8
Array Roaming
The RAID controller stores configuration information both in NVRAM and on the disk drives. It
can protect the configuration settings in the case of a disk drive or controller failure. Array
roaming allows the user to move a RAID set to another system without losing the RAID
configuration and data on that RAID set. If a server fails, the RAID set disk drives can be moved
to another server and inserted in any order.
Online Capacity Expansion
This feature allows the user to add one or more physical drives to a volume set without stopping
access to the unit or deleting the data. When disks are added to a RAID set, the unused
capacity is added to the end of the RAID set. Data on the existing volume sets residing on that
RAID set is redistributed evenly across all of the disks. A contiguous block of unused capacity is
then made available on the RAID set. The unused capacity can create additional volume sets.
RAID and Stripe Size Migration
Users can change both the RAID type and stripe size of an existing volume set while the server
is operational and the volume set is in use. This feature is helpful to tune the performance of the
unit as well as in the event that additional physical disks are added to the unit. For example, in a
system using two drives in RAID 1 configuration, you could add capacity and retain fault
tolerance by adding another drive. With the addition of the third disk, you have the option of
adding this disk to your existing RAID set and migrating from RAID 1 to RAID 5. The result
would be parity fault tolerance along with double the available capacity without taking the
system offline.
1.4 Terminology
RAID Sets
A RAID Set is a group of disks containing one or more volume sets, and defines the set of drives
that a volume set can use. A Volume Set must be created either on an existing RAID Set or on a
group of available individual disks (disks that are not yet a part of a RAID set). If there are pre-
existing RAID Sets with available capacity and enough disks for the specified RAID level, then
the volume set will be created in an existing raid set of the user’s choice. If the physical disks in
a RAID Set are of different sizes, then the capacity of the smallest disk will become the effective
capacity of all the disks in the RAID Set.
Volume Sets
A Volume Set is seen by the host system as a single logical device. It is organized in a RAID
configuration with one or more physical disks within a RAID set. The RAID configuration will
determine the performance, redundancy, and capacity of the Volume Set. A Volume Set can
consume all or a portion of the disk capacity available in a RAID Set. Multiple Volume Sets can
exist on a group of disks in a RAID Set.
Figure 1-1 below shows a RAID Set with two volumes. Volume 1 is using RAID 5 while Volume
0 uses RAID 0+1 and coexist in the same RAID Set.
