Okay so I have always loved the DRS rules, and find them so under utilised in almost every scenario.
So lets take this scenario on board.
1050 virtual servers spread across 30 vSphere 5.1 hosts on mixed vlans.
DRS Rules in place – none…?
Q. What would happen if we found the top talkers on the same vlans and placed them on the same hosts?
A. Well when two of the guests needed to talk to each other (provided they were on the same vlan) would talk through the virtual switch, whether standard or dv.
A. Yes really, unless they need to talk to another vlan (and of course users) they would not need to go over the physical network, physical switch.
Q. What are the implications of this?
A. Well for starters you are taking load off the physical switches (yes most physical switches are designed with capacity in mind and can handle the load easily) but we can still reduce that load so if on a particular day there was a high level of traffic (Backups, Vmotion, DRS actions for example) the physical switches would simply laugh at you saying is that all you got…
Q. So what you are saying is that if I have two sql servers configured with database mirroring then the network component will simply go over the virtual switch.
A. Yes, however in the scenario you describe you would have to put a lot of faith into vmware HA. This is not something I would advise, dont get me wrong HA is awesome if you lost a fan or anything on the physical host you would most likely drop at least one packet to each sql server and there are so many applications that cannot handle a drop out to their Database. But in theory yes, a better scenario is if you had an application server that fetched data from its sql server, if you put these on the same host you would reduce network traffic to the virtual switch only.
Q. That seems like a lot of effort, are there any tools to tell me what my top talkers are and to put them in groups for me?
A. Sadly there are a number of applications out there but they are high end and whilst they may tell you what the top talkers are they will not create the DRS groups for you.
Q. Is there a better way..
A. Yes there is, you can create a DRS group via powershell. So all we need to do is sort the guests by vlan or port group name and then add all those in the same port group to a DRS group labelled as the port group name. Then we create a rule “Virtual Machines to Hosts” , vlan 202 , should run on hosts in group , clu8ter host group name.
Q. Should run on hosts in group?
A. I try never to use the Must run on hosts in group as it restricts DRS options for moving machines around, and this type of rule applies even with DRS turned off, so it can be a bit dangerous in my opinion.