5.4. Hard Drive Performance Characteristics

5.4.1.1. Command Processing Time

All hard drives produced today have sophisticated embedded computer systems controlling their operation. These computer systems perform the following tasks:

• Interacting with the outside world via hard drive's interface

• Controlling the operation of the rest of the hard drive's components, recovering from any error conditions that might arise

• Processing the raw data read from and written to the actual storage media

Even though the microprocessors used in hard drives are relatively powerful, the tasks assigned to them take time to perform. On average, this time is in the range of .003 milliseconds.

5.4.1.2. Heads Reading/Writing Data

The hard drive's read/write heads only work when the disk platters over which they "fly" are spinning. Because it is the movement of the media under the heads that allows the data to be read or written, the time that it takes for media containing the desired sector to pass completely underneath the head is the sole determinant of the head's contribution to total access time. This averages .0086 milliseconds for a 10,000 RPM drive with 700 sectors per track.

5.4.1.3. Rotational Latency

Because a hard drive's disk platters are continuously spinning, when the I/O request arrives it is highly unlikely that the platter will be at exactly the right point in its rotation necessary to access the desired sector. Therefore, even if the rest of the drive is ready to access that sector, it is necessary for everything to wait while the platter rotates, bringing the desired sector into position under the read/write head.

This is the reason why higher-performance hard drives typically rotate their disk platters at higher speeds. Today, speeds of 15,000 RPM are reserved for the highest-performing drives, while 5,400 RPM is considered adequate only for entry-level drives. This averages approximately 3 milliseconds for a 10,000 RPM drive.

5.4.1.4. Access Arm Movement

If there is one component in hard drives that can be considered its Achilles' Heel, it is the access arm. The reason for this is that the access arm must move very quickly and accurately over relatively long distances. In addition, the access arm movement is not continuous — it must rapidly accelerate as it approaches the desired cylinder and then just as rapidly decelerate to avoid overshooting. Therefore, the access arm must be strong (to survive the violent forces caused by the need for quick movement) but also light (so that there is less mass to accelerate/decelerate).

Achieving these conflicting goals is difficult, a fact that is shown by how relatively much time the access arm movement takes when compared to the time taken by the other components. Therefore, the movement of the access arm is the primary determinant of a hard drive's overall performance, averaging 5.5 milliseconds.

5.4.2.1. Reads Versus Writes

For the average hard drive using magnetic media for data storage, the number of read I/O operations versus the number of write I/O operations is not of much concern, as reading and writing data take the same amount of time[1]. However, other mass storage technologies take different amounts of time to process reads and writes[2].

The impact of this is that devices that take longer to process write I/O operations (for example) are able to handle fewer write I/Os than read I/Os. Looked at another way, a write I/O consumes more of the device's ability to process I/O requests than does a read I/O.

5.4.2.2. Multiple Readers/Writers

A hard drive that processes I/O requests from multiple sources experiences a different load than a hard drive that services I/O requests from only one source. The main reason for this is due to the fact that multiple I/O requesters have the potential to bring higher I/O loads to bear on a hard drive than a single I/O requester.

This is because the I/O requester must perform some amount of processing before an I/O can take place. After all, the requester must determine the nature of the I/O request before it can be performed. Because the processing necessary to make this determination takes time, there is an upper limit on the I/O load that any one requester can generate — only a faster CPU can raise it. This limitation becomes more pronounced if the requester requires human input before performing an I/O.

However, with multiple requesters, higher I/O loads may be sustained. As long as sufficient CPU power is available to support the processing necessary to generate the I/O requests, adding more I/O requesters increases the resulting I/O load.

However, there is another aspect to this that also has a bearing on the resulting I/O load. This is discussed in the following section.

5.4.2.3. Locality of Reads/Writes

Although not strictly constrained to a multi-requester environment, this aspect of hard drive performance does tend to show itself more in such an environment. The issue is whether the I/O requests being made of a hard drive are for data that is physically close to other data that is also being requested.

The reason why this is important becomes apparent if the electromechanical nature of the hard drive is kept in mind. The slowest component of any hard drive is the access arm. Therefore, if the data being accessed by the incoming I/O requests requires no movement of the access arm, the hard drive is able to service many more I/O requests than if the data being accessed was spread over the entire drive, requiring extensive access arm movement.

This can be illustrated by looking at hard drive performance specifications. These specifications often include adjacent cylinder seek times (where the access arm is moved a small amount — only to the next cylinder), and full-stroke seek times (where the access arm moves from the very first cylinder to the very last one). For example, here are the seek times for a high-performance hard drive: