Product manual

Chapter 7: Technology Background
251
Media Patrol will also check spare drives, if those drives have Media Patrol
enabled. Media Patrol for spare drives is enabled by default. You can disable it in
VTrak’s Command Line Interface (CLI).
Unlike Synchronization and Redundancy Check, Media Patrol is concerned with
the condition of the media itself, not the data recorded on the media. If Media
Patrol encounters a critical error, it triggers PDM if PDM is enabled.
See “Physical Drive Media Patrol” on page 125.
Predictive Data Migration (PDM)
Predictive Data Migration (PDM) is the migration of data from the suspect disk
drive to a spare disk drive, similar to Rebuilding a Logical Drive. But unlike
Rebuilding, PDM constantly monitors your disk drives and automatically copies
your data to a spare disk drive before the disk drive fails and your Logical Drive
goes Critical.
After the data is copied from the suspect disk drive, the controller marks it with a
Stale configuration and a PFA error.
You can clear the Stale configuration and PFA error and put the disk drive back
into service. See “Clear Physical Drive Conditions” on page 124 for WebPAM
PROe or see “Clear Stale and PFA Conditions” on page 174 for the CLU. In most
cases, however, you will remove the disk drive for repair or replacement.
PDM Triggers
The 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 level
Because data would be lost if written to a bad sector, when a bad sector is
detected, the disk drive creates a map around it. These maps are saved in the
bad sector remapping table, which has a capacity of 32 entries.
When the table fills to a specified percentage of its capacity, PDM triggers a
migration of data from the suspect drive (the disk drive with the bad sectors) to a
spare disk drive.