Yellow Bricks

by Duncan Epping

u1

VMware vSphere Clustering Services (vCLS) considerations, questions and answers.

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In the vSphere 7.0 Update 1 release VMware introduced a new service called the VMware vSphere Clustering Service (vCLS). vCLS provides a mechanism that allows VMware to decouple both vSphere DRS and vSphere HA from vCenter Server. Niels Hagoort wrote a lengthy article on this topic here. You may wonder why VMware introduces this, well as Niels states. by decoupling the clustering services (DRS and HA) from vCenter Server via vCLS we ensure the availability of critical services even when vCenter Server is impacted by a failure.

vCLS is a collection of multiple VMs which, over time, will be the backbone for all clustering services. In the 7.0 U1 release a subset of DRS functionality is enabled through vCLS. Over the past week(s) I have seen many questions coming in and I wanted to create a blog with answers to these questions. When new questions or considerations come up, I will add these to the list below.

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Site locality in a vSAN Stretched Cluster?

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On the community forums, a question was asked around the use of site locality in a vSAN Stretched Cluster. When you create a stretched cluster in vSAN you can define within a policy how the data needs to be protected. Do you want to replicate across datacenters? Do you want to protect the “site local data” with  RAID-1 or RAID-5/6? All of these options are available within the UI.

What if you decide to not stretch your object across locations, is it mandatory to specify which datacenter the object should reside in?

The answer is simple: no it is not. The real question, of course is, would be: should you define the location? Most definitely! If you wonder how to do this, simplicy specify it within the policy you define for these objects as follows:

The above screenshot is taken from the H5 client, if you are still using the Web Client it probably looks slightly different (Thanks Seamus for the screenshot):

Why would you do this? Well, that is easy to explain. When the objects of a VM get provisioned the decision will be made per object where to place it. If you have multiple disks, and you haven’t specified the location, you could find yourself in the situation where disks of a single non-stretched VM are located in different datacenters. This is, first of all, terrible for performance, but maybe more importantly also would impact availability when anything happens to the network between the datacenters. So when you use site locality for non-stretched VMs, make sure to also configure the location so that your VM and objects will align as demonstrated in the below diagram.

 

Impact of adding Persistent Memory / Optane Memory devices to your VM

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I had some questions around this in the past month, so I figured I would share some details around this. As persistent memory (Intel Optane Memory devices for instance) is getting more affordable and readily available more and more customers are looking to use it. Some are already using it for very specific use cases, usually in situations where the OS and the App actually understand the type of device being presented. What does that mean? At VMworld 2018 there was a great session on this topic and I captured the session in a post. Let me copy/paste the important bit for you, which discusses the different modes in which a Persistent Memory device can be presented to a VM.

  • vPMEMDisk = exposed to guest as a regular SCSI/NVMe device, VMDKs are stored on PMEM Datastore
  • vPMEM = Exposes the NVDIMM device in a “passthrough manner, guest can use it as block device or byte addressable direct access device (DAX), this is the fastest mode and most modern OS’s support this
  • vPMEM-aware = This is similar to the mode above, but the difference is that the application understands how to take advantage of vPMEM

But what is the problem with this? What is the impact? Well when you expose a Persistent Memory device to the VM, it is not currently protected by vSphere HA, even though HA may be enabled on your cluster. Say what? Yes indeed, the VM which has the PMEM device presented to it will be disabled for vSphere HA! I had to dig deep to find this documented anywhere, and it is documented in this paper. (Page 47, at the bottom.) So what works and what not? Well if I understand it correctly:

  • vSphere HA >> Not supported on vPMEM enabled VMs, regardless of the mode
  • vSphere DRS >> Does not consider vPMEM enabled VMs, regardless of the mode
  • Migration of VM with vPMEM / vPMEM-aware >> Only possible when migrating to host which has PMEM
  • Migration of VM with vPMEMDISK >> Possible to a host without PMEM

Also note, as a result (data is not replicated/mirrored) a failure could potentially lead to loss of data. Although Persistent Memory is a great mechanism to increase performance, it is something that should be taken into consideration when you are thinking about introducing it into your environment.

Oh, if you are wondering why people are taking these risks in terms of availability, Niels Hagoort just posted a blog with a pointer to a new PMEM Perf paper which is worth reading.

 

 

DRS Advanced Setting IsClusterManaged

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On Reddit, someone asked what DRS advanced setting IsClusterManaged does and if it is even legit. I can confirm it is legit, it is a setting which was introduced to prevent customers from disabling DRS while the cluster is managed by vCloud Director for instance. As disabling DRS would lead to deleting resource pools, which would be a very bad situation to find yourself in when you run vCloud Director as it leans on DRS Resource Pools heavily. So if you see the advanced setting IsClusterManaged in your environment for DRS, just leave it alone, it is there for a reason. (Most likely because you are using something like vCloud Director…)

Mixing versions of ESXi in the same vSphere / vSAN cluster?

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I have seen this question being asked a couple of times in the past months, and to be honest I was a bit surprised people asked about this. Various customers were wondering if it is supported to mix versions of ESXi in the same vSphere or vSAN Cluster? I can be short whether this is support or not, yes it is. Would I recommend it? No, I would not!

Why not? Well mainly for operational reasons, it just makes life more complex. Just think about a troubleshooting scenario, you now need to remember which version you are running on which host and understand the “known issues” for each version. Also, for vSAN things are even more complex as you could have “components” running on a different version of ESXi. On top of that, it could even be the case that a certain command or esxcli namespace is not available on a particular version of ESXi.

Another concern is when doing upgrades or updates, you need to take the current version into account when updating, or more importantly when upgrading! Also, remember that firmware/driver combination may be different for a particular version of vSphere/vSAN as well, this could also make life more complex and definitely increases the chances of making mistakes!

Is this documented anywhere? Well, there are two KBs which hint at this, for vSAN specifically I have requested our team to document this in an upcoming document. The KBs that hint at this are below.