Specifications

DS300f G2 Series User Manual
184
PDm
Predictive Data Migration (PDM) is the migration of data from the suspect phyical drive to a spare drive, similar
to rebuilding a logical drive. But unlike Rebuilding, PDM constantly monitors your physical drives and automatically
copies your data to a spare drive before the physical drive fails and your logical drive goes Critical.
The following actions trigger PDM:
• A phyical drive with unhealthy status (see below)
• MediaPatrolndsa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llstothespeciedlevel.
Because data would be lost if written to a bad sector, when a bad sector is detected, the physical 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 2048 error blocks
YoucanspecifythemaximumlevelsforthereassignedanderrorblocksinPDMsettings.Whenthetablellsto
aspeciedvalue,PDMtriggersamigrationofdatafromthesuspectdrive(thediskdrive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 physical
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.
loGICal drIves
Logical drive technology includes:
• "RAID Levels" on page 184
• "RAID Level Migration" on page 189
• "Stripe Size" on page 192
• "Sector Size" on page 192
• "Initialization" on page 192
• "Partition and Format" on page 192
raID levelS
RAID (Redundant Array of Independent Disks) allows multiple physical drives to be combined together in a disk
array. Then all or a portion of the disk array is formed into a logical drive. The operating system sees the logical
drive as a single storage device, and treats it as such.
rAid 0 – striPe
When a logical drive is striped, the read and write blocks of data are interleaved between the sectors of multiple
physical drives. Performance is increased, since the workload is balanced between drives or “members” that form
thelogicaldrive.Identicaldrivesarerecommendedforperformanceaswellasdatastorageefciency.
The disk array’s data capacity is equal to the number of disk drive members multiplied by the smallest drive’s
capacity. For example, one 100 GB and three 120 GB drives form a 400 GB (4 x 100 GB) disk array instead of
460 GB.
If physical drives of different capacities are used, there is unused capacity on the larger drives.