3Ware 9550SXU-16ML User Guide - Page 182

Chapter 9. Maintaining Units When the initialization is complete, the unit is guaranteed to be redundant again. The initialization does not erase user data. About Rebuilds Fault tolerant RAID units provide data redundancy by duplicating information on multiple drives. „ RAID 1 and 10 units each use mirroring, where identical data is stored on two or more drives to protect against drive failure. „ RAID 5 and 50 units achieve fault tolerance by using a simple (exclusive OR) function to generate the parity data that is distributed on all drives. When one of the drives in a fault-tolerant unit is removed, unplugged or fails on read or write requests, the unit is said to be "degraded." Rebuilding is the process by which the RAID unit is made fault tolerant again. You can still read and write data from a degraded unit, but the unit will not be fault tolerant until it is rebuilt using the Rebuild feature. In all cases, when a RAID unit becomes degraded, it is marked as such, and the drive(s) that failed are marked as Not In Use in the 3BM screens and Degraded in the 3DM pages. If a hot spare is specified and a redundant unit degrades, an event notification is generated and the hot spare automatically replaces the failed drive in the redundant unit without user intervention. The rebuild process will automatically be launched as a background process at the next scheduled time. If scheduling is turned off, the rebuild process will start almost immediately (within a couple of minutes). If 3DM is running and E-mail notification is enabled, an event notification will be sent to specified users when the rebuild process is complete. For 9550SX controllers, if Auto-Rebuild is enabled, other drives in addition to spares may be used to rebuild degraded units. If no spare is available, the firmware will attempt to rebuild the unit using first any available drives and then any failed drives. This allows you to insert a new drive to use in the rebuild process. With the 3ware 9000 series controller, rebuilds on multiple units can take place simultaneously. About Verification For the 3ware 9000 series, the verify feature combines verification of redundant units (confirming the validity of the redundant data) and media scans for non-redundant units. (3ware software for the 7000 and 8000 series provided separate features for verify and media scan.) 172 3ware 9550SX Serial ATA RAID Controller User Guide