Specifications

SCSI Reservations

Storage Fence Using SCSI Reservations

While LifeKeeper for Linux supports both resource fencing and node fencing, its primary fencing

mechanism is storage fencing through SCSI reservations. This fence, which provides the highest

level of data protection for shared storage, allows for maximum flexibility and maximum security

providing very granular locking to the LUN level. The underlying shared resource (LUN) is the primary

quorum device in this architecture. Quorum can be defined as exclusive access to shared storage,

meaning this shared storage can only be accessed by one server at a time. The server who has

quorum (exclusive access) owns the role of “primary.” The establishment of quorum (who gets this

exclusive access) is determined by the “quorum device.”

As stated above, with reservations enabled, the quorum device is the shared resource. The shared

resource establishes quorum by determining who owns the reservation on it. This allows a cluster to

continue to operate down to a single server as long as that single server can access the LUN.

SCSI reservations protect the shared user data so that only the system designated by LifeKeeper can

modify the data. No other system in the cluster or outside the cluster is allowed to modify that data.

SCSI reservations also allow the application being protected by LifeKeeper to safely access the

shared user data when there are multiple server failures in the cluster. A majority quorum of servers is

not required; the only requirement is establishing ownership of the shared data.

Adding quorum/witness capabilities provides for the establishment of quorum membership. Without

this membership, split-brain situations could result in multiple servers, even all servers, killing each

other. Watchdog added to configurations with reservations enabled provides a mechanism to recover

114Configuration