Yellow Bricks

by Duncan Epping

drs

vSphere 6.5 what’s new – DRS

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Most of us have been using DRS for the longest time. To be honest, not much has changed over the past years, sure there were some tweaks and minor changes but nothing huge. In 6.5 however there is a big feature introduced, but lets just list them all for completeness sake:

  • Predictive DRS
  • Network-Aware DRS enhancements
  • DRS profiles

First of all Predictive DRS. This is a feature that the DRS team has been working on for a while. It is a feature that integrates DRS with VROps to provide placement and balancing decisions. Note that this feature will be in Tech Preview until vRealize Operations releases their version of vROPs which will be fully compatible with vSphere 6.5, hopefully sometime in the first half of next year. Brian Graf has some additional details around this feature here by the way.

Note that of course DRS will continue to use the data provided by vCenter Server, it will on top of that however also leverage VROps to predict what resource usage will look like, all of this based on historic data. You can imagine a VM currently using 4GB of memory (demand), however every day around the same time a SQL Job runs which makes the memory demand spike up to 8GB. This data is available through VROps now and as such when making placement/balancing recommendations this predicted resource spike can now be taken in to consideration. If for whatever reason however the prediction is that the resource consumption will be lower then DRS will ignore the prediction and simply take current resource usage in to account, just to be safe. (Which makes sense if you ask me.) Oh and before I forget, DRS will look ahead for 60 minutes (3600 seconds).

How do you configure this? Well that is fairly straight forward when you have VROps running, go to your DRS cluster and click edit settings and enable the “Predictive DRS” option. Easy right? (See screenshot below) You can also change that look ahead value by the way, I wouldn’t recommend it though but if you like you can add an advanced setting called ProactiveDrsLookaheadIntervalSecs.

One of the other features that people have asked about is the consideration of additional metrics during placement/load balancing. This is what Network-Aware DRS brings. Within Network IO Control (v3) it is possible to set a reservation for a VM in terms of network bandwidth and have DRS consider this. This was introduced in vSphere 6.0 and now with 6.5 has been improved. With 6.5 DRS also takes physical NIC utilization in to consideration, when a host has higher than 80% network utilization it will consider this host to be saturated and not consider placing new VMs.

And lastly, DRS Profiles. So what are these? In the past we’ve seen many new advanced settings introduced which allowed you to tweak the way DRS balanced your cluster. In 6.5 several additional options have been added to the UI to make it easier for you to tweak DRS balancing, if and when needed that is as I would expect that for the majority of DRS users this would not be the case. Lets look at each of the new options:

So there are 3 options here:

  • VM Distribution
  • Memory Metric for Load Balancing
  • CPU Over-Commitment

If you look at the description then I think they make a lot of sense. Especially the first two options are options I get asked about every once in a while. Some people prefer to have a more equally balanced cluster in terms of number of VMs per host, which can be done by enable “VM Distribution”. And for those who much rather load balance on “consumed” vs “active” memory you can also enable this. Now the “consumed” vs “active” is almost a religious debate, personally I don’t see too much value, especially not in a world where memory pages are zeroed when a VM boots and consumed is always high for all VMs, but nevertheless if you prefer you can balance on consumed instead. Last is the CPU Over-Commitment, this is one that could be useful when you want to limit the number of vCPUs per pCPU, apparently this is something that many VDI customers have asked for.

I hope that was useful, we are aiming to update the vSphere Clustering Deepdive at some point as well to include some of these details…

HA/DRS configuration with Virtual SAN Stretched Cluster environment

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This question is going to come sooner or later, how do I configure HA/DRS when I am running a Virtual SAN Stretched cluster configuration. I described some of the basics of Virtual SAN stretched clustering in a what’s new for 6.1 post already, if you haven’t read it then I urge you to do so first. There are a couple of key things to know, first of all the latency between data sites that can be tolerated is 5ms and to the witness location ~100ms.

