Teach Yourself Linux Virtualization and High Availability: prepare for the LPIC-3 304 certification exam by David Clinton

Author:David Clinton [Clinton, David]
Language: eng
Format: mobi
Publisher: Bootstrap IT
Published: 2017-03-23T04:00:00+00:00

Cluster: Two or more server nodes running in coordination with each other to

complete individual tasks as part of a larger service, where mutual awareness

allows one or more nodes to compensate for the loss of another.

Server failure: The inability of a server node to respond adequately to client

requests. This could be due to a complete crash, connectivity problems, or

because it has been overwhelmed by high demand.

Failover: The way a cluster tries to accommodate the needs of clients orphaned

by the failure of a single server node by launching or redirecting other nodes to

fill a service gap.

Failback: The restoration of responsibilities to a server node as it recovers from

a failure.

Replication: The creation of copies of critical data stores to permit reliable

synchronous access from multiple server nodes or clients and to ensure they will

survive disasters. Replication is also used to enable reliable load balancing.

Redundancy: The provisioning of multiple identical physical or virtual server

nodes of which any one can adopt the orphaned clients of another one that fails.

Split brain: An error state in which network communication between nodes or

shared storage has somehow broken down and multiple individual nodes, each

believing it's the only node still active, continue to access and update a common

data source. While this doesn't impact shared-nothing designs, it can lead to

client errors and data corruption within shared clusters.

Fencing: To prevent split brain, the stonithd daemon can be configured to

automatically shut down a malfunctioning node or to impose a virtual fence

between it and the data resources of the rest of a cluster. As long as there is a

chance that the node could still be active, but is not properly coordinating with

the rest of the cluster, it will remain behind the fence. Stonith stands for "Shoot

the other node in the head". Really.

Quorum: You can configure fencing (or forced shutdown) to be imposed on

nodes that have fallen out of contact with each other or with some shared

resource. Quorum is often defined as more than half of all the nodes on the total

cluster. Using such defined configurations, you avoid having two subclusters of

nodes, each believing the other to be malfunctioning, attempting to knock the

other one out.

Disaster Recovery: Your infrastructure can hardly be considered highly

available if you've got no automated backup system in place along with an

integrated and tested disaster recovery plan. Your plan will need to account for

the redeployment of each of the servers in your custer.

Active/Passive Cluster

The idea behind service failover is that the sudden loss of any one node in a

service cluster would quickly be made up by another node taking its place. For

this to work, the IP address is automatically moved to the standby node in the

event of a failover. Alternatively, network routing tools like load balancers can be

102 | 7. High Availability Concepts and Theory

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