IBM 86624RY Maintenance Manual - Page 77

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. Drive A Drive B Data Stripe 1 Data Stripe 2 Data Stripe 3 . . Block 1 Block 2 Block 3 Mirror Stripe 1 Mirror Stripe 2 Mirror Stripe 3 . . Block 1 Block 2 Block 3 If one of the hard disk drives fails, the ServeRAID controller switches read and write requests to the remaining functional drive in the array. Enhanced RAID level-1: When you group more than two physical drives into an array and 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. Netfinity 5500 M20 - Type 8662 69