Dell PowerVault MD3000i - Manual (Page 39)

Using Your RAID Enclosure
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Disk Group Defragmentation
Defragmenting consolidates the free capacity in the disk group into one
contiguous area. Defragmentation does not change the way in which the data
is stored on the virtual disks.
Disk Group Operations Limit
The maximum number of active, concurrent disk group processes per
controller is one. This limit is applied to the following disk group processes:
virtual disk RAID level migration, segment size migration, virtual disk
capacity expansion, disk group expansion, and disk group defragmentation.
If a redundant controller fails with an existing disk group process, the process
on the failed controller is transferred to the peer controller. A transferred
process is placed in a suspended state if there is an active disk group process
on the peer controller. The suspended processes is resumed when the active
process on the peer controller completes or is stopped.
NOTE:
If you try to start a disk group process on a controller that does not have an
existing active process, the start attempt will fail if the first virtual disk in the disk
group is owned by the other controller and there is an active process on the other
controller.
RAID Background Operations Priority
The controller supports a common configurable priority for the following
RAID operations: background initialization, rebuild, copy back, virtual disk
capacity expansion, RAID level migration, segment size migration, disk group
expansion, and disk group defragmentation.
The priority of each of these operations can be changed to address
performance requirements of the environment in which the operations are to
be executed.
NOTE:
Setting a high priority level will impact storage array performance. It is not
advisable to set priority levels at the maximum level. Priority should also be
assessed in terms of impact to host server access and time to complete an
operation. For example, the longer a rebuild of a degraded virtual disk takes, the
greater the risk for potential secondary disk failure.