vCenter is the management product used to manage VMware VMs, vNetworking, Storage, and more....
There are two things that you need to have in place in order for many of the available operations of VMware to function. The most important one is the licensing to support it. Most large organizations are paying for Enterprise or Enterprise Plus for their Virtual Infrastructure (VI), for those that pay for standard or advanced you are limited to the features you can use. A breakdown of the licensing model is below...
(VMware, 2010).
The other thing is the resources in your cluster to support the neat features that expand the availability and redundancy of your VI. This is very important as the larger your cluster (in terms of hosts) the more redundant your VI is without added features with vCenter, you can simply use High Availability (HA) coupled with Distributed Resource Scheduling (DRS) to ensure that your environment is not only utilizing resources in a balanced and efficient manner, but also that VMs can be moved to other hosts.
This is common for those running enterprise licenses or greater. For those running advanced or fewer, you just have the high availability function (advanced users have vMotion as well which performs live migrations of your VMs to other hosts with zero downtime)
This is just one option and one scenario. Other products like Fault Tolerance (VMware's Clustering product with other VMs in a cluster), vCenter Server Heartbeat (create a vCenter active/passive cluster), and Distributed Power Manager (this really isn't a high available option but is a great product to efficiently use your hosts).
What's really neat about all of the above mentioned products is that they can be used in combination with one another (given the appropriate licensing levels). For example, use DRS with vMotion and you'll not only have live migration but efficient balancing of resources in your cluster.
Now if you want to migrate files between datastores so you can patch a given storage device or appliance, you have Storage vMotion to allow you to do live migration of VM's (files) from one store to the next.
So the point of this is how redundant or high available do you want your systems to be? To me, it would be where you draw the line between effective and efficient utilization versus time spent maintaining the systems. If your environment has critical VMs that require 100% uptime due to critical SLA's then I'd say that you have a requirement to maintain the VI at the highest available level (vMotion with DRS in a FT cluster where vCenter is using Heartbeat). If not, then doing something simpler while providing that level of availability (perhaps HA only). This allows recoverability of VMs though you may experience minimal downtimes. Below is a layout on the migration and high availability products commonly used with VMware and how it is broken down.
(VMware, 2010)
(VMware,2010).
These products are all meant to be used to make VMware environment with vSphere more available. Additionally, they are also broken down to be geared towards anyone that could use any given module based on it's cost. i.e. Small Business owners could easily afford vSphere Standard or Advanced to use vMotion or High Availability. Where your corporate enterprises could easily affort Enterprise or Enterprise Plus and use the added functionality of DRS and even purchase Heartbeat if needed. This product is robust enough to allow you to mold your VI based on not just your requirement (say an SLA) for uptime, but your engineering and administration team's ease of mind knowing that the VI is easily recoverable or highly available so there won't be that wake up call at 3AM as often...
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