HP Serviceguard manual Failover, Shows what happens in a failover situation

control of the package to another cluster node, allowing services to remain available with minimal interruption.

•There are also packages that run on several cluster nodes at once, and do not fail over. These are called system multi-node packages and multi-node packages. Examples are the packages HP supplies for use with the Veritas Cluster Volume Manager and Veritas Cluster File System from Symantec (on HP-UX releases that support them; see “About Veritas CFS and CVM from Symantec” (page 23)).

A system multi-node package must run on all nodes that are active in the cluster. If it fails on one active node, that node halts. System multi-node packages are supported only for HP-supplied applications.

A multi-node package can be configured to run on one or more cluster nodes. It is considered UP as long as it is running on any of its configured nodes.

In Figure 1, node 1 (one of two SPU's) is running failover package A, and node 2 is running package B. Each package has a separate group of disks associated with it, containing data needed by the package's applications, and a mirror copy of the data. Note that both nodes are physically connected to both groups of mirrored disks. In this example, however, only one node at a time may access the data for a given group of disks. In the figure, node 1 is shown with exclusive access to the top two disks (solid line), and node 2 is shown as connected without access to the top disks (dotted line). Similarly, node 2 is shown with exclusive access to the bottom two disks (solid line), and node 1 is shown as connected without access to the bottom disks (dotted line).

Mirror copies of data provide redundancy in case of disk failures. In addition, a total of four data buses are shown for the disks that are connected to node 1 and node 2. This configuration provides the maximum redundancy and also gives optimal I/O performance, since each package is using different buses.

Note that the network hardware is cabled to provide redundant LAN interfaces on each node. Serviceguard uses TCP/IP network services for reliable communication among nodes in the cluster, including the transmission of heartbeat messages , signals from each functioning node which are central to the operation of the cluster. TCP/IP services also are used for other types of inter-node communication. (The heartbeat is explained in more detail in the chapter “Understanding Serviceguard Software.”)

Failover

Any host system running in a Serviceguard cluster is called an active node. Under normal conditions, a fully operating Serviceguard cluster monitors the health of the cluster's components on all its active nodes.

Most Serviceguard packages are failover packages. When you configure a failover package, you specify which active node will be the primary node where the package will start, and one or more other nodes, called adoptive nodes, that can also run the package.

Figure 2 shows what happens in a failover situation.

22 Serviceguard at a Glance