Whenever possible, you should use disks of the same speed in an aggregate. However, if needed, you can configure Data ONTAP to allow mixed speed aggregates based on the disk class.
When these options are set to on, Data ONTAP allows mixing speeds for the designated disk class. Performance disk types are FC and SAS; capacity disk types are BSAS, FSAS, MSATA, and ATA.
By default, raid.mix.hdd.rpm.performance is set to off, and raid.mix.hdd.rpm.capacity is set to on.
Even if Data ONTAP is not configured to allow mixing speeds, you can still create aggregates out of disks with different speeds by using the -f option of the aggr create or aggr add commands.