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.
188.8.131.52. 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. However, other mass storage technologies take different amounts of time to process reads and writes.
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.
 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.
 Some optical disk drives exhibit this behavior, due to the physical constraints of the technologies used to implement optical data storage.