# Details of the cluster overcommit algorithm in System Center 2012 R2 Virtual Machine Manager

### Overview of the approaches in the algorithm

**OK**. Otherwise, the cluster state is set to

**Overcommitted**.

The four approaches can be visualized in a table as follows:

Check methods | Proof method | Slot method |
---|---|---|

Simple check | Proof-simple | Slot-simple |

Full complexity check | Proof-full | Slot-full |

Note The full complexity check is only a marginal refinement over the simple check, and the simple proof check offers very similar results.

### Calculations and algorithms

### Value definitions and precalculations in the algorithm

Important The AdditionalMemory and AvailableSlots values cannot be calculated by using the values from a single host. The value of LargestClusterVM or SlotSize from the source failing hosts must be known. In a simple check, they are equal to the largest virtual machine in the cluster. In a full complexity check, they are equal to the largest HA virtual machine in the set of failing hosts. Some hosts will fail, and other hosts will receive that workload. The calculations of available space are incorrect unless they were completed for the failing hosts and not the receiving host.#### Cluster values

The follow table shows the definitions of cluster values:Value name | Definition |
---|---|

N | The total number of hosts in the cluster |

R | The cluster reserve value (that is, the maximum number of concurrent failures to model) |

H | The remaining healthy hosts to be used as targets for failover (H = N-R) |

#### Host values

The following values are precalculated for each host. When a value is calculated for the LargestClusterVMMB or SlotSizeMB, it is recalculated in each iteration of full complexity checks.Value name | Definition |
---|---|

AvailableMemory | This is the total usable host memory for failed-over virtual machines to use. AvailableMemory = Host Total Memory - Existing VMs - Host Reserve |

AdditionalMemory | This is the fill line after which a host will no longer be able to start the largest virtual machine that is failing over. AdditionalMemory = Max(AvailableMemory - LargestClusterVM,0) |

HAVMs | This is the total of high availability (HA) virtual machines on this host. |

AvailableSlots | This is the number of failing over virtual machines that this host is guaranteed to be able to start. AvailableSlots = AvailableMemory / SlotSize, rounded down |

UsedSlots | This is the number of HA virtual machines on this host. |

TotalSlots | Total number of slots on a host. TotalSlots = AvailableSlots + UsedSlots |

Notes

- A 64 megabyte (MB) buffer is added to each virtual machine's memory to account for hypervisor overhead.
- Stopped, saved state, paused, and running virtual machines (VMs) are all counted. A tenant user who is starting a stopped virtual machine should be accounted for when the algorithm calculates overcommits.
- If dynamic memory virtual machines are present in the cluster, their current memory demand is used.

### Algorithms in the four approaches

#### Slot-simple

The algorithms in the slot-simple approach are as follows:- SlotSize = largest HA virtual machine in the cluster.
- Calculate the AvailableSlots, UsedSlots, and TotalSlots values for each host.
- TotalSlotsRemaining = sum of the smallest H values of TotalSlots.
- If Sum(UsedSlots) <= TotalSlotsRemaining, cluster is not overcommitted.

#### Slot-full

Iterated over each set of R failing hosts. The algorithms in the slot-full approach are as follows:- SlotSize = largest HA virtual machines on the R failing hosts.
- Calculate the AvailableSlots, UsedSlots, and TotalSlots values for each host.
- TotalSlotsRemaining = sum of TotalSlots on all non-failing hosts.
- Compare the Sum(UsedSlots) and TotalSlotsRemaining values:
- If Sum(UsedSlots) > TotalSlotsRemaining, cluster may be overcommitted.
- If Sum(UsedSlots) <= TotalSlotsRemaining for every set of failing hosts, cluster is not overcommitted.

#### Proof-simple

The algorithms in the proof-simple approach are as follows:- LargestClusterVM = largest HA virtual machine in the cluster.
- Calculate AdditionalMemory, HAVMs for all hosts.
- TotalAdditionalSpace = sum of smallest H values of AdditionalMemory.
- TotalOrphanedVMs = (sum of largest R values of HAVMs) - LargestClusterVM.
- Compare the values:
- If TotalOrphanedVMs <= TotalAdditionalSpace, cluster is not overcommitted.
- If TotalOrphanedVMs is 0, LargestClusterVM > 0 and TotalAdditionalSpace = 0, cluster may be overcommitted.

#### Proof-full

Iterated over each set of R failing hosts. The algorithms in the proof-full approach are as follows:- LargestClusterVM = largest HA virtual machine on the R failing hosts.
- Calculate AdditionalMemory, HAVMs for all hosts.
- TotalAdditionalSpace = sum of AdditionalMemory on non-failing hosts.
- TotalOrphanedVMs = (sum of HAVMsMB on the R failing hosts) - LargestClusterVM.
- Compare the values:
- If TotalOrphanedVMs > TotalAdditionalSpace, cluster may be overcommitted.
- If TotalOrphanedVMs = 0, LargestClusterVM > 0 and TotalAdditionalSpaceMB = 0, cluster may be overcommitted.
- If TotalOrphanedVMs < TotalAdditionalSpace for every set of failing hosts, cluster is not overcommitted.

### Combining the approaches and example

**OK**, and calculation is stopped immediately.

However, for the full complexity analysis, if even a single set of R failing hosts shows that the cluster may be overcommitted, the method is immediately completed and does not flag the cluster as

**OK**.

##### Properties

Article ID: 3023928 - Last Review: Feb 12, 2015 - Revision: 1

Microsoft System Center 2012 R2 Virtual Machine Manager