IBM 86624RY Maintenance Manual - Page 75

Understanding RAID technology RAID is the technology of grouping several hard disk drives in a computer into an array that can be defined as one or more logical drives. Each logical drive appears to the operating system as a single drive. This grouping technique greatly enhances logical-drive capacity and performance beyond the physical limitations of a single hard disk drive. When multiple physical hard disk drives are grouped into a logical drive, the ServeRAID controller can transfer data in parallel from the multiple drives in the array. This parallel transfer yields data-transfer rates that are many times higher than with nonarrayed drives. This increased speed makes the system better able to meet the throughput (the amount of work in a given amount of time) or productivity needs of the multiple-user network environment. The ability to respond to multiple data requests provides not only an impressive increase in throughput, but also a decrease in response time. The combination of parallel transfers and simultaneous responses to multiple requests allows disk arrays to provide a high level of performance in network environments. Interleave depth and stripe-unit size: With RAID technology, data is striped across an array of hard disk drives. This data-distribution scheme complements the way the operating system requests data. The granularity at which data from one file is stored on one drive of the array before subsequent data is stored on the next drive of the array is called the interleave depth. You can control the interleave depth and maximize the performance of a ServeRAID controller by setting a stripe-unit size that is close to the size of the system I/O requests. You can set the stripe-unit size to 8 KB, 16 KB, 32 KB, or 64 KB. For example, performance in transaction-based environments, which typically involve large blocks of data, might be optimal when the stripe-unit size is set to 32 KB or 64 KB; however, performance in file and print environments, which typically involve multiple small blocks of data, might be optimal when the stripe-unit size is set to 8 KB or 16 KB. The collection, in logical order of these stripe units, from the first drive of the array to the last drive of the array, is called a stripe. (See page 90 for more information.) Supported RAID levels: Disk arrays are used to improve performance and reliability. The amount of improvement depends on the application programs that are run on the server and the RAID levels that are assigned to the logical drives. Netfinity 5500 M20 - Type 8662 67