A resource pool is a logical abstraction for flexible management of resources. Resource pools can be grouped into hierarchies and used to hierarchically partition available CPU and memory resources.
Each standalone host and each DRS cluster has an (invisible) root resource pool that groups the resources of that host or cluster. The root resource pool does not appear because the resources of the host (or cluster) and the root resource pool are always the same.
Users can create child resource pools of the root resource pool or of any user-created child resource pool. Each child resource pool owns some of the parent’s resources and can, in turn, have a hierarchy of child resource pools to represent successively smaller units of computational capability.
A resource pool can contain child resource pools, virtual machines, or both. You can create a hierarchy of shared resources. The resource pools at a higher level are called parent resource pools. Resource pools and virtual machines that are at the same level are called siblings. The cluster itself represents the root resource pool. If you do not create child resource pools, only the root resource pools exist.
Resource Pool - Shares, Limits and Reservations
Resource Pool provides better control of cluster resource by using Shares, Limits, and Reservations on CPU and RAM usage. Let's explain these parameters:
Shares
Limits
Reservations
Shares
Shares specify the relative priority or importance of a virtual machine (or resource pool). For example, if a virtual machine has triple as many shares of a resource as another virtual machine, it is entitled to consume triple as much of that resource when these two virtual machines are competing for resources. Shares are typically specified as High, Normal, or Low and these values specify share values with a 4:2:1 ratio, respectively. You can also specify Custom as well.
Shares only come into play when there is resource contention (Memory or CPU).
Limits
Limits specify the maximum resource usage. A server can allocate more than the reservation to a virtual machine, but never allocates more than the limit, even if there are unused resources on the system. Assigning a limit is useful if you start with a small number of virtual machines and want to manage user expectations. However, you could waste idle resources if you specify a limit.
Reservations
Reservations establish a minimum guarantee of resource usage. For example, if you have fours VMs belonging to Resource Pool with RAM reservation set to 4 GB, the resource pool guarantees the concurrent RAM usage of all VMs in the pool to a minimum of 4 GB jointly. It means that if VM1 is using only 0.5GB, VM2 and VM3 is using 2GB (1 GB each), VM4 can use 1.5GB.
Shares, Reservations, and Limits can also be set on a per-VM basis.
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