VERITAS Volume Manager 3.2 for HP-UX Release Notes

VxVM and Multi-Host Failover Configurations

Chapter 128

VxVM and Multi-Host Failover Configurations

VxVM disk groups can be imported (made available) from only one host at any given time.

When a host imports a disk group, that disk group’s volumes and configuration become

accessible to the host. If the administrator or some system software wants to 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 were somehow allowed to access a disk group concurrently, the configuration of

the disk group, and possibly the contents of volumes, could be corrupted. Similar corruption

would also occur if a file system or database on a raw disk partition were accessed

concurrently by two hosts, so this is not a problem limited to VxVM.

When a host 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

reimporting of disk groups after a reboot (called 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).

NOTE The import lock contains a host ID (with 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. 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. This can be followed with the

command vxdctl hostid new_hostname.

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 wishes to access the disk group from Node B if Node A crashes. This kind of

scenario (commonly called 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.