Appendix B: Selecting the Best RAID Level ● 155RAID 1 Logical DrivesA RAID 1 logical drive is built from two disk drives, where one disk drive is a mirror of theother (the same data is stored on each disk drive). Compared to independent disk drives, RAID 1logical drives provide improved performance, with twice the read rate and an equal write rateof single disks. However, capacity is only 50 percent of independent disk drives.If the RAID 1 logical drive is built from different-sized disk drives, drive segment size is the sizeof the smaller disk drive, as shown in this figure.RAID 1 Enhanced Logical DrivesA RAID 1 Enhanced (RAID 1E) logical drive—also referred to as a striped mirror—is similar toa RAID 1 logical drive except that data is both mirrored and striped, and more disk drives canbe included. A RAID 1E logical drive can be built from three or more disk drives.In this figure, the large bold numbers represent the striped data, and the smaller, non-boldnumbers represent the mirrored data stripes.Disk Drive 2Disk Drive 1 250 GB400 GBDrive Segment Size (Smallest Disk Drive)Disk Drive 1Disk Drive 2Disk Drives in Logical Drive RAID 1 Logical Drive = 250 GB1 – 2501 – 250Unused Space = 150 GBDisk Drive 2Disk Drive 1400 GBDisk Drives in Logical Drive RAID 1E Logical Drive = 600 GB400 GB400 GBDisk Drive 3Disk Drive 2Disk Drive 1Disk Drive 31 32345612645
