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When a dynamic pool is created or expanded, the selected drives are placed into drive partnership groups. A drive partnership group is a group of drives of the same drive type which are combined into a hidden dynamic pool object. The number of drives placed in each group directly depends on the number and type of drives selected at pool creation, or selected later when the pool is expanded. A dynamic pool always includes at least one drive partnership group, and each drive within a dynamic pool can only be part of a single drive partnership group. For the life of the pool, a drive never changes the drive partnership group that it is part of.
Each drive partnership group can only contain a single drive type, though different sizes of a particular drive type can be mixed within the group. Figure 1 shows an example of a dynamic pool created with different drive types and sizes. In this example, all SAS flash 3 drives are placed into a single drive partnership group, while all SAS flash 4 drives are placed into their own drive partnership group. A typical all-flash configuration contains a single drive type and size, but dynamic pools allow for different drive types and sizes to be mixed.
Figure 1 Drive partnership group example with SAS flash 3 and SAS flash 4 drives
Within a hybrid system, adding flash, SAS, and NL-SAS drives into a multi-tiered pool is common. As each drive partnership group can only contain a single drive type, flash, SAS, and NL-SAS drives will be placed into their own drive partnership groups. Furthermore, different speed spinning drives can be added to the same dynamic pool but will be placed into their own drive partnership groups for performance reasons. For example, 10K RPM and 15K RPM drives residing in the same pool will be placed into different drive partnership groups regardless of the drive size as shown in the figure below. In this example, the 6TB and 12TB NL-SAS drives are placed in the same drive partnership group as they have the same speed rating.
Figure 2 Drive partnership group example with multiple drive types
The maximum number of drives contained within a drive partnership group is 64. When more drives of the same type are included in the pool, more drive partnership groups are created. Each drive partnership group is started with a minimum number of drives, which is controlled by the RAID type and stripe width of the pool. Once the RAID type and stripe width are set for a particular drive type, all drive partnership groups containing that type of drive will use those settings. When a drive partnership group is full, another minimum number of drives is needed to create the next drive partnership group with the same RAID type and stripe width. This topic is covered in further detail in section 0 on supported configurations.
Figure 3 shows an example of multiple dynamic pools and drive partnership groups. In the left image, the dynamic pool has only six drives of the same type. All of these drives are placed into a single drive partnership group at time of pool creation. The middle image is a pool which contains 64 drives of the same type. The pool may have been created with 64 drives, or was created with less drives and later expanded. Since the total number of drives is 64, they are all placed into a single drive partnership group, and the group is considered full. The right image is an example of a dynamic pool which contains 70 drives. Since a drive partnership group is considered full at 64 drives, the remaining six drives are placed in a new drive partnership group. As with the middle pool, this pool may have been created using 70 drives, or was expanded after creation to reach this drive count.
Figure 3 Drive partnership group example
The drive partnership group within a dynamic pool is used to limit the amount of drives other dynamic pool objects can cross. These objects include drive extents, RAID extents, spare space extents, and dynamic pool private RAID groups, all of which are explained in the following subsections. This level of division ensures the data is not spread across a large amount of drives, which can increase the risk of encountering multiple drive failures and could degrade or break the RAID protection selected.