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vcenter moving hosts between datacenters and clusters

vcenter server 5.1 with 3x esxi 5.0 hosts connected.

Till now we've been using the three hosts just in the same datacenter.

Implementing a SAN, and been told we should put them all in one CLUSTER.  To do that i have to (to my understanding) remove the hosts from vcenter, and then add them again into the cluster I create.

Is it that easy?  Will VMs that are running on the hosts stop running at any point?  I get a warning that they will, but just want to make sure it's really that straightforward.

Any disadvantage to them being in a cluster?
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Mystical_Ice
Asked:
Mystical_Ice
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2 Solutions
 
xperttechCommented:
Mystical_Ice,

A "Datacenter" container only helps you identify your VM hosts that are in different sites. A "Cluster", which must be created inside a Datacenter has the purpose of resource sharing among multiple VM hosts. You can have multiple clusters in a Datacenter.

That being said, I don't believe VMs would be affected when you move the VM hosts into a cluster. I would create the cluster but would not enable HA, DRS and EV settings initially.

If you have resource pools in your individual hosts, these will be removed. If you enable DRS before moving in the hosts, then you will have the opportunity to choose whether to delete the resource pools or to create the them in the cluster. Be careful about creating the pools. Remember, the cluster resources will be the aggregate of the resources contributed by all hosts joining the cluster. If you create the resource pools based on the first VM host you add, you may need to revisit the settings later to confirm the pools have the proper settings.

Hope this helps!

-XT
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Mystical_IceAuthor Commented:
It does help - thanks!  How about shared storage?  Our SAN vendor added a LUN from our new SAN to one host, and it showed up on all of the host in vcenter.

What happens if vcenter fails or the cluster gets destroyed - all of the hosts will stop seeing the SAN LUN?
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Create a Cluster, and Drag and Drop the Hosts into the Cluster.

No need to remove the hosts from vCenter Server.

Clusters have no relationship to a SAN or Shared Storage or LUNs.
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xperttechCommented:
Your vendor did not filter out the rest of the hosts.

Now, it is pretty common and a good practice to allow the other VM hosts in the cluster see the same LUNs. This way, if HA or DRS need to take any action and move a VM that "lives" in the shared storage, then the VM can be brought up (powered on) in the next available host.

When VM hosts are part of a cluster, they are managed and constantly observed by DRS and HA for any events that would require actions. If vCenter crashes, no problem, just restart or recover your vCenter. The VM hosts and their VMs continue to run, but the DRS and HA features do not apply until vCenter regains control. In other words, the monitoring is suspended.

vCenter has nothing to do with your shared storage. Just to clarify. Your hosts are configured to mount shared storage regardless of vCenter involvement.

-XT
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Mystical_IceAuthor Commented:
Ohh ok, because when the one host was configured to see the storage on the HBA, the other hosts see it all too.  I haven't tried to actually add the datastore/LUN in the storage of the host, but if/when I do, will the other hosts see it too?

So that's not vcenter that does that?  I was thinking it was related to the hosts all being in a cluster in vcenter that they see the same storage.  I was then worried that if the cluster got broken or vcenter went down or something, that they suddenly wouldn't have the connection to the storage, and all our guest VMs would die.
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xperttechCommented:
Yes. If this is an unformatted LUN, you will need to initialize it for the VMFS system. Once it's ready, a rescan on the other hosts will show the system write/read-able to all hosts.

That is correct. vCenter has nothing to do with your storage, other than knowing it is there and allow you to do additional tasks with it. Like storage Vmotion, or Storage DRS is you are licensed for it. That's all.

See vCenter as a Management and feature-enhancing tool. But the VM hosts are not fully dependent on it. Once they have "their marching orders", hosts are pretty much sort-of independent.
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Mystical_IceAuthor Commented:
That does make sense - thanks.  How do the hosts all see the same shared storage?  That's got to be a piece of vcenter, right?  I add it on one, and because they're members of the same cluster, they see the same storage?
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xperttechCommented:
No, the cluster membership has nothing to do with that.
You could have shared storage without vCenter or vCenter without cluster and the hosts would still see it.

Network storage is an inherent feature of each host. That's why they would not depend on vCenter for "just seeing it". vCenter would handle storage Vmotion for instance, but it woudn't work if you did not have shared storage configured before hand.
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
its nothing todo with vCenter, your hosts ALL have a connection to the same storage LUNs
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Mystical_IceAuthor Commented:
How though - maybe i'm just not understanding how i can add a connection to shared storage on one host, and suddenly all the other hosts see it too
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xperttechCommented:
Mystical_Ice,

At the storage management in your SAN you allocate your LUN. If you do nothing else, all of your servers (VMware, Windows and else) can see it (I'm assuming iSCSI is what you're using). You can use CHAP to filter what servers have access to the LUN.

In your case, chances are, your vendor did not filter access (LUN mask), or created a CHAP setting and configured all of your hosts with the CHAP phrase to be able to connect.

vCenter is not involved in this.
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