Vmware HA - specify which host a VM fails over to

Hi guys, is there a way in VMware to specify which host a VM will failover to?

I have a number of VMs each allocated 32GB or 48GB memory (reserved) and I want to specify they fail over to a specific host.

Also, if a VM has reserved memory, how does it work when it fails over to another host? I assume it has to find a host that has that RAM available? Or is it more complicated than that?
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Andrew Hancock (VMware vExpert / EE MVE^2)Connect With a Mentor VMware and Virtualization ConsultantCommented:
VMware HA is designed, that when a Host fails, the VMs are restarted on available Hosts within the cluster.

If there is not the available resources available within your cluster, VMs will not be restarted.

So you need to ensure you have the correct capacity and resources available if a host should fail.

Further reading I would recommend

VMware vSphere 4.1 HA and DRS Technical deepdive (Volume 1)

By Duncan Epping and Frank Denneman


ISBN-10: 1456301446
ISBN-13: 978-1456301446

Written by Duncan Epping and Frank Denneman, both of whom are Consulting Architects at VMware and are perceived by the industry as Subject Matter experts on VMware High Availability and VMware Distributed Resource Scheduler.
This book zooms in on two key components of every VMware based infrastructure. It covers the basic steps needed to create a VMware HA and DRS cluster, and goes on to explain the concepts and mechanisms behind HA and DRS which will enable you to make well educated decisions. You will get the tools to understand and implement e.g. HA admission control policies, DRS resource pools and resource allocation settings and more.

On Amazon US



on Amazon UK



Available in Europe from

There is also a book for vSphere 5.0, vSphere 5.0 Clustering.
amaru96Author Commented:
Thanks for the info buddy, looks like I have some reading to do.

However, just going back to my last question, does a VM with reserved memory failover the same way a VM without reserved memory fails over? Is it just a matter of a host having that memory available? Cause I read something about "available slots", which didnt really make a lot of sense to me, but seems to indicate that failing over a VM with reserved memory requires the host to have that memory available as a row of slots, instead of fragmented.

Hope that makse sense :-)
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
that is correct.

So ensure ALL your hosts have adequate resources, if you were to lose a host.

is there a reason you are reserving so much memory for VMs?

32GB and 48GB for VM is alot, it's almost, worth setting up a physical server for them each! (when you consider the cost of the hypervisor!)
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amaru96Author Commented:
Not sure why they need to have it as reserved, but they are SAP servers and is what has been requested.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
okay, it's just that reserving memory does not make it available to any other VM. It's ring fenced specifically to that VM.

if 32GB and 48GB are not available on that host, they will not power on.

I understand that's what's been requested, it's a bit like saying we need a 48GB Physical Server, but you application only ever users 12GB! (so 36GB is wasted!). But in virtual world, that 36GB could be allocated elsewhere.

Again, it depends on your resources in the host, farm and how much memory you have per host?

How much do you have per host? 96GB+
amaru96Author Commented:
We are going to order 6 new blades, each with 128GB. The idea is to create a new cluster so the hosts are specifically available for these servers and not part of the general vmware environment. Any thoughts on this?
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
It depends on how you want to use your investment.

We prefer to use many hosts in clusters for resilience, availability and scalability and if we have a requirement for ring fencing resources, we create a Resource Pool for the VMs or VM.

Allocate CPU and Memory to the Resource Pool, this is then reserve out of the cluster, for the VM, the VM is protected, and cannot be starved of resources.

That's how we do it. But some organizations like to have a VMware SAP farm etc (so they can touch it!)
amaru96Author Commented:
Thanks for all the info, much appreciated.
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