RAID 1, or disk mirroring, is the process of replicating the data to more than one disk. Both the disks are operational at the same time so the system can read data from both simultaneously. This enhances the speed of read operations. However, the write operations are slower, as the system executes each write operation twice – once on each disk. A minimum of two disks are required for a RAID 1 array.
RAID 5 can tolerate failure of any drive in the array without losing a single byte of stored data. RAID 5 is block-level striping with distributed parity. Striping and data are distributed by independent read and write operations. In a “write” operation, data to be recorded is striped across all array members interspersed with a parity block and distributed in order to place one segment containing the parity checksum on a different drive within each striping cycle. Sometimes defined as rotating parity, this scheme is the reason why it doesn’t matter which drive in the array fails. A minimum of three disks are required for a RAID 5 array.
RAID 6 can tolerate the concurrent failure of two hard drives while precluding data loss and system downtime. RAID 6 provides a second layer of redundancy by means of two separate, independent parity blocks within each stripe written to the data storage subsystem array and distributed among each of the active members in the array. Thus, another moniker for RAID 6 is “double-parity RAID”.
RAID 6 requires a minimum of four drives, but it will typically have a significantly larger number of array members since performance, fault tolerance and cost efficiency are improved with a larger population.