Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Multi-Host Failover Configurations
368 VERITAS Volume Manager Administrator’s Guide
How DRL Works in a Cluster Environment
When one or more nodes in a cluster crash, DRL must handle the recovery of all volumes
that were in use by those nodes when the crashes occurred. On initial cluster startup, all
active maps are incorporated into the recovery map during the volume start operation.
Nodes that crash (that is, leave the cluster as dirty) are not allowed to rejoin the cluster
until their DRL active maps have been incorporated into the recovery maps on all affected
volumes. The recovery utilities compare a crashed node’s active maps with the recovery
map and make any necessary updates before the node can rejoin the cluster and resume
I/O to the volume (which overwrites the active map). During this time, other nodes can
continue to perform I/O.
VxVM tracks which nodes have crashed. If multiple node recoveries are underway in a
cluster at a given time, their respective recoveries and recovery map updates can compete
with each other. VxVM tracks changes in the state of DRL recovery and prevents I/O
collisions.
The master node performs volatile tracking of DRL recovery map updates for each
volume, and prevents multiple utilities from changing the recovery map simultaneously.
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 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 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 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.