Shulou(Shulou.com)06/03 Report--

The amount of physical memory not reserved for VM virtual machines in the Hyper-V cluster is used by the host, but as part of the cluster, the host usually receives VM from other nodes in the cluster to maintain its high availability, which can be user-initiated movements, such as real-time migration of VM from another node, or VM failover caused by a resource or hardware failure Therefore, initially the user does not have much control over the memory reserved by the host, and the VM from another node can easily move to a specific node and overwhelm its memory, so the cluster environment variable RootMemoryReserved is introduced to ensure that the cluster host has a minimum host physical memory reserve.

The RootMemoryReserved attribute was first introduced in 2008R2, existed in WSFC 2008R2 2012 2012R2, and disappeared in 2016

RootMemoryReserved

Despite the name, the RootMemoryReserved variable does not guarantee that the root partition will reserve a certain amount of physical memory for itself, but rather specifies the amount of memory that the host operating system will compare with the VM that the host operating system is about to start (moved to the node through user action or failover). By starting VM, the remaining physical memory of the host OS will be lower than the limit specified by RootMemoryReserved and the memory currently used by the host Then the VM startup operation will fail.

For example, on a cluster node with 16 GB physical memory and RootMemoryReserved set to 1024 MB (1 GB), the host is currently using 1 GB memory, and if each VM occupies 1 GB memory, the maximum number of virtual machines online is 14 virtual machines, because the host operating system already uses 1 GB of memory, while the host operating system retains another 1 GB of memory. This reserves 2 GB of memory for the host, and attempts to start the 15th virtual machine so that the physical memory usage of the virtual machine exceeds 15 GB, which causes the physical memory retention time of the host operating system to be lower than the 1 GB currently used by the host operating system. Therefore, the VM startup operation for the 15th VM will fail.

RootMemoryReserved is set to 512 MB by default, which should be sufficient for host virtual machines that do nothing but manage virtual machines, and this variable can be viewed by PowerShell cmdlet.

(get-cluster). RootMemoryReserved

To change the RootMemoryReserved, set the RootMemoryReserved to 1024 MB using the following PowerShell cmdlet:

(get-cluster) .RootMemoryReserved = 1024

Changing RootMemoryReserved will not affect any virtual machines that are already running. For example, on a node with 16 GB physical memory, if RootMemoryReserved is set to 512MB, the host uses 480MB at that time, and VM occupies 15 GB of memory, which will be feasible. The host gets the available memory of 512MB+480MB and does not reclaim memory from VM.

In addition, if the system slows down for some reason (such as another application running on the host), changing RootMemoryReserved to 2048 MB (2 GB) will not automatically free the host's physical memory, in which case, the way to free the host's physical memory is to offline the unimportant virtual machine, so it is recommended that you correctly set up the required RootMemoryReserved before bringing any virtual machine online

The maximum value of RootMemoryReserved is 4096 MB (4 GB), and any changes to values higher than 4 GB are ignored and the previous values are used. In addition, RootMemoryReserved as a cluster parameter applies to all nodes in the cluster, and the values in RootMemoryReserved are used to retain VM host memory on all nodes in the cluster.

The RootMemoryReserved variable does not limit the amount of memory a host can use. The purpose of this variable is to ensure that the host has a minimum physical memory reserve, which exceeds the amount currently used to control virtual machines. The host can use more memory than the value set aside by RootMemoryReserved. As a result, the amount of physical memory available to VM will be equal to or less than the amount of memory not reserved by RootMemoryReserved.

The memory currently being used by hosts that have not been reserved can be squeezed by VM over time. Setting RootMemoryReserved will ensure that in addition to the memory currently in use, the host will have part of the reserved memory that will never be occupied by VM and will only be used by the host.

Lao Wang's comments: RootMemoryReserved can be used to reserve host memory reasonably for cluster nodes in the 2008R2-2012R2 era, to prevent hosts from being completely flooded by VM or applications, and to ensure host performance. In the absence of SCVMM, host memory reservation can be natively controlled. If there is SCVMM, it is more intelligent. SCVMM can be set not only for the cluster, but also for a single host or the entire host group. The host reservation of SCVMM can be set from CPU, memory, disk space, disk ID O, and network capacity.

The function of host reservation and dynamic resource balance can be matched. When starting or moving a virtual machine, the host reservation takes effect. Once it is detected that the minimum reservation value is exceeded, the virtual machine is prevented from moving or starting. The dynamic resource balancing function is to set a balance value. When a host group or cluster is running, once it is detected that the load of a host exceeds the balance value, it will balance resources with other nodes with light load. The two do not conflict.

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