HP Surestore Disk Array FC60 HP SureStore E Disk Array FC60 Service Manual (A5 - Page 190

When selecting disks for a volume group, 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 3 or RAID 5 volume group vulnerable to an enclosure failure. A RAID 1 or 0/1 volume group 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 creating a volume group, you will be warned that doing so may compromise high availability. • Select disks of the same capacity. Creating a volume group using different size disks will result in unused capacity on the larger disks. For example, creating a 4-disk volume group that includes two 9-Gbyte disks and two 18-Gbyte disks will result in a volume group with the capacity of four 9-Gbyte disks. Only 9 Gbytes of the larger disks will be used when creating the volume group. • Consider using more disks for RAID 3 and RAID 5 volume groups. This increases both performance and storage efficiency. Because RAID 5 uses only one disk's worth of capacity for parity, more disks in the volume group will result in a greater percentage of the disk capacity used for data. For example, in a 4-disk RAID 5 volume group, 75% of the capacity is used for data (3 of 4). Adding another disk to the volume group will increase the percentage of disk capacity used for data to 80% (4 of 5). How are disk modules in the array identified? When performing tasks such as creating volume groups, disk modules are identified by a numbered pair of the form "channel number:SCSI ID". For example, 1:3 identifies the disk module on channel 1 with a SCSI ID of 3. Status information may also include the enclosure number and slot number. • Channel number indicates the SCSI channel that the disk enclosure containing the disk module is connected to. There are six channels available (1 - 6). If the disk enclosure is using full-bus configuration, all disk modules within the enclosure will be on the same SCSI channel. If the disk enclosure is configured for split-bus operation, disks in the even-numbered slots will be on one channel, and disks in the odd-numbered slots will be on a different channel. • SCSI ID identifies the ID assigned to the disk module. SCSI ID's are determined by the slot in which the disk module is installed in the disk enclosure. If the disk enclosure is 190 Managing Disk Array Capacity