HP XP P9500 Disk Array Configuration Guide (AV400-96496, May 2014)

D Using Veritas Cluster Server to prevent data corruption
Using VCS I/O fencing
By issuing a Persistent Reserve SCSI-3 command, VCS employs an I/O fencing feature that prevents
data corruption from occurring if cluster communication stops. To accomplish I/O fencing, each
node of VCS registers reserve keys for each disk in a disk group that is imported. The reserve key
consists of a unique value for each disk group and a value to distinguish nodes. When importing
a disk group, each node in a VCS cluster registers the reserve key for all paths of all disks (LUs)
in the disk group, but only 1 node will succeed in reserving the disk group. Example reserve keys
for Port 1A are shown in and example reserve keys for Port 2A are shown in . The format of the
reserve key is:
node number + disk group-unique information
When the Persistent Reserve command is received by the disk array, the reserve key and Port
WWN of node are registered in the key registration table for each port of the disk array. To avoid
duplicate registrations, the disk array checks the node Port WWN and reserve key and only adds
an entry in its table if the WWN/reserve key combination do not already exist in the table for the
port which received the registration request. Duplicate entries can exist across different ports.
The maximum number of reserve keys that can be registered per port for each disk array is 128.
Attempts to add additional registration keys will fail if the number exceeds this limit. Because of
this, try to limit the number of:
• Nodes
• Paths to each array port
• Disk groups
For example, when adding an LU to increase disk capacity, do not create a new disk group.
Instead, add the LU to one of the existing disk groups (see “Nodes and ports” (page 152)).
For each array port, calculate the number of VCS registration keys needed as follows:
number of WWNs visible to a P9000 port x number of disk groups = number of
registration keys
Where the number of WWNs visible to a P9000 port = number of hosts x number
of WWNs per P9000 port.
In “Nodes and ports” (page 152), the number of WWNs visible to either port (1A or 2A) is four
(WWNa0 and WWNb0 for port 1A and WWNa1 and WWNb1 for port 2A). Therefore, the
number of registration keys will be equal to 4.
Using VCS I/O fencing 151