Don’t add resource pools for fun, they’re dangerous

In this article I will explain the hidden dangers when adding a resource pool with default settings to a DRS cluster, but first off all kudos to my famous virtualization friends Duncan Epping, Frank Denneman and Bouke Groenescheij. They already have written several great articles about resource pools, shares and reservations. You could ask yourself is there anymore to say about using resource pools? Well there is. I often see production environments where people have added two or three resource pools just for fun saying; there’s no reservation or limit on it so it doesn’t matter. Well they’re just plain wrong! On top of that, they even use resource pools as folders to group their virtual machines. Let’s look at an example.

In my home environment I’ve created two resource pools and one vApp. I didn’t adjust the settings and left everything default. I’ve added 8 virtual machines to recourse pool “test 1”. The two resource pools and the vApp are sibling with three virtual machines in the root resource pool. Take a close look at the ”% Shares” column in the “Resource allocation tab”. The sibling virtual machines will get 18% of the resources when there’s contention. The vApp will get 26%, so that’s 13% for vCenter and 13% for SQL2008. The virtual machines within resource pool “test 1” will have to divide 26% among 8 virtual machines so that’s 3,25% per virtual machine. The sibling root resource pool virtual machines will get 6% each. So don’t use resource pools for cosmetic reasons, you better use virtual machine folders instead. As long as there’s no contention, there’s no problem. But as soon as you reach the max the resources might be divided in a way you didn’t expect.

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Tracked: Jun 04, 03:42

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Questions: If a VM that is not running doesn't use resources, does the same apply to shares etc. For instance, if I have a lot of test VMs in my environment (which I do) is it better not to have them registered with vCenter until I need them? Do they occupy resources simply by being included in available machines? Is there a way to make them available to be turned on when needed, yet remain completely off the resource grid when not in use? Could you clarify that?

#1 HughBorg707 (Homepage) on 2010-06-04 00:22 (Reply)

Resource Pools have become a hot topic due to the vSphere 4 Design class. This class apparently was codesigned with some VCDXs. It became clear during the design that even VCDXs had misconceptions about how RPs actually worked. This misconception has actually led to the coursework explicitly calling out RPs as something to be careful of, or even just flat our avoid. The rec is to use shares on VMs. If you have High (8000), Normal (4000), and Low (2000) RPs for the purpose of controlling shares and they each have 4 VMs in them, then the RPs will work the way most of us thought they would work. However, if you move 4 of the VMs to the High then you have 8H, 2N, and 2L. When there is contention the shares will look like this: H - 8000 shares / 8 VMs = 1000 shares per VM N - 4000 shares / 2 VMs = 2000 shares per VM L - 2000 shares / 2 VMs = 1000 shares per VM In this scenario your High RP VMs are acutally getting less then the Normal VMs and equal to the L VMs. So basically the only way for the RPs to work the way that WE want them to is to maintain a balanced # of VMs per RP or if they are unbalanced make sure that the lower tiered RPs contain more VMs than the higher tiers. Remmember shares only come into play during times of resource contention. So if you have no contention then the RPs are used for anything but organization. -MattG

#2 MattG on 2010-06-05 20:42 (Reply)

Aren't shares set on individual VMs cleared after a vMotion? In my tests they get reset because the porportions change. If you give a VM 1000 shares on host 1, and it vmotions to host 2, the shares on the hosts may add up differently and you could go from 2:1 to 1:2 just based on the number of VMs. What I fail to understand about VMware's implementation of resource pools is why you MUST dictate a share count. This forces me to take into account the shares involved across the board and removes a lot of the "dynamic" ability of DRS and just general usage. If I move a VM to a different resource pool or even add a new VM I have to figure out how this changes the whole scheme. In fact if I used low shares numbers (like the default values) then the addition/subtraction of a single VM could make a big difference. So if I want to create resource pools it is in my best interest to create them with a very high share count so a single addition or subtraction doesn't change the overall intended ratios by very much. I think an additional option when creating a resource pool/vApp should be avaiable that allows the division of shares to be divied from the parent without needing to add the additional level of concern. That way I can use all the benefits of a vApp without worrying about my 4 VMs getting screwed because I only have 4000 shares in times of contention.

#2.1 brandon rice on 2010-09-08 23:25 (Reply)

So looking at that picture this issue will also happen with vApps? would we have any control over this? or am I reading this wrong

#3 Simon Wilmann on 2010-09-09 06:17 (Reply)

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NTPRO.NL - Eric Sloof

Thursday, 3 June 2010

Don’t add resource pools for fun, they’re dangerous

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