User guide

Understanding Drive Arrays D-21
Controller Duplexing
IMPORTANT: Compaq strongly recommends using hardware-based fault tolerance (OS
independent) instead of controller duplexing, which is OS dependent. Hardware-based
fault tolerance provides a much more robust and controlled environment for fault
tolerance protection.
Some operating systems support controller duplexing, a fault tolerance feature
that requires two Smart Array 431 Controllers. With duplexing, the two
controllers each have their own drives that contain identical data. In the
unlikely event of a Smart Array 431 Controller failure, the remaining drives
and Smart Array 431 Controller service all requests.
Controller duplexing is not the same as mirroring the two SCSI buses on a
single dual channel controller. Controller duplexing is a function of the
operating system and takes the place of other fault tolerance methods. Refer to
the documentation included with your operating system for implementation.
Software-Based Drive Mirroring
IMPORTANT: Compaq strongly 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.
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 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.
For more information on hardware versus software fault tolerance, see the
White Paper section of the Compaq Storage website.