Page 1300 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Both software and hardware-based RAID solutions are available.
Software-based systems require the operating system to manage the

disks in the array and can reduce overall system performance. They
are relatively inexpensive since they don’t require any additional
hardware other than the additional disk(s). Hardware RAID systems
are generally more efficient and reliable. While a hardware RAID is
more expensive, the benefits outweigh the costs when used to increase
availability of a critical component.

Hardware-based RAID arrays typically include spare drives that can
be logically added to the array. For example, a hardware-based RAID-
5 could include five disks, with three disks in a RAID-5 array and two
spare disks. If one disk fails, the hardware senses the failure and
logically swaps out the faulty drive with a good spare. Additionally,
most hardware-based arrays support hot swapping, allowing
technicians to replace failed disks without powering down the system.
A cold swappable RAID requires the system to be powered down to

replace a faulty drive.

Protecting Servers

Fault tolerance can be added for critical servers with failover clusters.
A failover cluster includes two or more servers, and if one of the
servers fails, another server in the cluster can take over its load in an

automatic process called failover. Failover clusters can include
multiple servers (not just two), and they can also provide fault
tolerance for multiple services or applications.

As an example of a failover cluster, consider Figure 18.2. It shows
multiple components put together to provide reliable web access for a
heavily accessed website that uses a database. DB1 and DB2 are two
database servers configured in a failover cluster. At any given time,

only one server will function as the active database server, and the
second server will be inactive. For example, if DB1 is the active server
it will perform all the database services for the website. DB2 monitors
DB1 to ensure it is operational, and if DB2 senses a failure in DB1, it
will cause the cluster to automatically fail over to DB2.