Lacie 4big Quadra User Manual - Page 17

LaCie 4big Quadra Enterprise Class • Design by Neil Poulton User Manual RAID page 17 3.1.5. RAID 5 and RAID 5+Spare RAID 5 combines the striping of RAID 0 with data redundancy in an array with a minimum of three disks (Fig. 12). Data is striped across all disks and a parity block (P) for each data block is written on the same stripe. If one physical disk fails, the data from the failed disk can be rebuilt onto a replacement disk. No data is lost in the case of a single disk failure, but if a second disk fails before data can be rebuilt to a replacement drive, all data in the array will be lost. In RAID 5+Spare, one disk in the array is left empty. If a drive in the array fails, the data from the failed disk is automatically rebuilt to the empty or "spare" disk (Fig. 13). Applications RAID 5 combines data safety with efficient use of disk space. Disk failure does not result in a service interruption because data is read from parity blocks. RAID 5 is useful for people who need performance and constant access to their data, like video editors. In RAID 5+Spare disk failure does not require immediate attention because the system rebuilds itself using the hot spare, but the failed disk should be replaced as soon as possible. RAID 5 A1 B1 C1 Dp Disk 1 Fig. 12 A2 B2 Cp D1 Disk 2 A3 Bp C2 D2 Disk 3 Ap B3 C3 D3 Disk 4 RAID 5+Spare A1 B1 Cp D1 Disk 1 Fig. 13 A2 Bp C1 D2 Disk 2 Ap B2 C2 Dp Disk 3 Spare Disk 4