Administrator Guide

Virtual disk operations
Virtual disk initialization
Every virtual disk must be initialized. Initialization is done in the background automatically, however the priority can be modified by updating
the Change Modification Priority option. This change can affect the performance of the array until the initialization is complete. A
maximum of four virtual disks can be initialized concurrently on each RAID controller module.
The storage array executes a background initialization when the virtual disk is created to establish consistency, while allowing full host
server access to the virtual disks. Background initialization does not run on RAID 0 virtual disks. The background initialization rate is
controlled by MD Storage Manager. To change the rate of background initialization, you must stop any existing background initialization.
The rate change is implemented when the background initialization restarts automatically.
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 data
consistency check 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 consistency 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 RAID 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.
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