COSI 127b Lecture Notes - Lecture 6: Parity Bit, Disk Controller, Standard Raid Levels

• School: Brandeis University
• Department: Computer Science
• Course: COSI 127b
• Professor: Cherniack

Document Summary

Parity uses n + 1 disks, where n disks store data bits and 1 disk stores a parity bit. An odd parity bit means the sum of all n + 1 bits is odd. If 1 disk fails, parity can be used for recovery. Raid 3 uses block striping with a dedicated parity drive. Parity requires fewer disks but it is less reliable than mirroring. Raid 5 is block striping with distributed parity. A disk controller is a hardware interface between bus and disk. The elevator algorithm has the head sweep back and forth when no requests lie ahead; thereby minimizing latency time. A buffer is an area of process space where data resides before and after disk access. The buffer manager can clear space in the buffer by using a page replacement policy. Databases often know what blocks will next be needed. Thus, the query processor can provide hints to the buffer manager.