1.4. The Clustered Logical Volume Manager (CLVM)

The Clustered Logical Volume Manager (CLVM) is a set of clustering extensions to LVM. These extensions allow a cluster of computers to manage shared storage (for example, on a SAN) using LVM. CLVM is part of the Resilient Storage Add-On.
Whether you should use CLVM depends on your system requirements:
  • If only one node of your system requires access to the storage you are configuring as logical volumes, then you can use LVM without the CLVM extensions and the logical volumes created with that node are all local to the node. Additionally, if you are using a clustered system for failover where only a single node that accesses the storage is active at any one time, then you can also use LVM without the CLVM extensions. When configuring logical volumes in a cluster that will not require the CLVM extensions, you configure your system with the LVM high availability resource agent. For information on configuring resources in a cluster, see the High Availability Add-On Reference.
  • If more than one node of your cluster will require access to your storage which is then shared among the active nodes, then you must use CLVM. CLVM allows a user to configure logical volumes on shared storage by locking access to physical storage while a logical volume is being configured. LVM uses clustered locking services to manage the shared storage. When configuring logical volumes in a cluster that will require the CLVM extensions, you configure your system with a clvm resource agent. For information on configuring resources in a cluster, see the High Availability Add-On Reference.
In order to use CLVM, the High Availability Add-On and Resilient Storage Add-On software, including the clvmd daemon, must be running. The clvmd daemon is the key clustering extension to LVM. The clvmd daemon runs in each cluster computer and distributes LVM metadata updates in a cluster, presenting each cluster computer with the same view of the logical volumes.
Figure 1.2, “CLVM Overview” shows a CLVM overview in a cluster.
CLVM Overview

Figure 1.2. CLVM Overview

In Red Hat Enterprise Linux 7, clusters are managed through Pacemaker. Clustered LVM logical volumes are supported only in conjunction with Pacemaker clusters, and must be configured as cluster resources. For information on configuring LVM volumes in a cluster, see Section 3.1, “Creating LVM Volumes in a Cluster”.