When implementing the Cisco Nexus 1000v, High Availability (HA) Slot Sizes can be affected on your vSphere Cluster. HA slot sizes are used to calculate failover capacity for HA when the “Host Failures” HA setting is used. By default the slot size for CPU is the highest reservation of CPU among all VM’s in the cluster (or 256 MHz if no per VM reservations exist). The slot size for Memory is the highest reservation of Memory among all VM’s in the cluster (or 0 MB + Memory Overhead if no per VM reservations exist). If you want to really dive further into HA and slot size calculations, I would highly recommend reading Duncan Epping’s HA Deepdive at Yellow-Bricks.com.

Each Cisco Nexus 1000v Virtual Supervisor Module (VSM) has a reservation of 1500 MHz on the CPU and 2048 MB on the Memory so that these resources can be guaranteed for the VSM’s. So, the slot size will be affected accordingly (1500 MHz for CPU and 2048 MB for Memory).

If you do not want your slot size affected and you do not want to enable the advanced slot size parameters das.slotCpuInMHz or das.slotMemInMB there is another solution that will allow you to still guarantee resources for the Nexus 1000v VSM’s and maintain smaller slot sizes.

Create a separate resource pool for each VSM with a CPU reservation of 1500 MHz and Memory reservation of 2048 MB.

Since slot sizes are only affected by per VM reservations, the resource pools will give you the desired results without increasing your HA slot sizes. I wouldn’t recommend doing this with too many VM’s as this turn into a management nightmare very quickly. However, for the two VSM’s in a Cisco Nexus 1000v infrastructure it is manageable.

Now your Nexus 1000v VSM’s are happy and you are not wasting resources in your HA cluster by calculating larger slot sizes than are required.

References

http://www.yellow-bricks.com/vmware-high-availability-deepdiv/

http://frankdenneman.nl/2010/02/resource-pools-and-avoiding-ha-slot-sizing/