Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Multiple host failover configurations
Outside the context of CVM, VxVM disk groups can be imported (made available)
on only one host at any given time. When a host imports a (private) disk group,
the volumes and configuration of that disk group become accessible to the host.
If the administrator or system software wants to privately use the same disk group
from another host, the host that already has the disk group imported (importing
host) must deport (give up access to) the disk group. Once deported, the disk group
can be imported by another host.
If two hosts are allowed to access a disk group concurrently without proper
synchronization, such as that provided by Oracle RAC, the configuration of the
disk group, and possibly the contents of volumes, can be corrupted. Similar
corruption can also occur if a file system or database on a raw disk partition is
accessed concurrently by two hosts, so this problem in not limited to Veritas
Volume Manager.
Import lock
When a host in a non-CVM environment imports a disk group, an import lock is
written on all disks in that disk group. The import lock is cleared when the host
deports the disk group. The presence of the import lock prevents other hosts from
importing the disk group until the importing host has deported the disk group.
Specifically, when a host imports a disk group, the import normally fails if any
disks within the disk group appear to be locked by another host. This allows
automatic re-importing of disk groups after a reboot (autoimporting) and prevents
imports by another host, even while the first host is shut down. If the importing
host is shut down without deporting the disk group, the disk group can only be
imported by another host by clearing the host ID lock first (discussed later).
The import lock contains a host ID (the host name) reference to identify the
importing host and enforce the lock. Problems can therefore arise if two hosts
have the same host ID.
Since Veritas Volume Manager uses the host name as the host ID (by default), it
is advisable to change the host name of one machine if another machine shares
its host name. To change the host name, use the vxdctl hostid new_hostname
command.
Failover
The import locking scheme works well in an environment where disk groups are
not normally shifted from one system to another. However, consider a setup where
two hosts, Node A and Node B, can access the drives of a disk group. The disk
437Administering cluster functionality (CVM)
Multiple host failover configurations