break down into two groups: physical or positional, meaning how quickly the disk can find things; and transfer rates, meaning how fast the drive can receive, manipulate, and send data.
Access time is measured in milliseconds, or ms. It represents how long the drive takes to find a particular area. A normal desktop drive will have a random (not really random, disk areas are selected in a partly randomized attempt to replicate standard use patterns) access time from 10-15ms.
Access time has three minor components and one major one. The big contributor to access time is seek time. This is how long it takes the drives’ heads (like the needle and arm on a record player) to move across to the correct track of the disk. Unless otherwise noted, seek time (and the resulting access time calculations) are based on reading from the drive. The time taken to write is about half to 1 millisecond higher than that.
The next contributor, in terms of descending importance, is rotational latency or delay. This is how long it takes for the required part of the disc to spin around to the heads (like waiting for the beginning of a song to reach the needle on a record player).
The other two factors are very small. In fact they are falling out of use but you might still come across them. One is command overhead, or the time the drive spends thinking about what to do before it starts doing things. The other is settle time, or how long the drive takes to start reading once it finds what it’s looking for.
There are two main measurements for transfer rates: burst speed and read (more rarely write) speed. Both are reported in MBps or megabytes per second, but they mean different things.
Burst Speed is how quickly data can be sent to or from the hard drive cache (the bit of memory it uses to store things without having to record them on the drive), and is largely dependent on how your hard drive is connected. ATA drives have their maximum throughput in the name of the connector. For instance ATA133 can move 133MBps, but that is split over two devices if you have two hard drives, or a disk and an optical burner, on one cable. SATA drives each have their own cable, so they get access to 150MBps for SATA 1.5G and 300MBps for SATA 3G. Note that those are maximums, and burst speed performance is usually about 2/3s as fast.
Read and write speed are more comprehensive measurements. They establish how fast data can actually be read or written from or to the drive itself. Drive performance slows the larger the file it is working with. This is shown by the graph in the screenshot above. The speed across different sizes is summarized with average speed.
A Few More Things to Note
Cache size is a nice thing to have; bigger is better. The hard disk uses the cache to buffer instructions and store files it thinks you might want to use based on what you have used recently.
Spin up time or ready time is how long it takes the drive to go from a dead stop to being ready to use. This is an important number in terms of system start-up for all users. For laptop, and power or noise conscious desktop users, this is an even bigger concern, since their hard drives will often stop spinning. Power consumption is another specification that obviously impacts battery life for notebook users.
That’s far from every way to measure a hard drive’s speed, but they are the most common and important.
This post is part of the series: Understanding Hard Drive Specifications
Hard drive marketing materials and reviews offer a cornucopia of info. We help you find the important bits and tell you what they mean.