Chapter 5. Managing Storage

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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 to 

gether to satisfy the request. Because each of these components have different performance character 

istics, 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.

Tip

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 5.6 milliseconds. The following sections

break this figure down in more detail, showing how each component impacts the hard drive's overall

performance.

5.4.1.1. Command Processing Time

All hard drives produced today have sophisticated embedded computer systems controlling their op 

eration. 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 .00014 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