Redundant Array of Independent Disks (RAID) describes a storage solution in which part of the storage capacity stores redundant
information about user data that is stored on the remainder of the storage capacity. The redundant information enables regeneration
of user data if one of the disk drives in the fails.
RAID relies on a series of configurations, called levels, to determine how user and data is written and retrieved from the disk drives.
RAID Level 1, RAID Level 3, RAID Level 5, and RAID Level 6 write redundancy data to the disk drive media for fault tolerance.
The redundancy data might be a copy of the data (mirrored) or an error-correcting code derived from the data. You can use
the redundancy data to quickly reconstruct information on a replacement disk drive if a disk drive fails.
You configure a single RAID level across a single array. All redundancy data for that array is stored within the array. The
capacity of the array is the aggregate capacity of the member disk drives minus the capacity reserved for redundancy data.
The amount of capacity needed for redundancy depends on the RAID level used. Select Array >> Change >> RAID Level when you want to change the redundancy of the array, such as changing from RAID Level 1 to RAID Level 5 .
This storage management software offers the following RAID level configurations: RAID Level 0, RAID Level 1 or RAID Level
10, RAID Level 3, RAID Level 5, and RAID Level 6. These RAID levels are described in the sections that follow. Each RAID
level also provides different performance features. For more information about performance, refer to the RAID Levels and Performance
online help topic.
RAID Level 0
Short description – Non-redundant, striping mode.
How it works – RAID Level 0 stripes data across all of the disk drives in the array.
Data protection features –
RAID Level 0 is not recommended for high availability needs. RAID Level 0 is better for non-critical data.
If a single disk drive fails in the array, all of the associated logical drives fail, and all data is lost.
Disk drive number requirements – RAID Level 0 arrays can have more than 30 disk drives. You can create an array that includes all of the disk drives in
the storage subsystem.
RAID Level 1 or RAID Level 10
Short description – Striping/mirror mode.
How it works –
RAID Level 1 uses disk mirroring to write data to two duplicate disks simultaneously.
RAID Level 10 uses disk drive striping to stripe data across a set of mirrored disk drive pairs.
Data protection features –
RAID Level 1 and RAID Level 10 offer high performance and the best data availability.
RAID Level 1 and RAID Level 10 use disk drive mirroring to make an exact copy from one disk drive to another disk drive.
If one of the disk drives in a disk drive pair fails, the can instantly switch to the other disk drive without any loss of data or service.
A single disk drive failure causes associated logical drives to become degraded. The mirror disk drive allows access to the
data.
A disk drive-pair failure in an array causes all of the associated logical drives to fail, and data loss could occur.
Disk drive number requirements –
A minimum of two disk drives is required for RAID Level 1: one disk drive for the user data, and one disk drive for the mirrored
data.
If you select four or more disk drives, RAID Level 10 is automatically configured across the array: two disk drives for user
data, and two disk drives for the mirrored data.
You must have an even number of disk drives in the array. If you do not have an even number of disk drives and you have some
remaining unassigned disk drives, select Array >> Add Free Capacity to add additional disk drives to the array, and retry the operation.
RAID Level 1 and RAID Level 10 arrays can have more than 30 disk drives. An array can be created that includes all of the
disk drives in the storage subsystem.
RAID Level 3
Short description – High bandwidth mode.
How it works –
User data is striped across the disk drives and redundant information (parity) is written to a dedicated disk drive.
The equivalent of one disk drive’s worth of capacity is used for redundant information.
Data protection features –
If a single disk drive fails in a RAID Level 3 array, all of the associated logical drives become degraded, but the redundant
information allows the data to still be accessed.
If two or more disk drives fail in a RAID Level 3 array, all of the associated logical drives fail, and all data is lost.
Disk drive number requirements –
You must have a minimum of three disk drives in the array.
Typically, you are limited to a maximum of 30 disk drives in the array.
RAID Level 5
Short description – High I/O mode.
How it works –
User data and redundant information (parity) are striped across the disk drives.
The equivalent capacity of one disk drive is used for redundant information.
Data protection features –
If a single disk drive fails in a RAID Level 5 array, all of the associated logical drives become degraded. The redundant
information allows the data to still be accessed.
If two or more disk drives fail in a RAID Level 5 array, all of the associated logical drives fail, and all data is lost.
Disk drive number requirements –
You must have a minimum of three disk drives in the array.
Typically, you are limited to a maximum of 30 disk drives in the array.
RAID Level 6
Short description – High I/O mode.
How it works –
User data and redundant information (dual parity) are striped across the disk drives.
The equivalent capacity of two disk drives are used for redundant information.
Data protection features –
If one or two disk drives fail in a RAID Level 6 array, all of the associated logical drives become degraded, but the redundant
information allows the data to still be accessed.
If three or more disk drives fail in a RAID Level 6 array, all of the associated logical drives fail, and all data is lost.
Disk drive number requirements –
You must have a minimum of five disk drives in the array.
Typically, you are limited to a maximum of 30 disk drives in the array.
Note:
RAID Level 6 is available only to those controllers that are capable of supporting the P+Q calculation. The model DS4800 (Models
82, 84, 88) controller is not supported, however, the model DS4700 Model 70 controller and the model DS4700 Model 72 controller
are supported. A premium feature key allows customers to use RAID Level 6 and to use Dynamic RAID-Level Migration (DRM).