Chapter 9. Maintaining Units172 3ware 9550SX Serial ATA RAID Controller User GuideWhen the initialization is complete, the unit is guaranteed to be redundantagain. The initialization does not erase user data.About RebuildsFault tolerant RAID units provide data redundancy by duplicatinginformation on multiple drives. RAID 1 and 10 units each use mirroring, where identical data is stored ontwo or more drives to protect against drive failure. RAID 5 and 50 units achieve fault tolerance by using a simple (exclusiveOR) 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 failson read or write requests, the unit is said to be “degraded.” Rebuilding is theprocess 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 befault tolerant until it is rebuilt using the Rebuild feature.In all cases, when a RAID unit becomes degraded, it is marked as such, andthe drive(s) that failed are marked as Not In Use in the 3BM screens andDegraded in the 3DM pages.If a hot spare is specified and a redundant unit degrades, an event notificationis generated and the hot spare automatically replaces the failed drive in theredundant unit without user intervention. The rebuild process willautomatically be launched as a background process at the next scheduledtime. If scheduling is turned off, the rebuild process will start almostimmediately (within a couple of minutes). If 3DM is running and E-mailnotification is enabled, an event notification will be sent to specified userswhen the rebuild process is complete.For 9550SX controllers, if Auto-Rebuild is enabled, other drives in additionto spares may be used to rebuild degraded units. If no spare is available, thefirmware will attempt to rebuild the unit using first any available drives andthen any failed drives. This allows you to insert a new drive to use in therebuild process.With the 3ware 9000 series controller, rebuilds on multiple units can takeplace simultaneously.About VerificationFor the 3ware 9000 series, the verify feature combines verification ofredundant units (confirming the validity of the redundant data) and mediascans for non-redundant units. (3ware software for the 7000 and 8000 seriesprovided separate features for verify and media scan.)
