The volume groups as per the operations on them are concerned can be of three types:
- Non Concurrent VG
- Concurrent VG
- Enhanced Concurrent VG
Often the confusion arises among these types. I am trying to clarify the differences. The terms often apply in clustering scenario.
Non Concurrent VG
In Non Concurrent VG, application runs typically on a single node only and data of VG is accessible to that node only. If however present node fails then application moves to other nodes and VG is varied on that node. Then only present node can access the data on VG.
In concurrent VG, the application runs on all nodes of cluster simultaneously. All of the nodes can access the data at the same time. All of the nodes can read and write the data simultaneously. The data integrity is then the responsibility of the application.
Enhanced Concurrent VG
In enhanced concurrent VG, access to VG is allowed on all of the nodes of cluster. Certain restrictions apply in this case. The VG can be varied on different nodes in active or passibe states.
Active State Varyon
In active state varyon of VG,Â the following operations are allowed:
- Filesystem operations and mounting of filesystems.
- Operations on Logical Volumes.
- Operations on application.
- Syncing of VGs.
Passive State Varyon
In passive state varyon of VG certain restrictions apply:
- Filesystems mounting and operations on filesystems are not allowed.
- No operations on Lvs are allowed.
- Syncing of VGs is not allowed.
The following are allowed:
- LVM read only access to Vgs special file is allowed.
- LVM read only access to first 4k of all Lvs under VG is allowed.
The details have been provided with the help of two books:
- SC23-4862-08Â HACMP Administration Guide.
- SG24-6769-00 Implementing High Availability Cluster Multi-Processing (HACMP) Cookbook.
Both of the books are available for free download from redbooks.ibm.com.