How the VMware vSphere calculator works

01 Memory, reservation, limit and shares

• Reservation is the guaranteed minimum allocation, in megabytes. A VM whose reservation can't be met doesn't power on, and a reservation can't be larger than the VM's configured memory. vSphere docs
• Limit is the upper bound, in megabytes. With no limit, the VM's configured memory is its effective limit. vSphere docs In the API, a limit of −1 means exactly that. vSphere docs
• Shares set a VM's priority against the others when memory is contended: High, Normal and Low are 20, 10 and 5 shares per megabyte of configured memory. vSphere docs

02 VMware's advice, and how the calculator follows it

VMware's resource-management guidance: vSphere docs

• A limit can waste idle memory: the VM can't use more than the limit even when the host has plenty.
• Reserve the minimum acceptable amount of memory, not the amount you'd like to have available.
• Leave at least 10% of the host unreserved, so later changes still fit.

So the calculator sets no limit and sizes the VM's configured memory from the peak instead: that becomes the cap. The reservation comes from the average plus its margin, the memory the VM needs under normal load, which is how the calculator reads "minimum acceptable". Assumption

03 From usage to memory and reservation

reservation = average × (1 + request margin)                  rounded up to a whole MB
memory      = max(peak × (1 + limit margin), reservation)     rounded up to a multiple of 4 MB
limit       = Unlimited (−1)
shares      = Normal

The margins come from the sensitivity, workload type and environment you choose; the sizing model explains them, and they are assumptions too. vSphere's MB are taken as MiB, so values pass through unconverted. Assumption A reservation can't exceed the configured memory, so when the peak is so close to the average that the peak-based memory falls below the reservation, the memory is raised to it. The memory size must be a multiple of 4 MB, so it is rounded up to the next one; the reservation takes any whole MB. vSphere docs

Worked example

Average 390 MiB, peak 700 MiB, generic workload, medium sensitivity, production:

A steady cache, averaging 1000 MiB with a 1020 MiB peak, where the reservation decides the memory:

04 What the calculator writes

govc vm.change -vm "<vm-name>" -m 912 -mem.reservation 507 -mem.limit -1 -mem.shares normal

• govc vm.change takes the memory size with -m, and the reservation and limit with -mem.reservation and -mem.limit, all in MB; shares as a level or a number. govc docs The same values go in the vSphere Client under Edit Settings → Virtual Hardware → Memory.
• Memory hot add lets you add memory to a powered-on VM; turning it on needs the VM powered off. vSphere docs Lowering a VM's memory isn't covered by hot add, so it needs the VM off. Assumption
• Shares stay at Normal: the calculator has no way to know how this VM should rank against its neighbours, and doesn't claim Normal is vSphere's default. Assumption

05 Every warning, and why

06 What to paste

node_memory_MemTotal_bytes{instance="vm1:9100"}
  - node_memory_MemAvailable_bytes{instance="vm1:9100"}

• Measured inside the guest with node_exporter: total memory minus MemAvailable, the kernel's estimate of memory available for new applications without swapping. Kernel docs
• The calculator asks for guest-side numbers rather than vSphere's own "active" or "consumed" memory, which describe the host's view of the VM rather than what the guest needs. Assumption

The Proxmox VE guide sizes a memory value and a minimum memory from the same guest-side measurement.

07 All assumptions in one place

Besides the margins of the shared sizing model:

Our default: a value we chose. Our reading: how we read documentation that isn't explicit. About your setup: something we take for granted about your data or cluster.

Values you can change

The calculator’s Advanced: margins and defaults section, closed by default, lets you replace these values when your setup differs. Each field’s “?” says how it moves the result; a value outside the range is used at its nearest end, and “Reset to defaults” puts them all back.

08 References

• Configure resource allocation settings — reservation, limit, and VMware's advice on both.
• Allocate memory resources — units and the reservation's limits.
• Resource allocation constraints (SDK) — −1 for no limit.
• Resource allocation shares — High, Normal, Low.
• Change memory hot add settings — adding memory to a running VM.
• govc USAGE — vm.change flags.
• The /proc filesystem (kernel) — MemTotal and MemAvailable.

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