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Chapter 5. Managing Storage

RPM is considered adequate only for entry level drives. This averages .05 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. I/O Loads and Performance

The other thing that controls hard drive performance is the I/O load to which a hard drive is subjected.

Some of the specific aspects of the I/O load are:

The amount of reads versus writes

The number of current readers/writers

The locality of reads/writes

These are discussed in more detail in the following sections.

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

6

. However, other mass storage technologies take different amounts of time

to process reads and writes

7

.

The impact of this is that devices that take longer to process write I/O operations (for example) will

be able to handle fewer write I/Os than read I/Os. Looked at another way, a write I/O will consume

more of the device's ability to process I/O requests than will 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.

6. Actually, this is not entirely true. All hard drives include some amount of on board cache memory that is used

to improve read performance. However, any I/O request to read data must eventually be satisfied by physically

reading the data from the storage medium. This means that, while cache may alleviate read I/O performance

problems, it can never totally eliminate the time required to physically read the data from the storage medium.

7. Some optical disk drives exhibit this behavior, due to the physical constraints of the technologies used to

implement optical data storage.