Table of Contents
This chapter describes using DRBD as replicated storage for Red Hat Cluster Suite high availability clusters.
![[Note]](images/note.png) | Note |
|---|---|
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. 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 (rgmanager,
alternatively 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.
A single highly available application, filesystem, IP address and the like is referred to as a resource in Red Hat Cluster Suite terminology.
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.
Where resources form a co-dependent collection, that collection is called a service. This is different from Heartbeat, where such a collection is referred to as a resource group.
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 drbd.sh and
drbd.metadata.