HA Deep Dive - Part 1

      In this chapter, we are going to discuss about vsphere HA and its components. HA features and Mechanism varies on different vsphere version. In this article mostly we talk about vsphere 6.0 features and Mechanism.

What is Vsphere HA ?

    VMware vSphere High Availability, hereafter simply referred to as HA, provides a simple and cost effective solution to increase availability for any application running in a virtual machine regardless of its operating system.In the event of a failure of one of the hosts in the cluster, impacted virtual machines are automatically restarted on other ESXi hosts within that same VMware vSphere Cluster.

Components of HA:

  There are three major components that form the foundation for HA as of vSphere 6.0: 

• FDM 
• Hostd
• vCenter

FDM:

    The first and probably the most important component that forms HA is FDM (Fault Domain Manager). This is the HA agent. The FDM Agent is responsible for many tasks such as communicating host resource information, virtual machine states and HA properties to other hosts in the cluster. FDM also handles heartbeat mechanisms, virtual machine placement, virtual machine restarts, logging and much more.
    FDM uses a single-process agent. However, FDM spawns a watchdog process. In the unlikely event of an agent failure, the watchdog functionality will pick up on this and restart the agent to ensure HA functionality remains without anyone ever noticing it failed. The agent is also resilient to network interruptions and “all paths down” (APD) conditions. Inter-host communication automatically uses another communication path (if the host is configured with redundant management networks) in the case of a network failure.
   HA has no dependency on DNS as it works with IP addresses only. This is one of the major improvements that FDM brought. This does not mean that ESXi hosts need to be registered with their IP addresses in vCenter; it is still a best practice to register ESXi hosts by its fully qualified domain name (FQDN) in vCenter. Although HA does not depend on DNS, remember that other services may depend on it. On top of that, monitoring and troubleshooting will be much easier when hosts are correctly registered within vCenter and have a valid FQDN.
    vSphere HA also has a standardized logging mechanism, where a single log file has been created for all operational log messages; it is called fdm.log. This log file is stored under /var/log/

Hostd:

   One of the most crucial agents on a host is HOSTD. This agent is responsible for many of the tasks we take for granted like powering on virtual machines. FDM talks directly to HOSTD and vCenter, so it is not dependent on VPXA, like in previous releases. This is, of course, to avoid any unnecessary overhead and dependencies, making HA more reliable than ever before and enabling HA to respond faster to power-on requests. That ultimately results in higher VM uptime.
   FDM is dependent on HOSTD and if HOSTD is not operational, FDM halts all functions and waits for HOSTD to become operational.

Vcenter:

     vCenter is responsible for pushing out the FDM agent to the ESXi hosts when applicable. The push of these agents is done in parallel to allow for faster deployment and configuration of multiple hosts in a cluster. vCenter is also responsible for communicating configuration changes in the cluster to the host which is elected as the master.

    Although HA is configured by vCenter and exchanges virtual machine state information with HA, vCenter is not involved when HA responds to failure. It is comforting to know that in case of a host failure containing the virtualized vCenter Server, HA takes care of the failure and restarts the vCenter Server on another host, including all other configured virtual machines from that failed host.

That's all for this chapter folks. In next part will talk about HA admission control and policies.

Source: Duncan Epping