Physical array, Logical drive, Raid array – Dell PERC 4/SI User Manual
Page 10: Fault tolerance, Consistency check
Physical Array
A physical array is a group of physical disk drives. The physical disk drives are managed in partitions known as logical drives.
Logical Drive
A logical drive is a partition in a physical array of disks that is made up of contiguous data segments on the physical disks. A logical drive can consist of an
entire physical array, more than one entire physical array, a part of an array, parts of more than one array, or a combination of any two of these conditions.
RAID Array
A RAID array is one or more logical drives controlled by the PERC.
Channel Redundant Logical Drives
When you create a logical drive, it is possible to use disks attached to different channels to implement channel redundancy, known as Channel Redundant
Logical Drives. This configuration might be used for disks that reside in enclosures subject to thermal shutdown.
For more information refer to the Dell OpenManage Array Manager or Dell OpenManage Storage Management user guides located at:
Fault Tolerance
Fault tolerance is the capability of the subsystem to undergo a single drive failure per span without compromising data integrity, and processing capability. The
RAID controller provides this support through redundant arrays in RAID levels 1, 5, 10 and 50. The system can still work properly even with a single disk failure
in an array, through performance can be degraded to some extent.
Fault tolerance is often associated with system availability because it allows the system to be available during the failures. However, this means it is also
important for the system to be available during the repair of the problem. To make this possible, PERC 4/Di/Si and 4e/Di/Si support hot spare disks, and the
auto-rebuild feature.
A hot spare is an unused physical disk that, in case of a disk failure in a redundant RAID array, can be used to rebuild the data and re-establish redundancy.
After the hot spare is automatically moved into the RAID array, the data is automatically rebuilt on the hot spare drive. The RAID array continues to handle
requests while the rebuild occurs.
Auto-rebuild allows a failed drive to be replaced and the data automatically rebuilt by "hot-swapping" the drive in the same drive bay. The RAID array
continues to handle requests while the rebuild occurs.
Consistency Check
The Consistency Check operation verifies correctness of the data in logical drives that use RAID levels 1, 5, 10, and 50. (RAID 0 does not provide data
NOTE:
The maximum logical drive size for all supported RAID levels (0, 1, 5, 10, and 50) is 2 TB. You can create multiple logical drives within the same
physical array.
NOTE:
Channel redundancy applies only to controllers that have more than one channel and that attach to an external disk enclosure.
NOTE:
Make sure that the spans are in different backplanes, so that if one span fails, you do not lose the whole array.
NOTE:
RAID level 0 is not fault tolerant. If a drive in a RAID 0 array fails, the whole logical drive (all physical drives associated with the logical drive) will
fail.