Remember, both disks must be updated. Also, seek time is different between the two disks since they are not synchronized. Therefore, it is the most expensive solution. In this scheme, data is striped across disks in very small units . These units are so small that all disks must work together on both reads and writes because a single sector actually spans all of the disks.
Redundancy is implemented by calculating parity and storing it on the check disk.
This level can survive a single disk failure and reconstruct data using the remaining disks. This RAID level is somewhat limited since the entire disk system can only handle one request at a time. Thus, the sustainable request rate is no higher than that of a single disk. This means that each disk can handle an individual small read request if all disks are working. However, writes are still a problem since the parity disk must participate in all write operations, which creates a bottleneck.