Dell Storage with Microsoft Storage Spaces Best Practices Guide

The host OS must have sufficient memory to provide services such as I/O virtualization, VM snapshot, and management to support the child partitions. The host reserves a minimal amount of memory, called root reserve, which cannot be allocated to VMs. In general, this minimal amount is often too low. This lack of memory invites the risk of the VMs starving the host during periods of high activity, resulting in the host OS running poorly and impacting the Microsoft Storage Spaces and Hyper-V management functions.

Dell recommends that you reserve at least 8 GB of memory for the host OS on each compute node, by not allocating this memory to VMs.

Plan your Converged solution so that when VMs failover to the remaining nodes during a compute node failure, sufficient memory is available to manage the additional VM load. Dell recommends that the amount of memory specified for all VMs hosted by the compute cluster not exceed the memory available for each node multiplied by the number of nodes minus one. This recommendation maintains the guaranteed optimal performance levels in the event of a single compute node failure. This is shown in the following equation:

Total amount of Memory available for VMs=(Memory available Per Node – 8 GB)*(Total # of nodes–1)

For example, in a two-node Converged cluster with 128 GB of memory on each node, and 8 GB of RAM reserved for the host, the total memory available to be allocated to all VMs in the solution should not exceed (128–8)*(2–1) = 120 GB of memory. If each VM was configured to use 2 GB of RAM the solution could support up to 60 total VMs, or 30 VMs per node.

In another example, a three-node Converged cluster with 256 GB of memory on each node allows for 496 GB of memory that can be allocated to all VMs hosted by the compute cluster. In this example, if one node fails, then the 248 GB on each of the two remaining nodes is sufficient to provide the allocated memory to VMs and still have 8 GB in reserve for each of the node’s host OS.