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418 Administering cluster functionality
Multiple host failover configurations
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-clustered 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 (in Veritas Volume Manager, this is 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.
Note: 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 group is first imported by Node A, but the administrator wants to access the
disk group from Node B if Node A crashes. This kind of scenario (failover) can be
used to provide manual high availability to data, where the failure of one node
does not prevent access to data. Failover can be combined with a “high
availability” monitor to provide automatic high availability to data: when Node
B detects that Node A has crashed or shut down, Node B imports (fails over) the
disk group to provide access to the volumes.
Veritas Volume Manager can support failover, but it relies on the administrator
or on an external high-availability monitor to ensure that the first system is
shut down or unavailable before the disk group is imported to another system.