VCS学习笔记

1.What is a Cluster?--什么是Cluster?

VERITAS Cluster Server(VCS) connects, or clusters, multiple, independent systems into a management framework for increased availability. Each system, or node, runs its own operating system and cooperates at the software level to form a cluster. VCS links commodity hardware with intelligent software to provide application failover and control. When a node or a monitored application fails, other nodes can take predefined action to take over and bring up services elsewhere in the cluster.

2.Detecting Failure--失败检测 VCS can detect application failure and node failure among cluster members.

(1).Detecting Application Failure At the highest level, VCS is typically deployed to keep business-critical applications online and available to users. VCS provides a mechanism to detect failure of an application and any underlying resources or services supporting the application. VCS issues specific commands, tests, or scripts that monitor the overall health of an application. VCS also determines the health of underlying system resources supporting the application, such as file systems and network interfaces.

(2).Detecting Node Failure One of the most difficult tasks in clustering is correctly discriminating between loss of a system and loss of communication between systems. There are several technologies used for this purpose, including heartbeat networks between servers, quorum disks, and SCSI reservation. VCS uses a redundant network heartbeat along with SCSI III-based membership coordination and data protection for detecting failure on a node and on fencing.

3.Switchover and Failover

Failover and switchover are the processes of bringing up application services on a different node in a cluster. In both cases, an application and its network identity are brought up on a selected node. Client systems access a virtual IP address that moves with the service. Client systems are unaware of which server they are using.

A virtual IP address is an address brought up in addition to the base address of systems in the cluster. For example, in a 2-node cluster consisting of db-server1 and db-server2, a virtual address may be called db-server. Clients will then access db-server and be unaware of which physical server is actually hosting the db-server. Virtual IP addresses use a technology known as IP Aliasing.

(1)The Switchover Process A switchover is an orderly shutdown of an application and its supporting resources on one server and a controlled startup on another server. Typically this means unassigning the virtual IP, stopping the application, and deporting shared storage. On the other server, the process is reversed. Storage is imported, file systems are mounted, the application is started, and the virtual IP address is brought up.

(2)The Failover Process A failover is similar to a switchover, except the ordered shutdown of applications on the original node may not be possible. In this case services are simply started on another node. The process of starting the application on the node is identical in a failover or switchover. This means the application must be capable of restarting following a crash of its original host.

4.Cluster Control, Communications, and Membership (1)High-Availability Daemon (HAD) The high-availability daemon, or HAD, is the main VCS daemon running on each system. It is responsible for building the running cluster configuration from the configuration files, distributing the information when new nodes join the cluster, responding to operator input, and taking corrective action when something fails. It is typically known as the VCS engine. The engine uses agents to monitor and manage resources.

(2)Low Latency Transport (LLT) VCS uses private network communications between cluster nodes for cluster maintenance. The Low Latency Transport functions as a high-performance, low-latency replacement for the IP stack, and is used for all cluster communications. VERITAS requires two completely independent networks between all cluster nodes, which provide the required redundancy in the communication path and enable VCS to discriminate between a network failure and a system failure. LLT has two major functions.

(3)Group Membership Services/Atomic Broadcast (GAB) The Group Membership Services/Atomic Broadcast protocol (GAB) is responsible for cluster membership and cluster communications. ◆ Cluster Membership GAB maintains cluster membership by receiving input on the status of the heartbeat from each node via LLT. When a system no longer receives heartbeats from a peer, it marks the peer as DOWN and excludes the peer from the cluster. In most configurations, the I/O fencing module is used to prevent network partitions. ◆ Cluster Communications GAB’s second function is reliable cluster communications. GAB provides guaranteed delivery of point-to-point and broadcast messages to all nodes.

5.I/O Fencing Module The I/O fencing module implements a quorum-type functionality to ensure only one cluster survives a split of the private network. I/O fencing also provides the ability to perform SCSI-III persistent reservations on failover. The shared VERITAS Volume Manager disk groups offer complete protection against data corruption by nodes assumed to be excluded from cluster membership.