Managing Serviceguard 11th Edition, Version A.11.16, Second Printing June 2004

Understanding Serviceguard Software Components
Volume Managers for Data Storage
Chapter 3 115
system multi-node package known as VxVM-CVM-pkg, which runs on all
nodes in the cluster. The cluster must be up and must be running this
package in order to configure VxVM disk groups for use with CVM.
CVM allows you to activate storage on one node at a time, or you can
perform write activation on one node and read activation on another
node at the same time (for example, allowing backups). CVM provides
full mirroring and dynamic multipathing (DMP) for clusters.
CVM can be used in clusters that:
• will run applications that require fast disk group activation after
package failover.
• require activation on more than one node at a time, for example to
perform a backup from one node while a package using the volume is
active on another node. In this case, the package using the disk
group would have the disk group active in exclusive write mode while
the node that is doing the backup would have the disk group active in
shared read mode.
• are configured with only a single heartbeat subnet.
CVM is supported on 4 nodes or fewer at this release. Shared storage
devices must be connected to all nodes in the cluster, whether or not the
node accesses data on the device.
Cluster Startup Time with CVM
With CVM, all shared disk groups (DGs) are imported when the
VxVM-CVM-pkg control script completes. Depending on the number of
DGs, the number of nodes and the configuration of these (number of
disks, volumes, etc.) this can take some time (current timeout value for
this package is 3 minutes but for larger configurations this may have to
be increased). Any failover package that uses a CVM DG will not start
until the VxVM-CVM-pkg is up. Note that this delay does not affect
package failover time; it is a one-time overhead cost at cluster startup.
Propagation of Disk Groups with CVM
With CVM, disk groups are created on one cluster node known as the
CVM master node. The cluster has to be running to create shared disk
groups. CVM will validate that each node can see each disk and will not
allow invalid DGs to be created.