Yellow Bricks

by Duncan Epping

ha

How to login to the vCLS VMs!?

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I was asked this question this week, how you can login to the vCLS VMs. Now before I share the video, I want to mention that I do not encourage people doing this, but as it is documented and supported I do want to provide a simple “how to” for how this works. If you want to login to the vCLS VM, maybe for troubleshooting if needed or for auditing, you can do so by SSH’ing first into your vCenter Server. When logged in to the vCenter Server you run the following command, which then returns the password, this will then allow you to login to the console of the vCLS VM. Again, I do not want to encourage you to do this. Either way, below you find the command for retrieving the password, and a short demo of me retrieving the password and logging in.

/usr/lib/vmware-wcp/decrypt_clustervm_pw.py

 

VMware vSphere Cluster 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 Cluster Services (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.

[Read more…] about VMware vSphere Cluster Services (vCLS) considerations, questions and answers.

Running vSphere 6.7 or 6.5 and configured HA APD/PDL responses? Read this…

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If you are running vSphere 6.7 or 6.5 and have not installed 6.7 P02 yet (6.5 P05 is available soon) and you have APD/PDL responses configured within vSphere HA it could be that an issue causes VMs not to be failed over when an APD or PDL occurs. This is a known issue in the release, and P02 or P05 solves this problem. What is the problem? Well, a bug causes VMs which are listed in “VM overrides” to have settings that are not configured to be set to “disabled” instead of “unset”, in specific the APD/PDL setting.

This means that even though you have APD/PDL responses configured on a cluster level, the VM level configuration overrides it as it would be set to “disabled”. It doesn’t matter really why you added them to VM Overrides, could be to configure VM Restart Priority for instance. The frustrating part is that the UI doesn’t show you it is disabled as it looks like it is not configured.

If you can’t install the patch just yet, for whatever reason, but you do have VMs in VM Overrides, make sure to go to VM Overrides and explicitly configure the VMs to have the APD/PDL responses enabled similar to what it is configured to on a cluster level as shown in the screenshots below.

vSphere HA internals: restart placement changes in vSphere 7!

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Frank and I are looking to update the vSphere Clustering deep dive to vSphere 7. While scoping the work I stumbled on to something interesting, and this is the change that was introduced for the vSphere HA restart mechanism,  and specifically the placement of VMs in vSphere 7. In previous releases vSphere HA had a straight forward way of doing placement for VMs when VMs need to be restarted as a result of a failure. In vSphere 7.0 this mechanism was completely overhauled.

So how did it work pre-vSphere 7?

  • HA uses the cluster configuration
  • HA uses the latest compatibility list it received from vCenter
  • HA leverages a local copy of the DRS algorithm with a basic (fake) set of stats and runs the VMs through the algorithm
  • HA receives a placement recommendation from the local algorithm and restarts the VM on the suggested host
  • Within 5 minutes DRS runs within vCenter, and will very likely move the VM to a different host based on actual load

As you can imagine this is far from optimal. So what is introduced in vSphere 7? Well, we introduce two different ways of doing placement for restarts in vSphere 7:

  1. Remote Placement Engine
  2. Simple Placement Engine

The Remote Placement Engine, in short, is the ability for vSphere HA to make a call to DRS for the recommendation of the placement of a VM. This will take the current load of the cluster, the VM happiness, and all configured affinity/anti-affinity/vm-host affinity rules into consideration! Will this result in a much slower restart? The great thing is that the DRS algorithm has been optimized over the past years and it is so fast that there will not be a noticeable difference between the old mechanism and the new mechanism. Added benefit of course for the engineering team is that they can remove the local DRS module, which means there’s less code to maintain. How this works is that the FDM Master communicated with the FDM Manager which runs in vCenter Server. FDM Manager communicates with the DRS service to request a placement recommendation.

Now some of you will probably wonder what happens when vCenter Server is unavailable, well this is where the Simple Placement Engine comes into play. The team has developed a new placement engine that basically takes a round-robin approach, but does consider of course “must rules” (VM to Host) and the compatibility list. Note, affinity, or anti-affinity rules, are not considered when SPE is used instead of RPE! This is a known limitation, which is considered to be fixed in the future. If a host, for instance, is not connected to the datastore the VM is running on that needs to be restarted than that host is excluded from the list of potential placement targets. By the way, before I forget, version 7 also introduced a vCenter heartbeat mechanism as a result. HA will be heart beating the vCenter Server instance to understand when it will need to resort to the Simple Placement Engine vs the Remote Placement Engine.

I dug through the FDM log to find some proof of these new mechanisms, (/var/log/fdm.log) and found an entry that shows there are indeed two placement engines:

Invoking the RPE + SPE Placement Engine

RPE stands for “remote placement engine”, and SPE for “simple placement engine”. Where Remote of course refers to DRS. You may ask yourself, how do you know if DRS is being called? Well, that is something you can see in the logs in the DRS log files, when a placement request is received, the below entry shows up in the log file:

FdmWaitForUpdates-vim.ClusterComputeResource:domain-c8-26307464

This even happens when DRS is disabled and also when you use a license edition which does not include DRS even, which is really cool if you ask me. If for whatever reason vCenter Server is unavailable, and as a result DRS can’t be called, you will see this mentioned in the FDM log, and as shown below, it will use the Simple Placement Engine’s recommendation for the placement of the VM:

Invoke the placement service to process the placement update from SPE

A cool and very useful small HA enhancement if you ask me for vSphere 7.0!

 

** Disclaimer: This article contains references to the words master and/or slave. I recognize these as exclusionary words. The words are used in this article for consistency because it’s currently the words that appear in the software, in the UI, and in the log files. When the software is updated to remove the words, this article will be updated to be in alignment. **

vSphere HA internals: VMCP super aggressive option in vSphere 7

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Most of you probably heard about a feature called VMCP aka VM Component Protection. If not, this is the functionality in vSphere HA that enabled you to restart VMs which have been impacted by a PDL (permanent device loss) or APD (all paths down) scenario. (If you have no idea what I am talking about read this article first.)

When you configure the APD response you have four options:

  1. Disable
  2. Issue Event
  3. Power Off / Restart – Conservative
  4. Power Off / Restart – Aggressive

The main difference between Conservative and Aggressive is that if you find yourself in a situation where HA isn’t sure whether a VM can be restarted during an APD scenario it will not power off the VM when using Conservative. If you have it configured as Aggressive it will power off the VM. However, if HA is certain that a VM can’t be powered on it will not power off the VM. Basically it prefers availability of the VM.

As you can imagine, in certain scenarios having a VM running while it is impacted by an “APD” situation makes no sense. The VM has lost access to storage, and you simply may prefer to kill the workload. Why? Well, when it loses access to storage it can’t write to disk. You could find yourself in a situation where a change is acknowledged and you think it is written to disk but it somehow is sitting in a memory cache etc.

If you prefer the VM to be killed, regardless of whether it can be restarted or not, you can enable this via a vSphere HA advanced setting. Now before you implement this, do note that if a cluster-wide APD situation occurs, you could find yourself in the scenario where ALL virtual machines are powered off by HA and not restarted as the resources are not available. Anyway, if you feel this is a requirement, you can configure the following vSphere HA advanced setting in vSphere 7:

das.restartVmsWithoutResourceChecks = true