351Chapter 9: Technology BackgroundPredictive Data Migration (PDM)Predictive Data Migration (PDM) is the migration of data from the suspect diskdrive to a spare disk drive, similar to Rebuilding a Logical Drive. But unlikeRebuilding, PDM constantly monitors your disk drives and automatically copiesyour data to a spare disk drive before the disk drive fails and your Logical Drivegoes Critical. See ―Running PDM‖ on page 71 (ASUS Storage Management) orpage 233 (CLU).After the data is copied from the suspect disk drive, the controller marks thesuspect disk drive with a Stale configuration and a PFA error.You can clear the Stale configuration and PFA error and put the disk drive backinto service. See ―Clearing Stale and PFA Conditions‖ on page 143 (ASUSStorage Management) or page 223 (CLU). In some cases, however, you mightremove the disk drive for repair or replacement.PDM TriggersThe following actions trigger PDM:• A disk drive with unhealthy status (see below)• Media Patrol finds a disk critical error*• You initiate PDM manually*PDM also counts the number of media errors reported by Media Patrol.A disk drive becomes unhealthy when:• A SMART error is reported• The bad sector remapping table fills to the specified levelBecause data would be lost if written to a bad sector, when a bad sector isdetected, the disk drive creates a map around it. These maps are saved in thebad sector remapping table, which has a capacity of 512 reassigned blocks and1024 error blocks.When the table fills to a specified percentage of its capacity, PDM triggers amigration of data from the suspect drive (the disk drive with the bad sectors) to aspare disk drive.During data migration, you will have access to the Logical Drive but it will respondmore slowly to read/write tasks because of the additional operation. The timerequired for data migration depends on the size of the disk drive.See ―Making Background Activity Settings‖ on page 69 (ASUS StorageManagement) or page 257 (CLU).
