Yellow Bricks

by Duncan Epping

stretched cluster

VMs which are not stretched in a stretched cluster, which policy to use?

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I’ve seen this question popping up regularly. Which policy setting (“site disaster tolerance” and “failures to tolerate”) should I use when I do not want to stretch my VMs? Well, that is actually pretty straight forward, in my opinion, you really only have two options you should ever use:

  • None – Keep data on preferred (stretched cluster)
  • None – Keep data on non-preferred (stretched cluster)

Yes, there is another option. This option is called “None – Stretched Cluster” and then there’s also “None – Standard Cluster”. Why should you not use these? Well, let’s start with “None – Stretched Cluster”. In the case of “None – Stretched Cluster”, vSAN will per object decide where to place it. As you hopefully know, a VM consists of multiple objects. As you can imagine, this is not optimal from a performance point of view, as you could end up having a VMDK being placed in Site A and a VMDK being placed in Site B. Which means it would read and write from both locations from a storage point of view, while the VM would be sitting in a single location from a compute point of view. It is also not very optimal from an availability stance, as it would mean that when the intersite link is unavailable, some objects of the VM would also become inaccessible. Not a great situation. What would it look like? Well, potentially something like the below diagram!

Then there’s “None – Standard Cluster”, what happens in this case? When you use “None – Standard Cluster” with “RAID-1”, what is going to happen is that the VM is configured with FTT=1 and RAID-1, but in a stretched cluster “FTT” does not exist, and FTT automatically will become PFTT. This means that the VM is going to be mirrored across locations, and you will have SFTT=0, which means no resiliency locally. It is the same as “Dual Site Mirroring”+”No Data Redundancy”!

In summary, if you ask me, “none – standard cluster” and “none – stretched cluster” should not be used in a stretched cluster.

vSphere HA reporting not enough failover resources fault with stretched cluster failure scenario

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Last few months I had a couple of customers asking why vSphere HA was reporting “not enough failover resources” fault in a stretched cluster failure scenario for virtual machines that are still up and running. Now before I explain why, let’s paint a picture first to make it clear what is happening here. When you run a stretched cluster you can have a scenario where a particular VM (or multiple VMs) are not mirrored/replicated across locations. Now note, with vSAN you can specify for any given VM on a VM level how and if the VM should be available across locations. Typically you would see a VM with RAID-1 across locations, and then RAID-1/5/6 within a location. However, you can also have a scenario where a VM is not replicated across locations, but from a storage point of view only available within a location, this is depicted in the diagram below.

Now in this scenario, when Site A is somehow partitioned from Site B, you will see alarms/errors which indicate that vSphere HA has tried to restart the VM that is located in Site B in Site A and that is has failed as a result of not having enough failover resources.

This, of course, is not the result of not having sufficient failover resources, but it is the result of the fact that Site A does not have access to the required storage components to restart the VM. Basically what HA is reporting is that it doesn’t have the resources which have the ability to restart the impacted VM(s).

Now, if you have paid attention, you will probably wonder why HA tries to restart the VM in the first place, as the VM will still be running in this scenario. Why is it still running? Well the VM isn’t stretched, and this is a partition and not an isolation, which means the isolation response doesn’t kill the VM. So why restart it? Well, as Site A is partitioned from Site B, Site A does not know what the status is of Site B. Site A only knows that Site B is not responding at all, and the only thing it can do is assume the full site has failed. As a result it will attempt a failover for all VMs that were/are running in Site B and were protected by vSphere HA.

Hope that explains why this happens. If you are not sure you understand the full scenario, I recorded a quick five minute video actually walking through the scenario and explaining what happens. You can watch that below, or simply go to youtube.

Creating a vSAN 7.0 Stretched Cluster

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A while ago I wrote this article and created this demo showing the creation of a vSAN Stretched Cluster. I bumped into the article/video today when I was looking for something and figured that it is time to recreate the vSAN Stretched Cluster demo. I recorded it today, using vSphere / vSAN 7, and just wanted to share it with you here. Hope you enjoy it.

Can I still provision VMs when a vSAN Stretched Cluster site has failed? Part II

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3 years ago I wrote the following post: Can I still provision VMs when a VSAN Stretched Cluster site has failed? Last week I received a question on this subject, and although officially I am not supposed to work on vSAN in the upcoming three months I figured I could test this in the evening easily within 30 minutes. The question was simple, in my blog I described the failure of the Witness Host, what if a single host fails in one of the two “data” fault domains? What if I want to create a snapshot for instance, will this still work?

So here’s what I tested:

  • vSAN Stretched Cluster
  • 4+4+1 configuration
    • Meaning, 4 hosts in each “data site” and a witness host, for a total of 8 hosts in my vSAN cluster
  • Create a VM with cross-site protection and RAID-5 within the location

So I first failed a host in one of the two data sites. When I fail the host, the following is what happens when I create a VM with RAID-1 across sites and RAID-5 within a site:

  • Without “Force Provisioning” enabled the creation of the VM fails
  • When “Force Provisioning” is enabled the creation of the VM succeeds, the VM is created with a RAID-0 within 1 location

Okay, so this sounds similar to the originally described scenario, in my 2016 blog post, where I failed the witness. vSAN will create a RAID-0 configuration for the VM. When the host returns for duty the RAID-1 across locations and RAID-5 within each location is then automatically created. On top of that, you can snapshot VMs in this scenario, the snapshots will also be created as RAID-0. One thing to mind is that I would recommend removing “force provisioning” from the policy after the failure has been resolved! Below is a screenshot of the component layout of the scenario by the way.

I also retried the witness host down scenario, and in that case, you do not need to use the “force provisioning” option. One more thing to note. The above will only happen when you create a RAID configuration which is impossible to create as a result of the failure. If 1 host fails in a 4+4+1 stretched cluster you would like to create a RAID-1 across sites and a RAID-1 within sites then the VM would be created with the requested RAID configuration, which is demonstrated in the screenshot below.

Can you move a vSAN Stretched Cluster to a different vCenter Server?

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I noticed a question today on one of our internal social platforms, the question was if you can move a vSAN Stretched Cluster to a different vCenter Server. I can be short, I tested it and the answer is yes! How do you do it? Well, we have a great KB that actually documents the process for a normal vSAN Cluster and the same applies to a stretched cluster. When you add the hosts to your new vCenter Server and into your newly created cluster it will pull in the fault domain details (stretched cluster configuration details) from the hosts itself, so when you go to the UI the Fault Domains will pop up again, as shown in the screenshot below.

What did I do? Well in short, but please use the KB for the exact steps:

  • Powered off all VMs
  • Placed the hosts into maintenance mode (do not forget about the Witness!)
  • Disconnected all hosts from the old vCenter Server, again, do not forget about the witness
  • Removed the hosts from the inventory
  • Connected the Witness to the new vCenter Server
  • Created a new Cluster object on the new vCenter Server
  • Added the stretched cluster hosts to the new cluster on the new vCenter Server
  • Took the Witness out of Maintenance Mode first
  • Took the other hosts out of maintenance

That was it, pretty straight forward. Of course, you will need to make sure you have the storage policies in both locations, and you will also need to do some extra work if you use a VDS. Nevertheless, it works pretty much straight-forward and as you would expect it to work!