After spare space extents are reserved within the dynamic pool, RAID extents are created with the remaining drive extents within each tier of the pool. A RAID extent is a collection of drive extents from a common drive partnership group which complete the stripe width chosen for a particular RAID type. For example, if RAID 5 is selected as the RAID type, and 4+1 is selected as the stripe width for a particular tier in the pool, the RAID extent would contain 5 drive extents (4+1), one for each element of the stripe width. The RAID extent provides RAID protection for user data stored within the dynamic pool, and is later used to provide usable capacity to the pool for storage resource creation. A single RAID extent cannot contain two drive extents from a single drive for protection purposes. Also, RAID extents must contain drive extents from only a single drive partnership group.
Figure 6 shows an example of a dynamic pool created with six drives, assuming that RAID 5 (4+1) has been selected. In this example, a number of extents have already been reserved as spare space extents, and the first three RAID extents are shown. Each RAID extent in this example contains five drive extents, due to the 4+1 stripe width. The five-drive extents are selected from the drives within the drive partnership group, and no 2 extents are selected from the same drive. For ease of illustration, the drive extents selected are in order across the drives within the pool. Within a real system, the dynamic pool algorithm will select drive extents from different drives seemingly at random within the drive partnership group.
Figure 6 RAID extent example