Managing HP Serviceguard A.11.20.20 for Linux, May 2013

4.7.2 Heartbeat Subnet and Cluster Re-formation Time
The speed of cluster re-formation depends on the number of heartbeat subnets.
If the cluster has only a single heartbeat network, and a network card on that network fails,
heartbeats will be lost while the failure is being detected and the IP address is being switched to
a standby interface. The cluster may treat these lost heartbeats as a failure and re-form without
one or more nodes. To prevent this, a minimum MEMBER_TIMEOUT value of 14 seconds is required
for clusters with a single heartbeat network.
If there is more than one heartbeat subnet, and there is a failure on one of them, heartbeats will
go through another, so you can configure a smaller MEMBER_TIMEOUT value.
NOTE: For heartbeat configuration requirements, see the discussion of the HEARTBEAT_IP
parameter later in this chapter. For more information about managing the speed of cluster
re-formation, see the discussion of the MEMBER_TIMEOUT parameter, and further discussion under
“What Happens when a Node Times Out” (page 75), and, for troubleshooting, “Cluster
Re-formations Caused by MEMBER_TIMEOUT Being Set too Low” (page 258).
4.7.3 About Hostname Address Families: IPv4-Only, IPv6-Only, and Mixed Mode
Serviceguard supports three possibilities for resolving the nodes' hostnames (and Quorum Server
hostnames, if any) to network address families:
• IPv4-only
• IPv6-only
• Mixed
IPv4-only means that Serviceguard will try to resolve the hostnames to IPv4 addresses only.
IMPORTANT: You can configure an IPv6 heartbeat, or stationary or relocatable IP address, in
any mode: IPv4-only, IPv6-only, or mixed. You can configure an IPv4 heartbeat, or stationary or
relocatable IP address, in IPv4-only or mixed mode.
IPv6-only means that Serviceguard will try to resolve the hostnames to IPv6 addresses only.
Mixed means that when resolving the hostnames, Serviceguard will try both IPv4 and IPv6 address
families.
You specify the address family the cluster will use in the cluster configuration file (by setting
HOSTNAME_ADDRESS_FAMILY to IPV4, IPV6, or ANY), or by means of the -a of cmquerycl
(1m); see “Specifying the Address Family for the Cluster Hostnames” (page 154). The default is
IPV4. See the subsections that follow for more information and important rules and restrictions.
4.7.3.1 What Is IPv4–only Mode?
IPv4 is the default mode: unless you specify IPV6 or ANY (either in the cluster configuration file or
via cmquerycl -a) Serviceguard will always try to resolve the nodes' hostnames (and the Quorum
Server's, if any) to IPv4 addresses, and will not try to resolve them to IPv6 addresses. This means
that you must ensure that each hostname can be resolved to at least one IPv4 address.
NOTE: This applies only to hostname resolution. You can have IPv6 heartbeat and data LANs
no matter what the HOSTNAME_ADDRESS_FAMILY parameter is set to. (IPv4 heartbeat and data
LANs are allowed in IPv4 and mixed mode.)
4.7.3.2 What Is IPv6-Only Mode?
If you configure IPv6-only mode (HOSTNAME_ADDRESS_FAMILY set to IPV6, or cmquerycl -a
ipv6), then all the hostnames and addresses used by the cluster — including the heartbeat and
stationary and relocatable IP addresses, and Quorum Server addresses if any — must be or resolve
88 Planning and Documenting an HA Cluster