DRS - Host RAM Differences...

[rbmadison]

We have a cluster with hosts that have few different amounts of ram and I notice that DRS isn't balancing out the load correcty. I see hosts with ram warnings yet hosts with the extra barely ram being imacted at all. I have my DRS set to the middle to apply priority 1,2 and 3 automatically. Is there something else I need to do to get DRS to balance better with the differences in host ram?

[a_p_]

If DRS doesn't generate any recommendations (i.e. migrates VMs) there's usually no need to do that. DRS's responsibility is not balancing workloads, but rather migrate VMs in case of contention. So there's actually nothing you are doing wrong or what you should configure differently.

André

[rbmadison]

Thanks for the info but I am seeing host ram alerts on hosts with less ram but DRS isn't moving any VMs to the hosts with more RAM. Does this have something to do with the hosts having different RAM capacities and if so is there some way to get it to balance out? I would guess that if there is an alert about RAM usage on a host that it would vMotion a VM to another host with more available ram but doesnt seem to do this. Isn't an alert warrant a vMotion with DRS set to medium?

[a_p_]

DRS calculations take different factors into account and is very complex. An alarm does not necessarily require a VM migration. You may take a look at e.g http://www.yellow-bricks.com/drs-deepdive/ to get some information on what's taken into account.

André

[sparrowangelste]

have you tried fully agressive?

[rbmadison]

I took a look at the deep dive link and what seems strange to me is my DRS is set to medium yet it indicates load imbalanced? Shouldnt it be balanced? We have 33 hosts in our cluster. I dont want to move to agressive because I dont want vMotions to occur frequently, just when a host has a memory issue.

[weinstein5]

Keep in mind DRS is concerned about delivering resources to the VM so as long as there is not ceontention for memory or CPU the VM will not move even if the host is showing warning that it is almost out of memory - so it really does not balance the load -

[rbmadison]

I saw the issue again and ran esxtop on the hosts and things look good. I guess it's just something that is typical of DRS and alerts dont nessesarily mean there's contention. Thanks for the help!