Q: – What is clusterring and why we use clustering ?
The concept of a cluster is that the cluster itself appears on the outside as a single system. A cluster consists of two or more Real Computers referred to as nodes or members of a cluster. The components of a cluster are commonly, but not always, connected to each other through fast Local Area Networks.
Clusters are usually deployed to improve performance and/or availability over that provided by a single computer, while typically being much more cost-effective than single computers of comparable speed or availability. Clustering is all about the back-end operations being performed by the nodes or members which appear to the outside world as a single computational entity.
Q: – What are the different type of clusters ?
1. High Performance Clusters
2. High Availability Clusters
3. Load Balancing clusters
4. Storage Clusters
Q: – What is High Performance Clusters ?
In High Performance Clusters Multiple nodes in cluster perform concurrent calculations. There are two key benefits of High Performance (or grid) computing :
Resilience –> As long as even a single member of a cluster is running, services continue to be provided by the cluster
Increased Capacity –> The more nodes added to the cluster, the more computing horsepower is available and therefore very powerful computers can be built using
Q: – What is High Availabilty Clusters (HA) ?
High-availability clusters provide continuous availability of services by eliminating single points of failure and by failing over services from one cluster node to another in case a node becomes inoperative.High-availability clusters are sometimes referred to as failover clusters. Red Hat Cluster Suite provides high-availability clustering through its High-availability Service Management component.
Q: – What is Load Balancing Clusters ?
Load Balancing Clusters operate by having all workload come through one or more load balancing front-ends, which then distribute it to a collection of back end servers. If a node in a load-balancing cluster becomes inoperative, the load-balancing software detects the failure and redirects requests to other cluster nodes. Red Hat Cluster Suite provides load- balancing through LVS (Linux Virtual Server).
Q: – What is Storage Clusters ?
Storage clusters provide a consistent file system image across servers in a cluster, allowing the servers to simultaneously read and write to a single shared file system.With a cluster-wide file system, a storage cluster eliminates the need for redundantcopies of application data and simplifies backup and disaster recovery. Red Hat Cluster Suite provides storage clustering through Red Hat GFS.
Q: – What is Active Passive and Active Active Terminology in HA Clusters ?
Active/Passive Terminology generally refers to failover clusters running only one service.
Active/Active Terminology can be used to refer to :
Failover clusters with multiple servers such that all cluster members are hosting at least one service
Q: – What is Fencing in Clustering ?
Fencing is used to avoid data inconsistencies in shared storage by fencing or cutting off a failed member.When the cluster manager detects a member failure, it immediately cuts off the failed member from accessing shared resources.Fence Devices can either be shared or non-shared.
Q: – What are Clustering Hardware Requirements ?
Red Hat Certified Hardware capable of running the proposed OS (RHEL 3/4/5)
Ethernet Switch for Networking
Shared Storage (for services that require access to shared data)
Q: – What is GFS2 filesystem ?
GFS2 is a shared file system used by Red Hat Cluster node member simultaneously. GFS2 allows all nodes to have direct concurrent access to the same shared block storage. In addition, GFS2 can also be used as a local filesystem. The principle component to allow such access is lock management. GFS2 uses DLM or Distributed Lock Management to achieve this. Also clustered LVM is used to communicate LVM meta data changes across nodes.
Q: – What is fencing in clustering and why it is required ?
Fencing is the process of isolating a node of a computer cluster when the former is malfunctioning. Isolating a node means ensuring that I/O can no longer be done from it. Fencing is typically done automatically, by cluster infrastructure such as shared disk file systems, in order to protect processes from other active nodes modifying the resources during node failures.
Fencing is required because it is impossible to distinguish between a real failure and a temporary hang. If the alfunctioning node is really down, then it cannot do any damage, so theoretically no action would be required (it could simply be brought back into the cluster with the usual join process). However, because there is a possibility that a malfunctioning node could itself consider the rest of the cluster to be the one that is malfunctioning, a race condition could ensue, and cause data corruption. Instead, the system has to assume the worst scenario and always fence in case of problems
Q: – what is Quorum ?
Quorum is a voting algorithm used by the cluster manager. A cluster can only function correctly if there is general agreement between the members about things. We say a cluster has 'quorum' if a majority of nodes are alive, communicating, and agree on the active cluster members. So in a thirteen-node cluster, quorum is only reached if seven or more nodes are communicating. If the seventh node dies, the cluster loses quorum and can no longer function. It's necessary for a cluster to maintain quorum to prevent 'splitbrain' problems.
Q: – What is multipathing in clustering ?
Clusters should be designed with NSPOF(No Single Point Of Failure) considerations. That means that there should be no single device such that it's failure could impact the Cluster. As an example, when using Fibre HBAs, two HBAs must be put into each clustered Node to guard against card failure, link failure etc. Similarly, two Fibre Switches should be used to prevent cluster services disruption int he event of the switch failure. If iSCSI protocol is used to access the shared storage, then two network cards should be used for this purpose. Each network card should have a unique IP address and should be connected via different switched to the iSCSI Storage.
Q: – Describe how the storage will be accessed using the iSCSI protocol in clustered environment ?
iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. iSCSI can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable location-independent data storage and retrieval. The protocol allows clients (called initiators) to send SCSI commands (CDBs) to SCSI storage devices (targets) on remote servers. It is a popular Storage Area Network (SAN) protocol, allowing organizations to consolidate storage into data center storage arrays while providing hosts (such as database and web servers) with the illusion of locally-attached disks. Unlike traditional Fibre Channel, which requires special purpose cabling, iSCSI can be run over long distances using existing network infrastructure.
Q: – What are the Logical Steps for installing Red Hat Cluster Suite ?
a) Configure Storage/udev/multipathing
b) Test file systems by mounting on both nodes
c) Install application on both nodes and copy required folders/data to shared
d) Write the startup/shutdown/status scripts, if required (you can use standard sysv
inti scripts as well, if you application has one)
e) Mount the shared partition on node1 and test the app
f) Unmount shared partition from node1, mount in node2 and test the app
g) Test fencing from both nodes
h) install cman, rgmanager rpms
i) Ensure iptables and selinux do not interfere (setenforce 0 for selinux)
Submitted By:-Deepak Rana Email-ID: – email@example.com