Adaptec 44300 Installation And User Manual

Also see for 48300: Datasheet Installation and user guide

Contents

Adaptec 44300 Installation And User Manual Manual pdf 59 page image

Appendix B: Understanding RAID ● 59RAID Technology OverviewRAID is the technology of grouping several physical drives in a computer into an array that youcan define as one or more logical drives. Each logical drive appears to the operating system as asingle drive. This grouping technique greatly enhances logical-drive capacity and performancebeyond the physical limitations of a single physical drive.When you group multiple physical drives into a logical drive, the HostRAID controller cantransfer data in parallel from the multiple drives in the array. This parallel transfer yields data-transfer rates that are many times higher than with non-arrayed drives, enabling the system tobetter meet the throughput (amount of data processed in a given amount of time) orproductivity needs of the multiple-user network environment.The ability to respond to multiple data requests provides not only an increase in throughput,but also a decrease in response time. The combination of parallel transfers and simultaneousresponses to multiple requests enables disk arrays to provide a high level of performance innetwork environments.Understanding Drive SegmentsA drive segment is a disk drive or portion of a disk drive that is used to create an array. A diskdrive can include both RAID segments (segments that are part of an array) and availablesegments. Each segment can be part of only one logical device at a time. If a disk drive is notpart of any logical device, the entire disk is an available segment.Stripe-unit SizeWith RAID technology, data is striped across an array of physical drives. This data-distributionscheme complements the way the operating system requests data.The granularity at which data is stored on one drive of the array before subsequent data isstored on the next drive of the array is called the stripe-unit size.You can set the stripe-unit size to 16, 32, or 64 KB. You can maximize the performance of yourHostRAID controller by setting the stripe-unit size to a value that is close to the size of thesystem I/O requests. For example, performance in transaction-based environments, whichtypically involve large blocks of data, might be optimal when the stripe-unit size is set to 32 or64 KB. However, performance in file and print environments, which typically involve multiplesmall blocks of data, might be optimal when the stripe-unit size is set to 16 KB.The collection of stripe units, from the first drive of the array to the last drive of the array, iscalled a stripe.Selecting a RAID Level and Tuning PerformanceDisk arrays are used to improve performance and reliability. The amount of improvementdepends on the application programs that you run on the server and the RAID levels that youassign to the logical drives.Each RAID level provides different levels of fault-tolerance (data redundancy), utilization ofphysical drive capacity, and read and write performance. In addition, the RAID levels differ inregard to the minimum and maximum number of physical drives that are supported.