Adaptec 3405 User Guide - Page 84

Appendix B: Understanding RAID ● 84 RAID 1 Arrays A RAID 1 array is built from two disk drives, where one disk drive is a mirror of the other (the same data is stored on each disk drive). Compared to independent disk drives, RAID 1 arrays provide improved performance, with twice the read rate and an equal write rate of single disks. However, capacity is only 50 percent of independent disk drives. If the RAID 1 array is built from different- sized disk drives, the free space, drive segment size is the size of the smaller disk drive, as shown in this figure. Drive Segment Size (Smallest Disk Drive) Disk Drive 1 250 GB Disk Drive 2 400 GB Disk Drives in Logical Drive Disk Drive 1 1 - 250 Disk Drive 2 1 - 250 Unused Space = 150 GB RAID 1 Logical Drive = 250 GB RAID 1 Enhanced Arrays A RAID 1 Enhanced (RAID 1E) array-also known as a striped mirror-is similar to a RAID 1 array except that data is both mirrored and striped, and more disk drives can be included. A RAID 1E array can be built from three or more disk drives. In this example, the large bold numbers represent the striped data, and the smaller, non-bold numbers represent the mirrored data stripes. Disk Drive 1 400 GB Disk Drive 2 400 GB Disk Drive 3 400 GB Disk Drives in Logical Drive Disk Drive 1 1 3 4 6 Disk Drive 2 2 1 5 4 Disk Drive 3 3 2 6 5 RAID 1E Logical Drive = 600 GB