SLVM Single-Node Online Reconfiguration (SLVM SNOR)

Abstract
The SLVM Single Node Online volume Re-configuration (SNOR) feature allows
changing the configuration of an active shared volume group in a cluster.
SLVM (Shared Logical Volume Manager) is a mechanism that permits multiple
systems in a ServiceGuard cluster to share (read/write) disk resources in the form of
volume groups.
Using new options in LVM commands, SLVM SNOR allows the system administrator
to change the configuration of a shared volume group, and of logical and physical
volumes in that volume group, while keeping it active in a single node. Using this
procedure, applications on at least one node will be available during the volume group
re-configuration.
This document will first provide an overview of the SNOR operation, detail the
command changes, and describe the step-by-step procedure and the possible error
messages.
Overview
Prior to SLVM SNOR consider a ServiceGuard cluster set up on three nodes. Data is
shared in the cluster using common storage. SLVM is used to create and manage
volume groups shared across all nodes of the cluster. Applications access the volume
group for data read/write operations from disk. To change a shared VG configuration,
like creation or modification of logical volumes/physical volumes or a modification in
the volume group (VG) attributes, pre-SNOR it is required to:
1. Shutdown the applications using the VG.
2. Deactivate the VG on all nodes.
3. Activate the VG in exclusive mode on one of the nodes.
4. Change the configuration on the VG.
5. Deactivate the VG.
6. If required, generate the map file with the changed configuration, copy it over
and re-import it to all the other nodes of the cluster.
7. Activate the VG in shared mode on all the cluster nodes.
8. Restart the applications using the VG.
With the SNOR online volume re-configuration functionality in place, the configuration
changes to the VG can be achieved without having to bring down the applications that
access the VG. A new command line option to the vgchange command (-x) is
introduced, which in conjunction with the -a option, allows a single node to switch an
active shared VG between shared and exclusive modes.