Dell PowerVault MD3220 - Manual (Page 44)

44 Planning: MD3200 Series Storage Array Terms and Concepts
Consistency Check
A consistency check verifies the correctness of data in a redundant array
(RAID levels 1, 5, 6, and 10). For example, in a system with parity, checking
consistency involves computing the data on one physical disk and comparing
the results to the contents of the parity physical disk.
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.
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