HP Surestore Disk Array 12h and FC60 - Manual (Page 243)

Managing Disk Array Capacity 243
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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.