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Dell Unity storage systems support a variety of drive type and size options. Choosing between the options available depends what needs to be achieved in the solution. If density is the main concern, using a small amount of larger drives may be preferable. If performance is the main concern, then using a larger count of smaller drives may be selected. In most cases, a balance of the overall footprint of the system, performance, and overall cost of the solution is factored into the decision.
In a typical configuration for an all-flash system, all drives within the system are the same size and type. An all-flash system containing different drive sizes or types typically occurs when additional drives are purchased to expand the capacity of an all-flash system. In hybrid systems, a mix of flash, SAS, and NL-SAS drives is typical. Dynamic pools support mixing different drive sizes and types within a single pool. This allows flexibility in the solution. Within the pool, flash, SAS, and NL-SAS drives cannot exist within the same drive partnership group, as outlined in Figure 2. Each drive type is subject to the stripe width and minimum drive count requirements which is explained later. For hybrid systems, SAS and NL-SAS system drives, DPE drives 0, 1, 2 and 3, are not supported within a dynamic pool. System drives can still be placed in traditional pools.
Mixing different drive sizes of the same type and speed is supported within a dynamic pool, and the drives are placed within the same drive partnership group. This may occur when a pool’s capacity is critically low and the pool must be expanded to increase the capacity, or a spare of the same size is not available. When this occurs, there are situations where the entire capacity of the largest drive(s) may not be usable. This unusable space is not reported to the user. The entire capacity of a particular drive size is only available when the drive count within the drive partnership group for the drive in question matches or exceeds the stripe width plus one drive. It is not recommended to mix different size drives of the same type for this reason, unless an adequate number of drives are available to meet or exceed the stripe width plus one drive requirement.
Figure 12 shows an example of a dynamic pool created with 5 x 400 GB drives and 1 x 800 GB flash drive. This assumes there is a RAID 5 pool with a stripe width of 4+1. In this example, many of the drive extents on the 800 GB drive cannot be used. These drive extents are outlined in red. This situation can also occur with SAS and NL-SAS drives.
Figure 12 Dynamic pool with mixed drive sizes and space unavailable for use
Figure 13 shows an example of a dynamic pool created as a RAID 5 pool with a stripe width of 4+1. In this example all drive extents are available for use. This pool may have been created this way, or the pool shown in Figure 13 was expanded with additional 800 GB drives to recover the missing space. In either case, all drive extents are available for use, and can be provisioned as spare space extents or RAID extents. If the last 800 GB drive within the example below was a 1.6 TB drive of the same drive type, then only 800 GBs of space would be usable.
Figure 13 Dynamic pool with mixed drive sizes and space available for use