It uses two or three disks , and then partitions them equally to form a combination of RAID 0 , RAID 1 , RAID 5 or RAID 1 +0 levels .
Whether an array runs as RAID 0 +1 or RAID 1 +0 in practice is often determined by the evolution of the storage system . A RAID controller might support upgrading a RAID 1 array to a RAID 1 +0 array on the fly , but require a lengthy offline rebuild to upgrade from RAID 1 to RAID 0 +1 .