A logical device is a group of physical disk drives that appears to your operating system as a single drive that can be used for storing data.
A logical device can comprise one or more disk drives and can use part or all of each disk drive’s capacity.
It is possible to include the same disk drive in two different logical devices by using just a portion of the space on the disk drive in each. (Show Me!)
Disk drive space that has been assigned to a logical device is called a segment. A segment can include all or just a portion of a disk drive’s space. A disk drive with one segment is part of one logical device; a disk drive with two segments is part of two logical devices; and so on. A segment can be part of only one logical device. When a logical device is deleted, the segments that comprised it revert to available space (or free segments).
A logical device can include redundancy, depending on the RAID level assigned to it. (See How RAID Protects Your Data for more information.)
Once a logical device has been created, you can change its RAID level or increase its capacity to meet changing requirements. You can also protect your logical devices by assigning one or more hot spares to them.