If you look at the picture you below you can imagine that when a VM sits in Fault Domain A and is reading from Fault Domain B that it could incur a latency of 5ms for each read IO. From a performance perspective we would like to avoid this 5ms latency, so for stretched clusters we introduce the concept of read locality. We don’t have this in a non-stretched environment, as there the latency is microseconds and not miliseconds. Now this “read locality” is something we need to take in to consideration when we configure HA and DRS.

[Read more…] about HA/DRS configuration with Virtual SAN Stretched Cluster environment

Using VM-Host rules without DRS enabled

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This week I was playing with the VM-Host rules in my environment. In this particular environment I had DRS disabled and I noticed some strange things when I created the VM-Host rules. I figured it should all work in a normal way as I was always told that VM/Host rules can be configured without DRS being enabled. And from a “configuration” perspective that is correct. However there is a big caveat here, and lets look at the two options you have when creating a rule namely “should” and “must”.

When using a VM-Host “must” rule when DRS is disabled it all works as expected. When you have the rule defined then you cannot place the VM on a host which is not within the VM-Host group. You cannot power it on on those hosts, no vMotion and HA will not place the VM there either after a failure. Everything as expected.

In the case of a VM-Host “should” rule when DRS is disabled this is different! When you have a should rule defined and DRS is disabled then vCenter will allow you to power on a VM on a host which is not part of the rule. HA will restart VMs on hosts as well which are not part of the rule, and you can migrate a VM to one of those hosts. All of this without a warning that the host is not in the rule and that you are violating the rule. Even after explicitly defining an alarm I don’t see anything triggered. The alarm by the way is called “VM is violating a DRS VM-Host affinity rule”.

I reached out to the HA/DRS engineering team and asked them why that is. It appears the logic for the “should” rule, in contrary to the “must rule, is handled by DRS. This includes the alerting. It makes sense to a certain extent, but it wasn’t what I expected.  So be warned, if you don’t have DRS enabled, “VM-Host should rules” will not work. Must rules however will work perfectly fine. (Yes, I’ve asked them to look in to this and fix it so it behaves as you would expect it to behave and come with a warning when you try anything that violates a should rule.)

 

ForceAffinePowerOn what is it?

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I’ve seen a lot of confusion around the ForceAffinePowerOn setting, and even the VMware documentation is incorrect around what this feature is / does. First and foremost: ForceAffinePowerOn is an advanced DRS setting (Yes I filed a doc bug for it). I’ve seen many people stating it is an HA setting, but it is not. You need to configure this in the advanced settings section of your DRS configuration.

Secondly, ForceAffinePowerOn can be used to ensure VM to VM affinity rules are respected when powering on a VM. ForceAffinePowerOn has absolutely nothing to do with VM to VM anti-affinity rules, it only applies to “affinity”.

Lets be crystal clear:

  • When ForceAffinePowerOn is set to 0, it means that VM to VM affinity can be dropped if necessary to power on a VM.
  • When ForceAffinePowerOn is set to 1, it means that VM to VM affinity should not be dropped and power-on should fail if the rule cannot be respected.

I hope that helps!

DRS rules still active when DRS disabled?

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I just received a question around DRS rules and why they are still active when DRS is disabled. I was under the impression this was something I already blogged about, but I cannot find it. I know some others did, but they reported this behaviour as a bug… which it isn’t actually.

Below is a screenshot of the VM/Host Rules screen for vSphere 6.0, it allows you to create rules for clusters… Now note I said “clusters” not DRS in specific. In 6.0 the wording in the UI has changed to align with the functionality vSphere offers. These are not DRS rules, but rather cluster rules. Whether you use HA or DRS, these rules can be used when either of the two is configured.

Note that not all types of rules will automatically be respected by vSphere HA. One thing which you can now also do in the UI is specify if HA should ignore or respect rules, very useful if you ask me and makes life a bit easier: