Red Hat Cluster Suite And Selinux; Multicast Addresses - Red Hat ENTERPRISE LINUX 5 - ADMINISTRATION Manual

Fencing
To ensure reliable fencing when using qdiskd, use power fencing. While other types of fencing
(such as watchdog timers and software-based solutions to reboot a node internally) can be reliable
for clusters not configured with qdiskd, they are not reliable for a cluster configured with qdiskd.
Maximum nodes
A cluster configured with qdiskd supports a maximum of 16 nodes. The reason for the limit
is because of scalability; increasing the node count increases the amount of synchronous I/O
contention on the shared quorum disk device.
Quorum disk device
A quorum disk device should be a shared block device with concurrent read/write access by
all nodes in a cluster. The minimum size of the block device is 10 Megabytes. Examples of
shared block devices that can be used by qdiskd are a multi-port SCSI RAID array, a Fibre
Channel RAID SAN, or a RAID-configured iSCSI target. You can create a quorum disk device
with mkqdisk, the Cluster Quorum Disk Utility. For information about using the utility refer to the
mkqdisk(8) man page.
Note
Using JBOD as a quorum disk is not recommended. A JBOD cannot provide
dependable performance and therefore may not allow a node to write to it quickly
enough. If a node is unable to write to a quorum disk device quickly enough, the node
is falsely evicted from a cluster.

2.7. Red Hat Cluster Suite and SELinux

Red Hat Cluster Suite for Red Hat Enterprise Linux 5 requires that SELinux be disabled. Before
configuring a Red Hat cluster, make sure to disable SELinux. For example, you can disable SELinux
upon installation of Red Hat Enterprise Linux 5 or you can specify SELINUX=disabled in the /etc/
selinux/config file.

2.8. Multicast Addresses

Red Hat Cluster nodes communicate among each other using multicast addresses. Therefore, each
network switch and associated networking equipment in a Red Hat Cluster must be configured to
enable multicast addresses and support IGMP (Internet Group Management Protocol). Ensure that
each network switch and associated networking equipment in a Red Hat Cluster are capable of
supporting multicast addresses and IGMP; if they are, ensure that multicast addressing and IGMP are
enabled. Without multicast and IGMP, not all nodes can participate in a cluster, causing the cluster to
fail.
Note
Procedures for configuring network switches and associated networking equipment
vary according each product. Refer to the appropriate vendor documentation or other
information about configuring network switches and associated networking equipment to
enable multicast addresses and IGMP.
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