Configuring an optimum RAID group size for an aggregate made up of drives requires a trade-off of factors. You must decide which factor—speed of recovery, assurance against data loss, or maximizing data storage space—is most important for the aggregate that you are configuring.
You change the size of RAID groups on a per-aggregate basis. You cannot change the size of an individual RAID group.
You should follow these guidelines when sizing your RAID groups composed of HDDs:
If this is impossible, any RAID group with fewer disks should have only one less disk than the largest RAID group.
The reliability of SAS and FC disks can support a RAID group size of up to 28, if needed.
The SSD RAID group size can be different from the RAID group size for the HDD RAID groups in a Flash Pool aggregate. Usually, you should ensure that you have only one SSD RAID group for a Flash Pool aggregate, to minimize the number of SSDs required for parity.
You should follow these guidelines when sizing your RAID groups composed of SSDs:
If this is impossible, any RAID group with fewer drives should have only one less drive than the largest RAID group.