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When multiple systems have access to the data on shared storage, the integrity of the data depends on the systems communicating with each other so that each is aware when the other is writing data. Usually this communication occurs in the form of heartbeats through the private networks between the systems. If the private links are lost, or even if one of the systems is hung or too busy to send or receive heartbeats, each system could be unaware of the other's activities with respect to writing data. This is a split brain condition and can lead to data corruption.
The I/O fencing capability of the Storage Foundation for Oracle RAC, which is managed by Veritas Volume Manager, prevents data corruption in the event of a split brain condition by using SCSI-3 persistent reservations for disks. This allows a set of systems to have registrations with the disk and a write-exclusive registrants-only reservation with the disk containing the data. This means that only these systems can read and write to the disk, while any other system can only read the disk. The I/O fencing feature fences out a system that no longer sends heartbeats to the other system by preventing it from writing data to the disk.
VxVM manages all shared storage subject to I/O fencing. It assigns the keys that systems use for registrations and reservations for the disks—including all paths—in the specified disk groups. The vxfen driver is aware of which systems have registrations and reservations with specific disks. To protect the data on shared disks, each system in the cluster must be configured to use I/O fencing.