Understanding RAID Technology
RAID is the technology of grouping several hard disk drives in a computer into an array that you can define 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 you group multiple physical hard disk drives 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 your 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 Menu Bar 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 you run on the server and the RAID levels that you assign to the logical drives. Netfinity 5500-M10 - Type 8661
The ServeRAID adapters and controllers support RAID level-0, RAID level-1, Enhanced RAID level-1, and RAID level-5.
RAID Level-0: RAID level-0 stripes the data across all the drives in the array. This offers substantial speed enhancement, but provides for no data redundancy. RAID level-0 provides the largest capacity of the RAID levels offered, because no room is taken up for redundant data or data-parity storage.
The following illustration shows data arranged in an array of three hard disk drives defined as one RAID level-0 logical drive. Notice that the data is striped across all the drives in the array, but no copies of the data or parity information is stored.
When you replace a failed drive, the ServeRAID controller can rebuild all the RAID level-1 and RAID level-5 logical drives automatically onto the replacement hard disk drive. However, any data stored in a failed RAID level-0 logical drive is lost.
Though the risk of data loss is present, you might want to assign RAID level-0 to one of the logical drives to take advantage of the speed offered with this RAID level. You could use this logical drive to enter data that you back up each day and for which safety is not of primary importance, that is, data that you can re-create easily. You also might want to use a RAID level-0 logical drive when the work you are doing requires maximum capacity.
RAID Level-1: RAID level-1 provides 100% data redundancy and requires two hard disk drives. With RAID level-1, the first half of a stripe is the original data, the second half of a stripe is a mirror (copy) of the data, but written to another drive.
Because the data is mirrored, the capacity of the logical drive when assigned RAID level-1 is 50% of the physical capacity of the two hard disk drives in the array.
The following illustration shows data arranged in an array of two hard disk drives defined as one RAID level-1 logical drive. Notice that the data on Drive B is a mirror copy of the data on Drive A.
Enhanced RAID Level-1: When you group more than two physical drives into an array and you select RAID level-1, the ServeRAID controller automatically assigns the IBM Enhanced RAID level-1 to the array.
The Enhanced RAID level-1 combines mirroring with data striping. This RAID level stripes data and copies of the data across all the drives in the array. As with the standard RAID level-1, the data is mirrored, and the capacity of the logical drive is 50% of the physical capacity of the grouping of hard disk drives in the array.
The Enhanced RAID level-1 requires a minimum of three drives and, depending upon the stripe-unit size, supports a maximum of eight or 16 drives.
The following illustration shows data arranged in an array with three hard disk drives. The logical drive is assigned the Enhanced RAID level-1. Notice that the first stripe is the data stripe and the second stripe is the mirror (copy) of the first data stripe, but shifted one drive.
Please see the LEGAL - Trademark notice.
Feel free - send a for any BUG on this page found - Thank you.