How does DRS load balance?

I have a mix of hosts in 1 cluster (EVC enabled) and see some unexpected loads on the hosts. I'd like to understand how DRS determines how to load balance across the hosts.

Here's my configuration:

3 hosts - Dell PE R710 - Dual quad core X5550 @ 2.67GHz - 64GB memory

2 hosts - Dell PE R900 - Quad quad core E7330 @ 2.4GHz - 64GB memory

Migration threshold currently set to Aggressive

EVC mode - Intel Xeon Core 2

Here's the current load on each host:

R710 - CPU 13% - Memory 79% - 27 VMs

R710 - CPU 18% - Memory 75% - 40 VMs

R710 - CPU 27% - Memory 68% - 30 VMs

R900 - CPU 21% - Memory 50% - 27 VMs

R900 - CPU 20% - Memory 62% - 22 VMs

It seems like the R710's are always loaded more (in regards to memory) than the R900's even though there's available memory and CPU capacity to be able to handle more VMs on the R900's.

Can someone explain why more VMs aren't being migrated to the R900's?

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2 Replies

if your set to fully automated with a 3 star level, then you are probably balanced out according to DRS. An easy way to check is look at your cluster and

VMware DRS Resource Distribution Charts, it will give you a good idea of how your load is balanced.

My guess, without seeing your environment is that the cluster is providing 90 -100+ percent of entitled resources.

If you have any DRS rules setup as well, that may be why you think your resources look a little unbalanced to you

Also, check out


Hot Shot
Hot Shot


The default three star DRS threshold seems to tolerate quite a lot of host load deviation before acting upon it and moving VM's and in most environments it's probably best to keep vMotions to a minimum. In your case it might be a matter of CPU load on the hosts affecting DRS decisions. I can recommend reading Duncan Epping's post on DRS and the math behind it: http://www.yellow-bricks.com/drs-deepdive/

Hope it helps!