HP ProLiant 2500 Novell GroupWise Performance Management on Compaq Servers - Page 14

ECG007.0897 WHITE PAPER (cont.) 1...4 The following levels of fault tolerance support are available: • RAID 5 - Distributed Data Guarding • RAID 4 - Data Guarding • RAID 1 - Disk Mirroring • RAID 0 - No Fault Tolerance Support The Compaq Smart-2 Array Controller is needed to support hardware striping and all levels of fault tolerance support. Features offered by the Compaq Smart-2 Controllers that are not found with Fast-Wide SCSI-2 Controllers are: • Support for RAID 0, RAID 1, RAID 4 and RAID 5 Hardware Striping and Fault Tolerance • Dual Fast-Wide SCSI-2 channels on a single board support up to 14 drives (7 per channel) • Support for multiple logical drives per drive array • Removable Array Accelerator - battery-backed 4 MB Read/Write cache with Error Checking and Correcting (ECC) • Read-ahead caching • Online Capacity Expansion and Disk Drive Upgrades • Fault Management Features RAID 5 - Distributed Data Guarding RAID 5 is also referred to as distributed data guarding because it uses parity data to guard against the loss of data. The parity data is distributed or striped across all the drives in the array. RAID 5 provides very good data protection because if a single drive fails, the parity data and the data on the remaining drives is used to reconstruct the data on the failed drive. With Compaq Smart-2 controller technology, this reconstruction process allows the failed drive to be replaced while the system continues to operate at a slightly reduced performance. RAID 5 also offers good performance because spreading the parity across all the drives allows more simultaneous read operations. The usable disk space when using RAID 5 depends on the total number of drives in the array. If there are three drives, 67 percent of the disk space is usable for data, with the remainder being used to support fault tolerance. If there are fourteen drives, 93 percent of the disk would be available. The tests that follow used seven drives. RAID 1 - Drive Mirroring RAID 1 is also referred to as drive mirroring. This is typically the highest performance fault tolerance method. RAID 1 is the only option for fault tolerance if no more than two drives are selected. Drive mirroring works as its name implies, storing two sets of duplicate data on a pair of disk drives. Therefore, RAID 1 always requires an even number of disk drives. From a cost standpoint, RAID 1 is the most expensive because 50 percent of the drive capacity is used for fault tolerance. If a drive fails, the mirror drive provides a backup copy of the data and normal system operation is not interrupted. A system with more than two drives may be able to withstand multiple drive failures as long as the failed drives are not mirrored to one another.