Yellow Bricks

by Duncan Epping

hci mesh

Cross connecting vSAN Datastores with HCI Mesh in vSAN 8 OSA

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Yesterday I had a discussion internally on Slack about a configuration for a customer. The customer had multiple locations and had the potential to create various clusters within locations, or across locations. Now, as most of you know, I have been talking a lot about stretched clusters for the past decade. However, stretched clusters are not the answer to every problem. Especially not in situations where you end up with unbalanced configurations, or where you lack the ability to place a witness in a third location. Customers tend to gravitate towards stretched clusters as it provides them resiliency, and pooling of all resources even though they are across locations.

With vSAN when you have multiple clusters, you also have multiple vSAN Datastores. Having that separation of resources is typically appreciated. However, in some cases, customers prefer the flexibility of movement with limited overhead. Sure if you have multiple clusters you can simply Storage vMotion VMs from source cluster to destination cluster, but it does mean you need to move ALL the data with the VM, where in some cases you may not care where the data resides.

This is where HCI Mesh comes into play. With HCI Mesh you have the ability to mount a vSAN Datastore. Meaning, you have a “client” and a “server” cluster, and the client mounts the server. Within our documentation on core.vmware.com this is demonstrated as follows:

If you look at this diagram then the top two clusters are “client” clusters and the bottom one is the “server” cluster. This basically means that the “client” cluster consumed the datastore capacity from the “server” cluster. The above diagram however resulted in a bit of confusion as it does not show a situation where your client cluster can simultaneously be a server cluster. This is something I want to point out. You can create a true “mesh” with HCI Mesh! If you have two clusters, let’s say Cluster A and Cluster B, then you can mount the vSAN Datastore from Cluster A to Cluster B and the datastore from Cluster B to Cluster A. This is fully supported, and works great, as demonstrated in the below two screenshots. I tested this with vSphere/vSAN 8.0 OSA, but it is also supported with vSAN 7.0 U1 and up. Do note, vSAN ESA today does not support HCI Mesh just yet, hopefully it will in the near feature!


So before you decide how to configure vSAN, please look at all the capabilities provided, write down your requirements, and see what helps solving the challenges you are facing while meeting those requirements (in a supported way)!

vSAN Storage Rules policy capability allows to set dedupe per VM?

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There was a question posted on the VMware Community Forums, and as this is something I have been asked regularly, I figured I would do a quick blog post about it. Although I have covered this before, it doesn’t hurt to repeat, as it appears to be somewhat confusing for people. When you create a VM Storage Policy, starting with vSAN 7.0 U2 you have the ability to specify if a VM needs to be Encrypted, have Dedupe and Compression enabled, have Compression-Only enabled, and/or needs to be stored on all-flash vSAN or Hybrid. Never noticed it? Look at the screenshot below.

In the screenshot, you see that you have the ability to specify which data service needs to be enabled. I guess this is where the confusion comes into play, as this functionality is not about enabling the data service for the VM to which you assign the policy. This is about which data service needs to be enabled on the datastore to which the VM can be provisioned. Huh, what? Okay, let’s explain.

If you are using vSAN as your storage platform, and you are sharing vSAN Datastores between clusters leveraging the HCI Mesh feature, then you could find yourself in a situation where some clusters are hybrid and some are all-flash. Some may have data services enabled like Encryption or Deduplication, some may not. In that scenario you want to be able to specify which features need to be enabled for the datastore the VM is provisioned to. So what this “storage rules” feature does is that it ensure that the datastore which is shown as “compatible” actually has the specified capabilities enabled! In other words, if you tick “data-at-rest encryption” in a policy and assign the policy to a VM, then only the datastores which have “data-at-rest encryption” enabled will be shown as compatible with your VM!

So again, “storage rules” apply to the data services that should be enabled on the vSAN Datastore, and do not enable data services on a per VM/VMDK basis.

<Update for vSAN 8.0 ESA>

With vSAN 8.0 ESA the above has changed. With vSAN 8.0 ESA you can set compression per VM, and you actually do that using the policy storage rules. I discussed this in this blog post.

HCI Mesh error: Failed to run the remote datastore mount pre-checks

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I had two comments on my HCI Mesh compute only blogpost where both were reporting the same error when trying to mount a remote datastore. The error that popped up was the following:

Failed to run the remote datastore mount pre-checks.

I tried to reproduce it in my lab, as both had upgraded from 7.0 to U2 I did the same, that didn’t result in the same error though. The error doesn’t provide any details around why the pre-check fails, as shown below in the screenshot. After some digging I found out that the solution is simple though, you need to make sure IPv6 is enabled on your hosts. Yes, even when you are not using IPv6, it still needs to be enabled to pass the pre-checks. Thanks, Jiří and Reza for raising the issue!

HCI Mesh error: Failed to run the remote datastore mount pre-checks

Can 2-node or Stretched be used as client or server cluster with vSAN HCI Mesh?

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I had this question today, probably sparked by my post earlier where a showed a single-node cluster being able to leverage HCI Mesh to mount a remote vSAN Datastore. The question was if this also works with 2-node vSAN or with a vSAN Stretched Cluster. Unfortunately, the answer is no, a 2-node cluster or a vSAN Stretched Cluster is not supported with HCI Mesh today. Yes, this is a hard limit today, meaning that the “health check”, which is done before mounting the datastore, will actually report the issue and not allow it to progress. You can imagine that this is the result of the latency and bandwidth/throughput requirements there are in place for vSAN HCI Mesh today. This may, or may not, change over time.

Using HCI Mesh with a stand-alone vSphere host?

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Last week at the French VMUG we received a great question. The question was whether you can use HCI Mesh (datastore sharing) with a stand-alone vSphere Host. The answer is simple, no you cannot. VMware does not support enabling vSAN, and HCI Mesh, on a single stand-alone host. However, if you still want to mount a vSAN Datastore from a single vSphere host, there is a way around this limitation.

First, let’s list the requirements:

  1. The host needs to be managed by the same vCenter Server as the vSAN Cluster
  2. The host needs to be under the same virtual datacenter as the vSAN Cluster
  3. Low latency, high bandwidth connection between the host and the vSAN Cluster

If you meet these requirements, then what you can do to mount the vSAN Datastore to a single host is the following:

  1. Create a cluster without any services enabled
  2. Add the stand-alone host to the cluster
  3. Enable vSAN, select “vSAN HCI Mesh Compute Cluster”
  4. Mount the datastore

Note, when you create a cluster and add a host, vCenter/EAM will try to provision the vCLS VM. Of course this VM is not really needed as HA and DRS are not useful with a single host cluster. So what you can do is enable “retreat mode”. For those who don’t know how to do this, or those who want to know more about vCLS, read this article.

As I had to test the above in my lab, I also created a short video demonstrating the workflow, watch it below.