Making The Storage Usable - Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

Chapter 5. Managing Storage 69
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 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
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:
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 is only possible by using
sector addresses (either geometrical or logical).