SCSI-3 Persistent Reservation (SCSI-3 PR) supports device access from multiple systems, or from multiple paths from a single system. At the same time it blocks access to the device from other systems, or other paths.
VCS logic determines when to online a service group on a particular system. If the service group contains a disk group, the disk group is imported as part of the service group being brought online. When using SCSI-3 PR, importing the disk group puts registration and reservation on the data disks. Only the system that has imported the storage with SCSI-3 reservation can write to the shared storage. This prevents a system that did not participate in membership arbitration from corrupting the shared storage.
SCSI-3 PR ensures persistent reservations across SCSI bus resets.
Use of SCSI 3 PR protects against all elements in the IT environment that might be trying to write illegally to storage, not only VCS related elements.
Membership arbitration combined with data protection is termed I/O fencing.