The generic SCSI target subsystem for Linux (SCST) allows creation of sophisticated storage devices from any Linux box. Those devices can provide advanced functionality, like replication, thin provisioning, deduplication, high availability, automatic backup, etc. Another class of such devices are Virtual Tape Libraries (VTL) as well as other disk-based backup solutions.
SCST devices can use any link which supports SCSI-style data exchange: iSCSI, Fibre Channel, FCoE, SAS, InfiniBand (SRP), Wide (parallel) SCSI, etc.
It might well be that your favorite storage appliance is running SCST in the firmware.
SCST project consists from a set of subprojects: generic SCSI target mid-layer itself (SCST core) with a set of device handlers as well as target drivers and user space utilities.