HP Surestore Disk Array FC60 HP SureStore E Disk Array FC60 - (English) Advanc - Page 243

Selecting Disks for a LUN When binding a LUN , you must select the disks that will be used. The capacity of the LUN is determined by the number and capacity of the disks you select, and the RAID level. When selecting disks for a LUN , consider the following: • To maximize high availability, select disks in different disk enclosures or on different channels. Multiple disks in the same enclosure make a RAID 5 LUN vulnerable to an enclosure failure. A RAID 1 or 0/1 LUN can survive an enclosure failure, as long as both disks of a mirrored pair are not in the same enclosure. If you attempt to select disks in the same enclosure or on the same channel when binding a LUN , you will be warned that doing so may compromise high availability. • Select disks of the same capacity.Binding a LUN using different size disks will result in unused capacity on the larger disks. For example, binding a 4-disk LUN that includes two 9-Gbyte disks and two 18-Gbyte disks will result in a LUN with the capacity of four 9-Gbyte disks. Only 9 Gbytes of the larger disks will be used when creating the LUN . • Consider using more disks for RAID 5LUNs . This increases both performance and storage efficiency. Because RAID 5 uses only one disk's worth of capacity for parity, more disks in the LUN will result in a greater percentage of the disk capacity used for data. For example, in a 4-disk RAID 5LUN , 75% of the capacity is used for data (3 of 4). Adding another disk to the LUN will increase the percentage of disk capacity used for data to 80% (4 of 5). Selecting Disks for a RAID 0/1 LUN The order in which you select disks is important when creating a RAID 0/1 LUN. The first half of the disks you select will be the primary disks, and the second half of the disks will be the disk mirrors. To maintain data availability, the disk mirrors must be in a different enclosure than the primary disks. This applies regardless of which tool you are using to bind a RAID 0/1 LUN. For example, in Figure 85 a 4-disk RAID 0/1 LUN is being bound using one disk enclosure on channel 1, and a second disk enclosure on channel 2. The correct order for selecting disks is 1:2, 1:3, 2:2, 2:3. This selection order creates mirrored pairs of 1:2/2:2 and 1:3/2:3. This maintains high availability because the primary disks are on channel 1, and the mirror disks are on channel 2. Managing Disk Array Capacity 243 Managing the Disk Array on HP-UX