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

Understanding Serviceguard Software Components
Volume Managers for Data Storage
Chapter 3116
Single Heartbeat Subnet Required with CVM
It is normally recommended that you configure all subnets that
interconnect cluster nodes as heartbeat networks, since this increases
protection against multiple faults at no additional cost. However, if you
will be using the VERITAS Cluster Volume Manager (CVM), only a
single heartbeat subnet is allowed. When the VxVM-CVM-pkg is added
(as shown in Chapter 5), cmcheckconf and cmapplyconf will check to
ensure that only a single heartbeat subnet is configured. If more
heartbeat subnets are configured, the cmcheckconf and cmapplyconf
will fail.
This restriction is due to the design of the VERITAS Cluster Daemon
(vxclustd), which is started within the VxVM-CVM-pkg. This daemon is
only able to communicate information to cluster nodes on a single IP
connection on a single subnet. Moreover, communication between nodes
in the cluster must be the same for the Veritas Cluster Daemon as for the
Serviceguard cluster daemon cmcld. Therefore, cmcld must also only
have the use of a single heartbeat subnet.
There are tradeoffs in only using a single subnet. If the heartbeat is not
heard for more than a NODE_TIMEOUT interval, the cluster will re-form.
Depending on the length of the disruption, this could cause one or all of
the cluster nodes to TOC. This is most commonly seen when there is
heavy traffic on the subnet preventing the heartbeat packets from being
transmitted. This can also been seen when there are network
configuration problems or network hardware problems that are not
easily recovered and may not be detected by Serviceguard or APA. The
most common way to avoid problems in these scenarios is to utilize
multiple heartbeat networks.
In the case of CVM, the single heartbeat subnet should be configured
with standby LANS or as a group of aggregated ports to help minimize
the possibility of system or cluster failures.
Comparison of Volume Managers
The following table summarizes some of the advantages and
disadvantages of the volume managers that are currently available.