- <title>3.1 What is Failover?</title>
- <para>A computer system is ''highly available'' when the services it provides are available with minimal downtime. In a highly-available system, if a failure condition occurs, such as the loss of a server or a network or software fault, the system's services continue without interruption. Generally, we measure availability by the percentage of time the system is required to be available.</para>
- <para>Availability is accomplished by replicating hardware and/or software so that when a primary server fails or is unavailable, a standby server can be switched into its place to run applications and associated resources. This process, called <emphasis role="italic">failover</emphasis>, should be automatic and, in most cases, completely application-transparent.</para>
- <para>A failover hardware setup requires a pair of servers with a shared resource (typically a physical storage device, which may be based on SAN, NAS, hardware RAID, SCSI or FC technology). The method of sharing storage should be essentially transparent at the device level; the same physical logical unit number (LUN) should be visible from both servers. To ensure high availability at the physical storage level, we encourage the use of RAID arrays to protect against drive-level failures.</para>
+ <title>
+ <indexterm>
+ <primary>failover</primary>
+ </indexterm>What is Failover?</title>
+ <para>In a high-availability (HA) system, unscheduled downtime is minimized
+ by using redundant hardware and software components and software components
+ that automate recovery when a failure occurs. If a failure condition
+ occurs, such as the loss of a server or storage device or a network or
+ software fault, the system's services continue with minimal interruption.
+ Generally, availability is specified as the percentage of time the system
+ is required to be available.</para>
+ <para>Availability is accomplished by replicating hardware and/or software
+ so that when a primary server fails or is unavailable, a standby server can
+ be switched into its place to run applications and associated resources.
+ This process, called
+ <emphasis role="italic">failover</emphasis>, is automatic in an HA system
+ and, in most cases, completely application-transparent.</para>
+ <para>A failover hardware setup requires a pair of servers with a shared
+ resource (typically a physical storage device, which may be based on SAN,
+ NAS, hardware RAID, SCSI or Fibre Channel (FC) technology). The method of
+ sharing storage should be essentially transparent at the device level; the
+ same physical logical unit number (LUN) should be visible from both
+ servers. To ensure high availability at the physical storage level, we
+ encourage the use of RAID arrays to protect against drive-level
+ failures.</para>