1.what is a cluster? - What is Cluster?
VERITAS Cluster Server (VCS) connects, or clusters, multiple, independent systemsinto a management framework for increased availability. Each system, or node, runs itsown operating system and cooperates at the software level to form a cluster. VCS linkscommodity hardware with intelligent software to Provide Application Failover andControl. When a node or a monTiled Application Fails, Other Nodes Can Take PredefinedAction To Take over and bruster.
2. DETECTING FAILURE - Failed Detection VCS CAN Detect Application Failure and Node Failure Among Cluster MEMBERS.
(1) .Detecting Application FailureAt the highest level, VCS is typically deployed to keep business-critical applicationsonline and available to users. VCS provides a mechanism to detect failure of anapplication and any underlying resources or services supporting the application. VCSissues specific commands, tests .
(2) .Detecting Node FailureOne of the most difficult tasks in clustering is correctly discriminating between loss of asystem and loss of communication between systems. There are several technologies usedfor this purpose, including heartbeat networks between servers, quorum disks, and SCSIreservation. VCS uses A Redundant Network Heartbeat Along with SCSI III-Basedmembership Coordination and Data Protection for Detecting Failure ON A Node and Onfencing.
3.Switchover and Failover
Failover and switchover are the processes of bringing up application services on adifferent node in a cluster. In both cases, an application and its network identity arebrought up on a selected node. Client systems access a virtual IP address that moves withthe 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 inthe cluster. For example, in a 2-node cluster consisting of db-server1 and db-server2, avirtual address may BE CALLED DB-Server. Clients Will Then Access DB-Server and BeunaWare Of Which Physical Server. Virtual IP Addressesuse A Technology Known As IP AliaSing.
(1) The Switchover ProcessA switchover is an orderly shutdown of an application and its supporting resources onone server and a controlled startup on another server. Typically this means unassigningthe virtual IP, stopping the application, and deporting shared storage. On the other server, the Process is Reverse, File Systems Are Mounted, The Application ISSTARTED, AND THE VIRTUAL IP Address IS BROUGHT UP.
(2) The Failover ProcessA failover is similar to a switchover, except the ordered shutdown of applications on theoriginal node may not be possible. In this case services are simply started on anothernode. The process of starting the application on the node is identical in a Failover Orswitchover. This Means The Application Must Be Capable of Restarting Following a Crash Officer 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 configurationfiles , distributing the information when new nodes join the cluster, responding to operatorinput, and taking corrective action when something fails. It is typically known as the VCSengine. The engine uses agents to monitor and manage resources. (2) Low Latency Transport (LLT) VCS uses private network communications between cluster nodes for clustermaintenance. The Low Latency Transport functions as a high-performance, low-latencyreplacement for the IP stack, and is used for all cluster communications. VERITASrequires two completely independent networks between all cluster nodes, which providethe Required Redundancy in the Communication path and enable vcs to discriminatebetween a network failure and a system failure. LLT HAS Two Major Funct Ions.
(3) Group Membership Services / Atomic Broadcast (GAB) The Group Membership Services / Atomic Broadcast protocol (GAB) is responsible forcluster membership and cluster communications. ◆ Cluster MembershipGAB maintains cluster membership by receiving input on the status of the heartbeatfrom each node via LLT . When a system no longer receives heartbeats from a peer, itmarks the peer as DOWN and excludes the peer from the cluster. In mostconfigurations, the I / O fencing module is used to prevent network partitions. ◆ Cluster CommunicationsGAB's second function is reliable cluster communications Gab Provides GuaranteedDelivery of Point-to-Point and Broadcast Messages To All Nodes.
