Veritas Storage Foundation 5.1 SP1 for Oracle RAC Administrator"s Guide (5900-1512, April 2011)

Figure 1-8
Private network disruption and I/O fencing solution
Disk Array
Public Network
Private
Network
(Heartbeats)
Data Corruption
Node A
Order
Entry
Node B
Order
Entry
Node C
Order
Entry
Node D
Order
Entry
Disk I/O
I/O fencing
solution
Disk I/O
About SCSI-3 Persistent Reservations
SCSI-3 Persistent Reservations (SCSI-3 PR) are required for I/O fencing and resolve
the issues of using SCSI reservations in a clustered SAN environment. SCSI-3 PR
enables access for multiple nodes to a device and simultaneously blocks access
for other nodes.
SCSI-3 reservations are persistent across SCSI bus resets and support multiple
paths from a host to a disk. In contrast, only one host can use SCSI-2 reservations
with one path. If the need arises to block access to a device because of data integrity
concerns, only one host and one path remain active. The requirements for larger
clusters, with multiple nodes reading and writing to storage in a controlled manner,
make SCSI-2 reservations obsolete.
SCSI-3 PR uses a concept of registration and reservation. Each system registers
its own "key" with a SCSI-3 device. Multiple systems registering keys form a
membership and establish a reservation, typically set to "Write Exclusive
Registrants Only." The WERO setting enables only registered systems to perform
write operations. For a given disk, only one reservation can exist amidst numerous
registrations.
With SCSI-3 PR technology, blocking write access is as easy as removing a
registration from a device. Only registered members can "eject" the registration
of another member. A member wishing to eject another member issues a "preempt
and abort" command. Ejecting a node is final and atomic; an ejected node cannot
eject another node. In SF Oracle RAC, a node registers the same key for all paths
Overview of Veritas Storage Foundation for Oracle RAC
About preventing data corruption with I/O fencing
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