Users guide

5 Intel® RAID Software User Guide
1.3 RAID Terminology
RAID is a group of physical disks put together to provide increased I/O (Input/Output)
performance (by allowing multiple, simultaneous disk access), fault tolerance, and
reliability (by reconstructing failed drives from remaining data). The physical drive group
is called an array, and the partitioned sets are called virtual disks. A virtual disk can
consist of a part of one or more physical arrays, and one or more entire arrays.
Using two or more configured RAID arrays in a larger virtual disk is called spanning. It is
represented by a double digit in the RAID mode/type (10, 50, 60).
Running more than one array on a given physical drive or set of drives is called a sliced
configuration.
The only drive that the operating system works with is the virtual disk, which is also
called a virtual drive. The virtual drive is used by the operating system as a single drive
(lettered storage device in Microsoft Windows*).
The RAID controller is the mastermind that must configure the physical array and the
virtual disks, and initialize them for use, check them for data consistency, allocate the data
between the physical drives, and rebuild a failed array to maintain data redundancy. The
features available per controller are highlighted later in this document and in the hardware
guide for the RAID controller.
The common terms used when describing RAID functions and features can be grouped
into two areas: fault tolerance (data protection and redundancy) and performance.
1.3.1 Fault Tolerance
Fault tolerance describes a state in which even with a drive failure, the data on the virtual
drive is still complete and the system is available after the failure and during repair of the
array. Most RAID modes are able to endure a physical disk failure without compromising
data integrity or processing capability of the virtual drive.
RAID mode 0 is not fault tolerant. With RAID 0, if a drive fails, then the data is no longer
complete and no longer available. Backplane fault tolerance can be achieved by a spanned
array where the arrays are on different backplanes.
True fault tolerance includes the automatic ability to restore the RAID array to
redundancy so that another drive failure will not destroy its usability.
1.3.1.1 Hot Spare
True fault tolerance requires the availability of a spare disk that the controller can add to
the array and use to rebuild the array with the data from the failed drive. This spare disk is
called a hot spare. It must be a part of the array before a disk failure occurs. A hot-spare
drive is a physical drive that is maintained by the RAID controller but not actually used