Dell PowerVault MD3000 Hardware Owner's Manual page 35

When considering a segment-size change, two scenarios illustrate different approaches to the
limitations:
• If I/O activity stretches beyond the segment size, you can increase it to reduce the number of disks
required to satisfy a single I/O. Using a single physical disk for a single request frees other disks to
service other requests, especially when you have multiple users accessing a database or storage
environment.
• If you are using the virtual disk in a single-user, large I/O environment (such as for multimedia
application storage), performance can be optimized when a single I/O request is serviced with a single
data stripe (the segment size multiplied by the number of physical disks in the disk group used for data
storage). In this case, multiple disks are used for the same request, but each disk is only accessed once.
Virtual Disk Capacity Expansion
When you configure a virtual disk, you select a capacity based on the amount of data you expect to store.
For example, if a disk group will contain a virtual disk that stores larger multimedia files and another
virtual disk that stores smaller text files, the multimedia file virtual disk will obviously require more
capacity.
However, you may need to eventually increase the virtual disk capacity for a standard virtual disk by
adding free capacity to the disk group. This creates more unused space for you to create new virtual disks,
or to expand your existing virtual disks.
Disk Group Expansion
Because the storage array supports hot pluggable physical disks, you can add two physical disks at a time
for each disk group while the storage array remains online. Data remains accessible on virtual disk groups,
virtual disks, and physical disks throughout the entire modification operation. The data and increased
unused free space are dynamically redistributed across the disk group. RAID characteristics are also
reapplied to the disk group as a whole.
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.
Using Your RAID Enclosure
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