In this video, Russ Long discusses in depth the different ways DRS initially places a virtual machine. Learn what happens when we power on a VM within the DRS cluster.
- [Narrator] When a DRS cluster is enabled, any power-on attempt triggers a recommendation by DRS as to where the virtual machine, or machines, should be placed within the cluster. Now, before we go further, I want to clear something up about admission control. There are three types of admission control. There's the host version, resource pools, and HA admission control. They are different. DRS performs a host admission control check when a VM is powered on.
This is to ensure that there enough resources available to provide to the virtual machine. This check is performed by our vCenter server. If there are not enough resources to power on a virtual machine, a failure notification is given and the VM is not powered on. DRS recommendations are configuration-based. We will have different, and sometime varying, recommendations based upon how we configure DRS.
For initial placement, we have two important options, or configurations, to consider. First, when automated, DRS will place the VM within the cluster without any manual action. Basically, this a hands-free VM deployment to a host that DRS deems would lead to the most balanced resource utilization. A manual implementation will recommend a VM's placement, but require you, as an administrator, to step in to accept or override DRS's choice.
When powering on a VM outside of the DRS cluster, no recommendations are given even when that VM is powered on at the same time as other VMs that are in the DRS cluster. So, let's talk about powering on a single VM. When we power on a single VM, we are going to receive our placement recommendation, or if it's automated, it'll just place it. But, if we're in manual, we'll receive the recommendation which tells us which host the VM should be placed on.
Now, that's if everything goes right. If there is something that needs to be done, your cluster will inform you by giving you a prerequisite list. This is a list of actions that DRS thinks are necessary in order to power on a virtual machine. This could be things such as take a host out of standby mode or migrate virtual machines from one host to another host. This will often be multiple lines of actions required and you will be forced to accept all those prerequisite actions or cancel powering on the virtual machine.
Now, things become a little more complicated when we attempt to power on multiple virtual machines at the same time. They don't even all have to be in the same cluster to power on together, just the same data center. We could even power on VMs outside of the DRS cluster along with VMs that are within the DRS cluster, but we need to note that when we do this, recommendations will only be issued for those VMs that are participating within the DRS cluster.
So, when we power on multiple VMs in two separate DRS clusters at the same time, notifications such as power-ons and failures will be issued per cluster. Any DRS clusters that are not automated will request administrator acceptance for the placement recommendations. A single recommendation per cluster will be given for all the VMs contained. Now, non-clustered VMs that are included will have the power-on results listed in the started or failed VM Power-Ons tab.
- An overview of DRS
- Migrating virtual machines and thresholds
- Configuring DRS
- Creating and editing clusters
- Working with Storage DRS cluster features
- Managing DRS cluster resources
- Enabling Distributed Power Management
- Troubleshooting DRS