You create a Flash Pool aggregate by enabling the feature on an existing 64-bit aggregate composed of HDD RAID groups, and then adding one or more SSD RAID groups to that aggregate. This results in two sets of RAID groups for that aggregate: SSD RAID groups (the SSD cache) and HDD RAID groups.
Using fewer RAID groups in the SSD cache reduces the number of parity disks required.
After you add an SSD cache to an aggregate to create a Flash Pool aggregate, you cannot remove the SSD cache to convert the aggregate back to its original configuration.
You can change the RAID group size of the SSD cache, but you cannot make this change until after SSDs have been added. After disks have been added to a RAID group, they cannot be removed. If you know that you want to use a different RAID group size than the default SSD RAID group size, you can add a small number SSDs at first. Then, after you update the RAID group size, you can add the rest of the SSDs.
By default, the RAID level of the SSD cache is the same as the RAID level of the HDD RAID groups. You can override this default selection by specifying the -t option when you add the first SSD RAID groups. Although the SSD cache is providing caching for the HDD RAID groups, the SSD cache is integral to the health of the aggregate as a whole. An SSD RAID group that experiences a failure that exceeds the RAID protection capability of the RAID level in use takes the aggregate offline. For this reason, it is a best practice to keep the RAID level of the SSD cache the same as that of the HDD RAID groups.
There are platform- and workload-specific best practices for Flash Pool SSD cache size and configuration. For information about these best practices, see Technical Report 4070: NetApp Flash Pool Design and Implementation Guide.
If you plan to change the RAID group size for the SSD cache, you should add only a small number SSDs in this step. (You must add at least three.)
If the HDDs and the SSDs do not have the same checksum type, or if the aggregate is a mixed-checksum aggregate, then you must use the -c parameter to specify the checksum type of the disks you are adding to the aggregate.
You can specify a different RAID type for the SSD cache by using the -t option.