Home > Storage > Unity XT > Storage Admin > Dell Unity Dynamic Pools > Dynamic pool private RAID groups and LUNs
Within a pool on a Dell Unity system, whether Traditional or Dynamic, exists one or more private RAID groups and a single private LUN created on each. These objects, as the names suggest, are underlying components of the system that are not shown in Unisphere, CLI, or REST API. The private RAID group is used to provide space to the private LUN, which provides space in the form of 256 MB slices to the user for storage resource allocation. In a traditional pool, the private RAID group is created by combining a number of drives into a physical RAID group within the system, whose RAID protection and width matches the settings chosen at time of creation or pool expansion. In the case of dynamic pools, the private RAID group concept and the creation of the private LUN and 256 MB slices on top of the usable capacity remain the same. What is different is how space is provided to the private RAID group.
Within dynamic pools, a private RAID group is created using a combination of RAID extents. Depending on the number of drives selected at time of pool creation or expansion, the dynamic pool software will determine how many RAID extents and private RAID groups to create. Later, a single private LUN is created on each private RAID group.
Figure 7 shows an example of a dynamic pool and the private RAID groups created in it. In this example, the dynamic pool consists of six drives of the same size and type. Once the drives extents are combined into RAID extents, the RAID extents are added to dynamic pool private RAID groups. The number of RAID extents contained in each private RAID group are equal, though this is not always the case. For illustration purposes, the RAID extents were created by taking drive extents in order across the drives, though this is not necessarily the order the dynamic pool algorithm will take them. Each tier within a multi-tiered pool will have its own private RAID groups and LUNs.
Figure 7 Dynamic pool private RAID group example
After RAID extents are assigned to a private RAID group, the usable space for each extent is combined into a private LUN. The private LUN is later carved into 256 MB slices for storage resource allocation. To ensure user data is spread across as many drives possible, the RAID extents are broken into pieces, a few MBs in size, and combined through concatenation into the private LUN. This ensures that each 256 MB slice contains pieces of multiple RAID extents within the private RAID group.
Figure 8 is an example of a private LUN created using RAID extents within a private RAID group. Pieces of each RAID extent are concatenated together to form the useable capacity of the private LUN. Later, the private LUN is carved into 256 MB slices, which are used to provide space for storage resources created on the dynamic pool. The benefit of allocating space in this manner is that a single 256 MB slice worth of data is spread across multiple RAID extents. These RAID extents may be spread across all drives within a single drive partnership group, which can contain up to 64 drives. This spreading out of data helps to not only spread the workload throughout the drive partnership group, but also spread out wear as much as possible due to the workload. When creating a storage resource on a dynamic pool, such as a LUN or file system, 256 MB slices for the resource will be taken from the different private LUNs within the dynamic pool to further spread out the workload.