NonStop Clusters (NSC) was an add-on package for SCO UnixWare that allowed creation of fault-tolerant single-system image clusters of machines running UnixWare.
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The disk subsystem was either accessible from multiple nodes (using a Fibre Channel SAN or dual-ported SCSI) or used cross-node mirroring in a similar fashion to DRBD.
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In 2000 NSC was modified to allow standard Fast Ethernet and later Gigabit Ethernet switches as the cluster interconnect and commercialized by SCO as UnixWare NonStop Clusters 7.1.1+IP.