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Technology BackgroundPegasus3 Product Manual
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.
The following actions trigger PDM:
• A disk drive with unhealthy status (see below)
• MediaPatrolndsadiskcriticalerror
• You initiate PDM manually
See “Running PDM on a Logical Drive” on page 146.
PDM also counts the number of media errors reported by Media Patrol. A disk drive becomes unhealthy
when:
• A SMART error is reported
• Thebadsectorremappingtablellstothespeciedlevel.
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 have a
capacity of 512 reassigned blocks and 2048 error blocks. See “PDM” on page 91.
You can specify the maximum levels for the reassigned and error blocks in PDM settings. When the
tablellstoaspeciedvalue,PDMtriggersamigrationofdatafromthesuspectdrive(thediskdrive
with the bad sectors) to a replacement physical drive.
During data migration, you have access to your logical drives but they respond more slowly to read/
write tasks because of the additional operation. The time required for data migration depends on the
size of the disk drives.
PDM is enabled on all disk arrays by default. You can disable PDM in the disk array settings; however
that action is not recommended. See “Making Disk Array Settings” on page 120.
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