DEC VAX 4000 300 Manual page 36

NOTE: Dual-host capability for VAX 4000 systems is only supported under
VMS (Version 5.3–2 and later) when the two systems are configured into the
same VAXcluster.
The benefits of a dual-host configuration are:
• VAXcluster features such as shared data across systems and satellite
nodes.
• Higher system availability — If one of the DSSI-based systems is
unavailable, for example, due to a system malfunction, the satellites
booted through it are able to continue operating through the other
system.
If one of the DSSI-based systems fails, all satellite nodes booted through
that system lose connections to the system disk. Each satellite node knows,
however, that the system disk is also available through a second path. The
satellite nodes establish a new connection through the other system and
continue operation.
To increase system availability, a second system disk may be added to each
boot node. In the event of one system disk failure, the remaining system
disk would continue to serve one DSSI-based system and the satellite nodes
booted through it. As with any VAXcluster, however, a second system disk
improves availability while increasing system management tasks.
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