Comments on: Using limits instead of downscaling….

By: Duncan

Duncan — Thu, 01 Oct 2009 03:41:40 +0000

It would not affect performance until for one reason or the other the memory gets increased and the limit stays at 4GB. That’s why I also never set limits to the same amount of memory as provisioned. There’s no point to do this.

By: Nil Paul

Nil Paul — Thu, 01 Oct 2009 01:48:32 +0000

What would happen if you have set the limit same as the memory being allocated to the VM, eg. 4gb VM has a limit of also 4GB? Would that affect performance. Going via what Scott at VMguru says, I would think not?

By: Duncan Epping

Duncan Epping — Tue, 29 Sep 2009 12:11:49 +0000

Euuuh Admission Control is not affected by limits, it’s affected by reservations.

By: Aubrey Williams

Aubrey Williams — Tue, 29 Sep 2009 09:52:01 +0000

Limits are dangerous when one lacks the understanding of how and when to apply them. If you have a VM that, by analysis of historical data of a month or more, never uses more than 2GB, it’s wise to set a memory limit to 3GB. Limits, used wisely makes more resources available for not only future VM deployments, but for current VMs with variable performance characteristics.

Keep in mind that components like DRS and admission control are impacted by limites as well. When used wisely, this impact is positive.

By: Paul Nothard

Paul Nothard — Mon, 28 Sep 2009 21:13:45 +0000

Limits can be a Machiavellian way for admins to prove a point where they have been unable to prove their point by giving the insistent developer team less memory. Instead of monitoring the obviously reduced memory over a period of time, they monitor the memory, which has been transparently limited, and present their findings to the developers at the end of the process. If the admin is correct, the limit will have had no impact on matters. It’s more politics than technology that drives this approach though.

Strongly agree that limits should be an exception rather than the norm. As you say, the OS paging algorithm is cognisant of what it’s pushing to disk, whereas ESX is not as aware.

By: Bouke Groenescheij

Bouke Groenescheij — Mon, 28 Sep 2009 07:38:21 +0000

I totally agree! When I was reading the title of this post I was thinking: ‘Oh no, do not use limits, please scale down!’, but after reading the full post I was totally relieved. Great article!

By: nate

nate — Fri, 25 Sep 2009 16:04:20 +0000

Does the balloon driver kick in when the memory limit is reached on a particular VM? I was under the impression at least that it only kicked in when there was memory pressure on the host itself. Also of course swapping has other downsides, if VMs are aggressively swapping they can degrade your storage performance by quite a bit impacting everyone else at the same time regardless of whether or not they are swapping too.

By: Matt

Matt — Fri, 25 Sep 2009 12:38:08 +0000

I agree that limits can be dangerous but I think they have their place. In situations where the server is at a lower tier level than normal production servers, imposing limits can be a helpful way to stack more servers per host. As long as the end user understands that performance may not be optimal, that is. Generally speaking this is good for dev/test environments where there isn’t a large budget for hardware/software and you want to utilize as many VMs per host as possible.

If I have to do this, I use Resource Pools with limits imposed rather than imposing the limits directly at the VM level. This way if one of those VMs needs the memory while the others do not it will have a better chance of accessing it.

Good article as always.