RAID level 1 is the most expensive solution to achieve the redundancy. In this system, two identical copies of the data on two different disks are maintained. This type of redundancy is called mirroring.
Data striping is used here similar to RAID 0.
Every write of a disk block involves two write due to the mirror image of the disk blocks.
These writes may not be performed simultaneously, since a global system failure may occur while writing the blocks and then leave both copies in an inconsistent state. Therefore, write a block on a disk first and then write the other copy on the mirror disk.
A read of a block can be scheduled to the disk that has the smaller access time. Since we are maintaining the full redundant information, the disk for mirror copy may be less costly one to reduce the overall cost.