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Robuea
Enthusiast
Enthusiast

Stretched-Clusters, VM->Host affinity and Resource Pool Question.

I wonder if anyone can help me on this one.

We've spent a long time planning the implementation of Resource-Pools, and have digested all there is to consume on the pitfalls and gotchas of the unintended consequences they bring.  We even wrote in-depth powershell scripts which query the VM's in the resource pools, calculate the number of vCPU's present and work out the correct ratio of shares to allocate to the pool to avoid this.  It all seems sound. (off topic - but looks like scalable shares may make all that redundant!!).

One thing I've never been able to solve though, is Resource-Pool management on stretched-cluster scenarios that use site-local vm-host affinity DRS rules.

e.g:

a cluster has 8 hosts over 2 sites - 4 hosts in each site.  DRS VM->Host affinity rules exist (using vm groups and host groups) to work around active-active/passive storage limitations.  i.e. vm's could run from non-site local storage, but there is a large performance penalty - the rules are therefore in place to keep VM's at their preferred site.  However, there is always the (likely) potential of an imbalance of VM's (more importantly, vCPU's) across sites.  Because of this, and that fact that shares are a percentage of the total cluster capacity - not site capacity, there is the potential for the number of shares allocated to a resource group to exceed the physical capacity of a site (i.e. 50% of the cluster).

Does anyone know how Resource Pool scheduling deals with this when it see's it?  It obviously cant allocate more than it has, but what happens to the other competing resource groups?