The Linux-HA project maintains a set of building blocks for high availability cluster systems, including a cluster messaging layer, a huge number of resource agents for a variety of applications, and a plumbing library and error reporting toolkit.
You usually want to deploy the Pacemaker Cluster Resource Manager on top of the Heartbeat infrastructure layer. If you happen to be still under the impression that this CRM would be part of Heartbeat, please see Why was the Pacemaker Project Created?
KTCPVS stands for Kernel TCP Virtual Server. It implements application-level load balancing inside the Linux kernel, so called Layer-7 switching. Since the overhead of layer-7 switching in user-space is very high, it is good to implement it inside the kernel in order to avoid the overhead of context switching and memory copying between user-space and kernel-space. Although the scalability of KTCPVS is lower than that of IPVS (IP Virtual Server), it is flexible, because the content of request is known before the request is redirected to one server.
"The main goal of the keepalived project is to add a strong & robust keepalive facility to the Linux Virtual Server project. This project is written in C with multilayer TCP/IP stack checks. Keepalived implements a framework based on three family checks : Layer3, Layer4 & Layer5. This framework gives the daemon the ability of checking a LVS server pool states. When one of the server of the LVS server pool is down, keepalived informs the linux kernel via a setsockopt call to remove this server entrie from the LVS topology. In addition keepalived implements a VRRPv2 stack to handle director failover. So in short keepalived is a userspace daemon for LVS cluster nodes healthchecks and LVS directors failover.
DRBD® refers to block devices designed as a building block to form high availability (HA) clusters. This is done by mirroring a whole block device via an assigned network. DRBD can be understood as network based raid-1.
In the illustration above, the two orange boxes represent two servers that form an HA cluster. The boxes contain the usual components of a Linux™ kernel: file system, buffer cache, disk scheduler, disk drivers, TCP/IP stack and network interface card (NIC) driver. The black arrows illustrate the flow of data between these components.
The orange arrows show the flow of data, as DRBD mirrors the data of a high availably service from the active node of the HA cluster to the standby node of the HA cluster.
