drs

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

Duncan Epping · Oct 9, 2020 ·

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.

vSphere 7 and DRS Scalable Shares, how are they calculated?

Duncan Epping · Mar 16, 2020 ·

I wrote a post and recorded a short demo that explained this cool new feature called Scalable Shares, part of vSphere 7 / DRS, last week. I didn’t want to go too deep in the post, but now that I am getting more questions about how this actually works, I figured I would provide some examples to explain it. As mentioned in my previous post, Scalable Shares solves a problem many have been facing over the last decade or so, which is that DRS does not take the number of VMs in the pool into account when it comes to allocating resources. So as an example:

Just imagine you have a resource pool called “Test”, it is a resource pool with “normal” shares and has 4 VMs. Let’s say this resource pool has 4000 shares.

Now compare that resource pool to the “Production” pool, which has shares set to “high” but has 24 VMs. Let’s say this resource pool has 8000 shares.

This is a very extreme example, but it shows you immediately what the problem is. On a per VM basis, the VMs in the production resource pool, would receive far less resources when there’s contention as DRS would simply divvy up the resources based on the assignment of the shares of the resource pool. Test would receive 1/3 of the resource (4000 of 12000 total shares), and production would receive 2/3 of the resources (8000 of 12000 total shares). If you would divide that by the number of VMs in each pool, it is obvious that the VMs in Test are in a better situation than the VMs in Production.

So how would this work if you Scalable Shares enabled? Well, let’s list some facts first:

A resource pool looks like a VM with 4 vCPUs and 16GB of memory to DRS
Scalable Shares looks at the total amount of shares in the Resource Pool (all vCPUs!)
For a resource pool high is 8000 shares, normal is 4000 shares and low is 2000 shares
- Note, that this is based on 4 vCPUs, so the real values are 2000, 1000, 500.

The calculation would be as following:

Resource Pool Shares = (4 vCPU * Shares of Pool )* (Total number of shares of all vCPUs in resource pool)

So as an example, in the case I have Test with normal shares and 4 VMs, and Production with high shares and 24 VMs, and all VMs have a single vCPU with normal priority the calculation for those two resource pools would be:

Test = (4 * 1000) * (4 * 1000) = 16,000,000 shares

Production = (4 * 2000) * (24 * 1000) = 192,000,000 shares

In other words, Production has 12 times more the number of shares as Test has when Scalable Shares is enabled. I hope that clears things up!

Introducing Scalable Shares – vSphere 7

Duncan Epping · Mar 12, 2020 ·

Early 2015 Frank Denneman and I had a discussion during a flight to San Francisco. We came up with this concept for Resource Pools where the number of shares would be determined by the number of VMs and the priority of the pool. In other words, we wanted to avoid the dilution of shares in an environment with resource pools and basically solve the resource pool pie paradox problem described here. We worked with the DRS team on describing the concept, we filed a patent for it and got the patent granted in 2019. Today I am happy to share that the feature made it into a release and will be part of vSphere 7.0.

Scalable Shares is a feature that is part of DRS. You either enable it on the Cluster or you enable it on the Resource Pool. Personally I would always enable it on the Resource Pool level. So how does it work? Well normally when you create a resource pool with High Priority and one with Normal Priority, the RP with Priority High will have 8000 shares and the RP with Priority Normal will get 4000 shares. In other words, a 2:1 ratio between High and Normal. Now if you have 8 VMs in High and 1 in Normal, you can imagine that this single VM in Normal will get more resources than the 8 in High when there is contention simply as the 8 share the resources of the pool while the single VM doesn’t share the resources.

When Scalable Shares is enabled, DRS does a calculation using the ratio (4:2:1 – High:Normal:Low) and the number of VMs. In other words, 8 VMs with a 1000 shares in High would be *4, and 1 VM with a 1000 shares in Normal would be *2. The result being:

Normal = (1*1000)*2=2000
High = (8*1000)*4=32000

I hope that makes a bit sense? It took me a while to fully grasp. If you are wondering what this looks like in the product, and what the impact could/would be when you switch from “traditional” to “scalable shares”, I created a demo that shows this below.

VMworld Reveals: DRS 2.0 (#HBI2880BU)

Duncan Epping · Sep 3, 2019 ·

At VMworld, various cool new technologies were previewed. In this series of articles, I will write about some of those previewed technologies. Unfortunately, I can’t cover them all as there are simply too many. This article is about DRS 2.0, which was session HBI2880BU. For those who want to see the session, you can find it here. This session was presented by Adarsh Jagadeeshwaran and Sai Inabattini. Please note that this is a summary of a session which is discussing a Technical Preview, this feature/product may never be released, and this preview does not represent a commitment of any kind, and this feature (or it’s functionality) is subject to change. Now let’s dive into it, what is DRS 2.0 all about?

The session started with an intro, DRS was first introduced in 2006. Since then datacenters, and workloads (cloud-native architectures), have changed a lot. DRS, however, has remained largely the same over the past 10 years. What we need is a resource management engine which is more workload-centric than it is cluster-centric, that is why we are planning on introducing DRS 2.0

What has changed? In general, the changes can be placed in 3 categories:

New cost-benefit model
Support for new resources and devices
Faster and scalable

DRS Advanced Setting IsClusterManaged

Duncan Epping · May 7, 2019 ·

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…)