Dell PowerVault MD3820f - Manual (Page 26)

A consistency check is similar to a background initialization. The difference is that background
initialization cannot be started or stopped manually, while consistency check can.
NOTE: It is recommended that you run data consistency checks on a redundant array at least once
a month. This allows detection and automatic replacement of unreadable sectors. Finding an
unreadable sector during a rebuild of a failed physical disk is a serious problem, because the system
does not have the redundancy to recover the data.
Media Verification
Another background task performed by the storage array is media verification of all configured physical
disks in a disk group. The storage array uses the Read operation to perform verification on the space
configured in virtual disks and the space reserved for the metadata.
Cycle Time
The media verification operation runs only on selected disk groups, independent of other disk groups.
Cycle time is the time taken to complete verification of the metadata region of the disk group and all
virtual disks in the disk group for which media verification is configured. The next cycle for a disk group
starts automatically when the current cycle completes. You can set the cycle time for a media verification
operation between 1 and 30 days. The storage controller throttles the media verification I/O accesses to
disks based on the cycle time.
The storage array tracks the cycle for each disk group independent of other disk groups on the controller
and creates a checkpoint. If the media verification operation on a disk group is preempted or blocked by
another operation on the disk group, the storage array resumes after the current cycle. If the media
verification process on a disk group is stopped due to a RAID controller module restart, the storage array
resumes the process from the last checkpoint.
Virtual Disk Operations Limit
The maximum number of active, concurrent virtual disk processes per RAID controller module installed in
the storage array is four. This limit is applied to the following virtual disk processes:
• Background initialization
• Foreground initialization
• Consistency check
• Rebuild
• Copy back
If a redundant RAID controller module fails with existing virtual disk processes, the processes on the failed
controller are transferred to the peer controller. A transferred process is placed in a suspended state if
there are four active processes on the peer controller. The suspended processes are resumed on the peer
controller when the number of active processes falls below four.
Disk Group Operations
RAID Level Migration
You can migrate from one RAID level to another depending on your requirements. For example, fault-
tolerant characteristics can be added to a stripe set (RAID 0) by converting it to a RAID 5 set. The MD
Storage Manager provides information about RAID attributes to assist you in selecting the appropriate
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