Appendix A: Capacity Growth Technologies; Array Expansion - HP 273914-B21 - Smart Array 6404/256 RAID Controller Technology Brief

Appendix A: Capacity growth technologies

The Smart Array controller family includes a standard toolset that administrators can use to configure
array controllers, expand an existing array configuration by adding disk drives, or reconfigure an
array by extending logical drive sizes. Before this innovation, expanding the storage capacity
attached to an array controller required a time-consuming backup-reconfigure-restore cycle.

Array expansion

Array expansion is the process of adding physical drives to an array that has already been
configured. The logical drives that exist in the array before the expansion takes place are unchanged;
only the amount of free space in the array changes.
For example, suppose an existing array consists of four physical drives and the administrator wants to
expand the array to six physical drives. This is like having four glasses full of water and dividing that
water from the original four glasses among six glasses (Figure A1). The amount of water (the size of
the logical drive) has not changed—it has merely been redistributed, or expanded, into a larger
number of containers (drives).
Figure A1. Array expansion redistributes an array into a larger number of physical drives. The size of the logical
drive does not change.
Thus, if an existing array is nearly filled with logical drives, an administrator can add new physical
drives and initiate an array expansion using the ACU. The ACU automatically checks the drive
hardware configuration to ensure the array expansion. Then, the existing logical drive is distributed
across all physical drives in the expanded array without affecting any existing data. If the array being
expanded contains more than one logical drive, data is redistributed one logical drive at a time.
The expansion process is entirely independent of the operating system. For example, if a
10-gigabyte (GB) logical drive is expanded from four drives to six, the operating system is unaware
of the change.
The amount of time it takes to complete an online array expansion depends on subsystem I/O activity,
the size of the logical drives on the array, fault-tolerance level, the number of disk drives in the new
array, disk drive performance, and the priority level of the array expansion. Administrators can
monitor array expansion completion progress within the ACU.
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