Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 13, Administering Cluster Functionality
Overview of Cluster Volume Management
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Whether all members of the cluster have simultaneous read and write access to a
cluster-shareable disk group depends on its activation mode setting as discussed in
“Activation Modes of Shared Disk Groups.” The data contained in a cluster-shareable
disk group is available as long as at least one node is active in the cluster. The failure of a
cluster node does not affect access by the remaining active nodes. Regardless of which
node accesses a cluster-shareable disk group, the configuration of the disk group looks the
same.
Note Applications running on each node can access the data on the VM disks
simultaneously. VxVM does not protect against simultaneous writes to shared
volumes by more than one node. It is assumed that applications control consistency
(by using a distributed lock manager, for example).
Activation Modes of Shared Disk Groups
A shared disk group must be activated on a node in order for the volumes in the disk
group to become accessible for application I/O from that node. The ability of applications
to read from or to write to volumes is dictated by the activation mode of a shared disk
group. Valid activation modes for a shared disk group are exclusive-write,
read-only, shared-read, shared-write, and off (inactive). These activation modes
are described in detail in the table “Activation Modes for Shared Disk Groups.”
Note The default activation mode for shared disk groups is off (inactive).
Special uses of clusters, such as high availability (HA) applications and off-host backup,
can use disk group activation to explicitly control volume access from different nodes in
the cluster.
Activation Modes for Shared Disk Groups
Activation Mode Description
exclusive-write (ew) The node has exclusive write access to the disk group. No other
node can activate the disk group for write access.
read-only (ro) The node has read access to the disk group and denies write
access for all other nodes in the cluster. The node has no write
access to the disk group. Attempts to activate a disk group for
either of the write modes on other nodes fail.
shared-read (sr) The node has read access to the disk group. The node has no
write access to the disk group, however other nodes can obtain
write access.
shared-write (sw) The node has write access to the disk group.