VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Overview of Cluster Volume Management
248 VERITAS Volume Manager Administrator’s Guide
Private and Shared Disk Groups
Two types of disk groups are defined:
◆ Private disk groups—belong to only one node. A private disk group is only imported
by onesystem. Disks in a privatedisk group maybe physically accessible from one or
more systems, but access is restricted to one system only. The root disk group
(rootdg) is always a private disk group.
◆ Shared disk groups—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.
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 volumein 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 264. 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 systemadministrator, youcan alsoimport ordeport a
shared disk group at any time; the operation takes places in a distributed fashion on all
nodes.
Each physicaldisk ismarked with a unique diskID. When cluster functionality for VxVM
starts on the master, it imports all shared disk groups (except for any that have the
noautoimport attribute set). Whena slave triesto join acluster, the mastersends it alist
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 imports the same shared disk groups as the master
and joins the cluster. When a node leaves the cluster, 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 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 acluster-shareabledisk group, the configurationof thedisk group looks the
same.