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E-28 Understanding Drive Arrays
Writer: Rickard Project: Compaq Smart Array 3200 Controller Reference Guide Comments: 340862-002
File Name: L-APPE.DOC Last Saved On: 12/7/98 9:30 AM
COMPAQ CONFIDENTIAL - NEED TO KNOW REQUIRED
Software-Based Drive Mirroring
Some operating systems support software-based drive mirroring as a fault-
tolerance feature. Software drive mirroring resembles hardware-based drive
mirroring (RAID 1), except that the operating system mirrors logical drives
instead of physical drives.
A pitfall of software-based drive mirroring is that the operating system believes
each logical drive is a separate physical drive. If you mirror logical drives in the
same array and a physical drive fails, both logical drives in the mirrored pair
will also fail. You will not be able to retrieve your data.
Because software-based drive mirroring is a function of the operating system,
refer to the documentation included with your operating system for
implementation.
Compaq recommends using hardware-based fault tolerance instead of software-
based fault tolerance. Hardware-based fault tolerance provides a much more
robust and controlled environment for fault-tolerance protection.
If you choose software-based drive mirroring, create at least two arrays with
RAID 0 to achieve maximum storage capacity. Then, when you configure the
drive mirroring through the operating system, mirror the logical drives residing
in different arrays. In addition, the following fault-tolerant features will not be
available: online spare, auto-reliability monitoring, interim data recovery, and
automatic data recovery. See Chapter 5 for information on your drive array(s)
and fault tolerance.