Hard drive performance characteristics have already been introduced in Section 4.2.4, “Hard Drives”
; this section discusses the matter in more depth. This is important for system administrators to understand, because without at least basic knowledge of how hard drives operate, it is possible to unwittingly making changes to your system configuration that could negatively impact its performance.
The time it takes for a hard drive to respond to and complete an I/O request is dependent on two things:
The following sections explore these aspects of hard drive performance in more depth.
5.4.1. Mechanical/Electrical Limitations
Because hard drives are electro-mechanical devices, they are subject to various limitations on their speed and performance. Every I/O request requires the various components of the drive to work together to satisfy the request. Because each of these components have different performance characteristics, the overall performance of the hard drive is determined by the sum of the performance of the individual components.
However, the electronic components are at least an order of magnitude faster than the mechanical components. Therefore, it is the mechanical components that have the greatest impact on overall hard drive performance.
The most effective way to improve hard drive performance is to reduce the drive's mechanical activity as much as possible.
The average access time of a typical hard drive is roughly 8.5 milliseconds. The following sections break this figure down in more detail, showing how each component impacts the hard drive's overall performance.
220.127.116.11. 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.
18.104.22.168. 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.
22.214.171.124. 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.
126.96.36.199. 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.