VERITAS Volume Manager 4.1 Release Notes (5900-0591, March 2010)

VERITAS Volume Manager 4.1 Release Notes
Known Problems and Workarounds
Chapter 1 23
Base-VXVM and Multi-Host Failover Configurations
Outside the context of clustering functionality, VxVM disk groups can be imported (made
available) from only one host at any given time. When a host imports a disk group as private,
the volumes and configuration of that disk group becomes 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 the Oracle Parallel Server, 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 is not
a problem limited to VxVM.
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 VxVM, 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 VxVM 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