Drive Arrays and Fault Tolerance
C-4
Compaq Smart Array 5i Plus Controller and Battery Backed Write Cache Enabler User Guide
COMPAQ CONFIDENTIAL
Writer: Amy L. Laffitte File Name: l-appc Drive Arrays and Fault Tolerance
Codename: Executor Part Number: 266328-001 Last Saved On: 4/18/02 2:51 PM
L4
L5
L3
A1
L1
L2
A2
Figure C-4: Two arrays (A1, A2) containing five logical
drives spread over five physical drives
Each logical drive in an array is distributed over all of the physical drives within the
array. A logical drive can also extend over more than one port on the same controller,
but it cannot extend over more than one controller.
Drive failure, although rare, is potentially catastrophic. In Figure C-4, for example,
failure of
any
physical drive causes
all
logical drives in the same array to fail, and all
data on the drives is lost.
To protect against data loss due to physical drive failure, logical drives are
configured with
fault tolerance.
There are several fault-tolerance methods; those
supported by current Compaq controllers (and described in the “Fault Tolerance
Methods” section) are:
•
RAID 0—Data Striping only (no fault tolerance)
•
RAID 1+0—Drive Mirroring
•
RAID 5—Distributed Data Guarding