Information Storage and Management: Storing, Managing, and Protecting Digital Information in Classic, Virtualized, and Cloud Environments, Second Edition

Appendix A

Application I/O Characteristics

Application I/O characteristics influence the overall performance of storage system and storage solution design. This appendix describes key application I/O characteristics.

Random and Sequential

I/O is characterized as either random or sequential. Random I/O refers to successive read/write operations from noncontiguous addresses—accesses that are spread across the addressable capacity of the LUN. Examples of applications that largely generate random I/O include messaging, OLTP (online transaction processing) applications.

Sequential I/O refers to successive read/write operations from contiguous addresses—one logical block address after another. In sequential I/O access, disk seek time is reduced because the read/write head moves little to access the next block. Examples of sequential I/O include data backup.

Reads and Writes

Another aspect of the I/O workload is the ratio of read I/Os to write I/Os generated by an application. The sum of the read rate and the write rate is the I/O rate (the number of I/O operations per second). The application's I/O rate is one of the important factors that determine the minimum number of disks required for the application. In storage systems, cache plays an important role to improve the system performance. Table A-1 summarizes how read I/O and write I/O interact with the cache.

Read/Write Interactions with Cache

I/O Type	Read	Write
Random	Hard to effectively cache (difficult to predict prefetch); ...