VessRAID 1740s, 1840s Product Manual
246
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. See “Running PDM” on page 54 (WebPAM PROe) or page 148
(CLU).
After the data is copied from the suspect disk drive, the controller marks the
suspect disk drive 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 “Clearing Stale and PFA Conditions” on page 95 (WebPAM
PROe) or page 138 (CLU). In some cases, however, you might 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 512 reassigned blocks and
1024 error blocks.
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.
During data migration, you will have access to the Logical Drive but it will 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 drive.
See “Making Background Activity Settings” on page 52 (WebPAM PROe) or
page 160 (CLU).
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