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401Administering cluster functionality
Overview of cluster volume management
Private and shared disk groups
Two types of disk groups are defined:
In a cluster, most disk groups are shared. Disks in a shared disk group are
accessible from all nodes in a cluster, allowing applications on multiple cluster
nodes to simultaneously access the same disk. A volume in a shared disk group
can be simultaneously accessed by more than one node in the cluster, subject to
licensing and disk group activation mode restrictions.
You can use the
vxdg command to designate a disk group as cluster-shareable as
described in “Importing disk groups as shared” on page 423. When a disk group
is imported as cluster-shareable for one node, each disk header is marked with
the cluster ID. As each node subsequently joins the cluster, it recognizes the disk
group as being cluster-shareable and imports it. As system administrator, you
can also import or deport a shared disk group at any time; the operation takes
place in a distributed fashion on all nodes.
Each physical disk is marked with a unique disk ID. When cluster functionality
for VxVM starts on the master, it imports all shared disk groups (except for any
that have the noautoimport attribute set). When a slave tries to join a cluster,
the master sends it a list of the disk IDs that it has imported, and the slave
checks to see if it can access them all. If the slave cannot access one of the listed
disks, it abandons its attempt to join the cluster. If it can access all of the listed
disks, it joins the cluster and imports the same shared disk groups as the master.
When a node leaves the cluster gracefully, it deports all its imported shared disk
groups, but they remain imported on the surviving nodes.
Reconfiguring a shared disk group is performed with the cooperation of all
nodes. Configuration changes to the disk group happen simultaneously on all
nodes and the changes are identical. Such changes are atomic in nature, which
means that they either occur simultaneously on all nodes or not at all.
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
Private disk group Belongs to only one node. A private disk group can only be
imported by one system at a time. Disks in a private disk group
may be physically accessible from one or more systems, but
access is restricted to one system only. The boot disk group
(usually aliased by the reserved disk group name bootdg) is
always a private disk group.
Shared disk group Can be shared by all nodes. A shared (or cluster-shareable) disk
group is imported by all cluster nodes. Disks in a shared disk
group must be physically accessible from all systems that may
join the cluster.