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. 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[19]. However, other mass storage technologies take different amounts of time to process reads and writes[20].
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.

[19] 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.
[20] Some optical disk drives exhibit this behavior, due to the physical constraints of the technologies used to implement optical data storage.