RAID

Redundant Arrays of Independent Disks

• RAID is a disk organisation technique that manages many redundant disks to store the same piece of information.
• Having multiple disks in parallel gives higher read speed.
• Having redundancy gives high reliability, allowing data to be recovered even when a disk fails.

Mirroring Technique

• In this technique, each disk is duplicated. A logical disk has two physical disks.
• Each write is carried out on both disks, each read is carried out on either disk.
• If a disk fails, we restore from the other disk. Theoretically both disks can fail simultaneously, but the probability of that is very small.

Mean Time to Repair

• MTR is the average amount of time required to repair a failed disk.

Mean Time to Data Loss

• MTTDL is the amount of time required for one block of data to be unretrievable.
• \(MTTDL = MTTF \cdot MTR \cdot disk\_count\)