Stable-Storage Implementation To implement stable storage, we need to replicate the needed information in several nonvolatile storage media (usually disk) with independent failure modes, and
to update the information in a controlled manner to ensure that failure during data transfer does not damage the needed information.

The simplest and fastest form of RAID is the mirrored disk, which keeps two copies of each block, on separate disks. Other forms of RAID offer lower costs, but at the expense of lower performance.