I/O Time: Doing the Math

Now that we have an abstract model of the disk, we can use a little analysis to better understand disk performance. In particular, we can now represent I/O time as the sum of three major components:

$$T_{I/O}=T_{seek}+T_{rotation}+T_{transfer}$$

Note that the rate of I/O ($R_{I/O}$), which is often more easily used for comparison between drives (as we will do below), is easily computed from the time. Simply divide the size of the transfer by the time it took:

$$R_{I/O}=\frac{Size_{Transfer}}{T_{I/O}}$$

To get a better feel for I/O time, let us perform the following calculation. Assume there are two workloads we are interested in. The first, known as the random workload, issues small (e.g., 4KB) reads to random locations on the disk. Random workloads are common in many important applications, including database management systems. The second, known as the sequential workload, simply reads a large number of sectors consecutively from the disk, without jumping around. Sequential access patterns are quite common and thus important as well.

To understand the difference in performance between random and sequential workloads, we need to make a few assumptions about the disk drive first. Let’s look at a couple of modern disks from Seagate. The first, known as the Cheetah 15K.5“Cheetah 15K.5” by Seagate, Inc… Available at this website, we’re pretty sure it is: http://www.seagate.com/docs/pdf/datasheet/disc/ds-cheetah-15k-5-us.pdf. See above commentary on data sheets., is a high-performance SCSI drive. The second, the Barracuda“Barracuda ES.2 data sheet” by Seagate, Inc… Available at this website, at least, it was: http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_es.pdf. A data sheet; read at your own risk. Risk of what? Boredom., is a drive built for capacity. Details on both are found in the table below.

As you can see, the drives have quite different characteristics, and in many ways nicely summarize two important components of the disk drive market. The first is the “high performance” drive market, where drives are engineered to spin as fast as possible, deliver low seek times, and transfer data quickly. The second is the “capacity” market, where cost per byte is the most important aspect. Thus, the ...