3.2 I/O Size Requirements
To discover the characteristic I/O size, gather information on data access. If possible, also gather information on data access patterns with respect to read-and-write ratios. If the access patterns cannot be discovered by looking at application specificssuch as logs, in the case of Web servers, or transactions, in the case of OLTP serversthen gather the data from host system tools. On UNIX systems use the sar command to look at the raw disk I/O behavior. On Windows NT systems, use the perfmon command. This data will shed some light on typical I/O sizes and the access patterns.
The raw data can be processed to show some additional interesting I/O behaviors. With a few simple rules of thumb applied to the raw data and the processed data, the analysis can provide all of the information necessary for the design of the SAN. This process yields a set of boundaries for the SAN design goals with respect to the application. At completion, the analysis provides requirements for maximum bandwidth, maximum IOPS, and the amount of storage space. The next two sections take a detailed look at the examination of I/O workload on host systems.
With definite requirements in hand, hardware and software can be selected and integrated to meet specific application needs. Evaluation follows to determine whether the SAN meets expectations or requires any additional changes.