Chapter 5. Managing Storage

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This is because, short of the I/O requester being an I/O benchmarking tool that does nothing but

produce I/O requests as quickly as possible, some amount of processing must be done before an I/O

is performed. 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 will be 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 reqester 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 will continue to increase 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 perfor 

mance 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 will

be 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:

Adjacent Cylinder

Full Stroke

0.6

8.2

Table 5 4. Adjacent Cylinder and Full Stroke Seek Times (in Milliseconds)

5.5. Making the Storage Usable

Once a mass storage device is in place, there is little that it can be used for. True, data can be written

to it and read back from it, but without any underlying structure data access will only be possible by

using sector addresses (either geometrical or logical).

What is needed are methods of making the raw storage a hard drive provides more easily usable. The

following sections explore some commonly used techniques for doing just that.

5.5.1. Partitions/Slices

The first thing that often strikes a system administrator is that the size of a hard drive may be much

larger than necessary for the task at hand. Many operating systems have the capability to divide a hard

drive's space into various partitions or slices.

Because they are separate from each other, partitions can have different amounts of space utilized,

and that space will in no way impact the space utilized by other partitions. For example, the partition