Table of Contents
This chapter describes using DRBD as replicated storage for Red Hat Cluster Suite high availability clusters.
This guide deals primarily with Red Hat Cluster Suite as found in Red Hat Enterprise Linux (RHEL 5). If you are deploying DRBD on earlier versions such as RHEL 4, configuration details and semantics may vary.
The Service Availability Forum is an industry consortium with the purpose of developing high availability interface definitions and software specifications. The Application Interface Specification (AIS) is one of these specifications, and OpenAIS is an open source AIS implementation maintained by a team staffed (primarily) by Red Hat employees. OpenAIS serves as Red Hat Cluster Suite's principal cluster communications infrastructure.
Specifically, Red Hat Cluster Suite makes use of the Totem group communication algorithm for reliable group messaging among cluster members.
Red Hat Cluster Suite in Red Hat Enterprise Linux (RHEL) version 5 adds an abstraction and convenience interface layer above OpenAIS named
cman also serves as a compatibility layer to RHEL 4, in which
cman behaved similarly, albeit without utilizing OpenAIS.
The Cluster Configuration System (CCS) and its associated daemon,
ccsd, maintains and updates the cluster configuration. Management applications utilize
ccsd and the CCS libraries to query and update cluster configuration items.
Red Hat Cluster Suite, originally designed primarily for shared storage clusters, relies on node fencing to prevent concurrent, uncoordinated access to shared resources. The Red Hat Cluster Suite fencing infrastructure relies on the fencing daemon
fenced, and fencing agents implemented as shell scripts.
Even though DRBD-based clusters utilize no shared storage resources and thus fencing is not strictly required from DRBD's standpoint, Red Hat Cluster Suite still requires fencing even in DRBD-based configurations.
The resource group manager (
clurgmgr) is akin to the Cluster Resource Manager in Heartbeat. It serves as the cluster management suite's primary interface with the applications it is configured to manage.
Where resources depend on each other — such as, for example, an NFS export depending on a filesystem being mounted — they form a resource tree, a form of nesting resources inside another. Resources in inner levels of nesting may inherit parameters from resources in outer nesting levels. The concept of resource trees is absent in Heartbeat.
The resource agents invoked by
rgmanager are similar to those used by the Heartbeat CRM, in the sense that they utilize the same shell-based API as defined in the Open Cluster Framework (OCF), although Heartbeat utilizes some extensions not defined in the framework. Thus in theory, the resource agents are largely interchangeable between Red Hat Cluster Suite and Heartbeat — in practive however, the two cluster management suites use different resource agents even for similar or identical tasks.
Red Hat Cluster Suite resource agents install into the
/usr/share/cluster directory. Unlike Heartbeat OCF resource agents which are by convention self-contained, some RHCS resource agents are split into a
.sh file containing the actual shell code, and a
.metadata file containing XML resource agent metadata.
Starting with version 8.3, DRBD includes a Red Hat Cluster Suite resource agent. It installs into the customary directory as