In the case of VMware HA, there is still a period of downtime when a server fails. Unfortunately, the duration of the downtime is not a value that can be calculated because it is unknown ahead of time how long it will take to boot a series of virtual machines. From this you can gather that, at this point in time, VMware HA does not provide the same level of high availability as found in a Microsoft server cluster solution.
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- VMware vCenter Server Version 5 Security Technical Implementation Guide.
Your mileage might vary but should give you a reasonable expectation of what to expect. This node crashed sometime during the night, and when the host went down, it took anywhere from 15 to 20 virtual machines with it. HA kicked in and restarted all the virtual machines as expected.
What made this an interesting experience is that the crash must have happened right after the polling of the monitoring and alerting server. All the virtual machines that were on the general alerting schedule were restarted without triggering any alerts. We did have some of those virtual machines with a more aggressive monitoring that did trip off alerts that were recovered before anyone was able to log on to the system and investigate.
I tried to argue the point that if an alert never fired, did the downtime really happen? I did not get too far with that argument but was pleased with the results.
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In another case, during testing I had a virtual machine running on a two-node cluster. I pulled the power cords on the host that the virtual machine was running to create the failure. My time to recovery from pull to ping was between five and six minutes. That's not too bad for general use but not good enough for everything. VMware Fault Tolerance can now fill that gap for even the most important and critical servers in your environment. I'll talk more about FT in a bit. By editing the cluster settings, you can enable the VMware HA feature for a cluster.
The HA cluster then determines the number of hosts failures it must support. After the nodes have been updated with the information about the cluster, vCenter Server no longer maintains a persistent connection with each node. Each node continues to function as a member of the HA cluster independent of its communication status with vCenter Server.
These components, shown in Figure The AAM, effectively the engine or service for HA, is a Legato-based component that keeps an internal database of the other nodes in the cluster.
Implementing VMware vCenter Server by Konstantin Kuminsky
The AAM is responsible for the intracluster heartbeat used to identify available and unavailable nodes. Each node in the cluster establishes a heartbeat with each of the other nodes over the Service Console network, or you can use or define another VMkernel port group for the HA heartbeat. As a best practice, you should provide redundancy to the AAM heartbeat by establishing the Service Console port group on a virtual switch with an underlying NIC team.
Though the Service Console could be multihomed and have an AAM heartbeat over two different networks, this configuration is not as reliable as the NIC team. The AAM is extremely sensitive to hostname resolution; the inability to resolve names will most certainly result in an inability to execute HA. When problems arise with HA functionality, look first at hostname resolution.
Having said that, during HA troubleshooting, you should identify the answers to questions such as these:. Adding a Host to vCenter Server When a new host is added into the vCenter Server inventory, the host must be added by its hostname, or the HA will not function properly.
VMware vCenter Server Version 5 Security Technical Implementation Guide
As just noted, HA is heavily reliant on successful name resolution. All hosts in a cluster are considered either a primary host or a secondary host. The primary host functions as the source of information for all new hosts and defaults to the first host added to the cluster. If the primary host experiences failure, the HA cluster will continue to function. In fact, in the event of primary host failure, one of the secondary hosts will move up to the status of primary host. The process of promoting secondary hosts to primary host is limited to four other hosts.
Only five hosts could assume the role of primary host in an HA cluster. The vpxa service communicates to the AAM through a third component called the Vmap.
vCenter Server for Windows and vCSA compared
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VMware vCenter Server is a necessary component of any professional vSphere implementation.
This book is a practical and hands-on guide to VMware vCenter Server, providing a description of its features and capabilities as well as useful tips on doing day-to-day administrative tasks. This book starts with an introduction to VMware vCenter Server, describing requirements and deployment steps along the way. It takes you through a description of product features and different aspects of administration giving useful tips for day-to-day tasks. You will learn how to deploy VMware vCenter Server, and how to manage hosts and virtual machines. You will also take a look at security features, availability, resource management, and discuss monitoring and automation topics.
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