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Sizing a virtualization server: processor, memory and storage

List your virtual machines and containers, starting from common templates (domain controller, Docker host, Home Assistant, Jellyfin, TrueNAS, firewall…): the tool calculates the cores, memory (ZFS cache included) and storage you need with a margin, and flags the classic traps.

Virtualization Tool Intermediate Published October 21, 2026 Translated October 27, 2026 5 min read Version française
Close-up of a RAM module
Photo: Liam Briese on Unsplash

In plain words

A virtualization server is a building divided into flats. Each virtual machine claims its rooms (processor), its furniture (memory) and its cellar (storage). Rooms can be shared, because not everyone is at home at the same time; furniture cannot. The tool adds up each tenant's needs, adds the building's common areas and a margin for future arrivals.

The three resources are not shared the same way

Resource Shared between machines Consequence
Processor Yes: machines do not all compute at the same time You can assign more vCPUs than there are threads, within reason (2 to 4 vCPUs per thread)
Memory Hardly: a machine keeps the memory it was given Add up the memory of all the machines, plus the host's and the ZFS cache
Storage Yes with thin provisioning, but the disk eventually fills up Plan for the full size of the disks, plus snapshots and growth

The most common mistakes

  • Too many vCPUs per machine. An 8-vCPU machine that only uses one still has a cost: the guest system manages all its processors, the vCPUs must stay in step, and on a busy host the machine waits for its turn more often (waiting time visible as steal on Linux or ready on VMware). Start small (1 or 2 vCPUs) and increase if monitoring shows a saturated processor.
  • Forgetting ZFS memory. ZFS uses memory as a read cache. The Proxmox VE documentation recommends at least 2 GiB base plus 1 GiB per TiB of storage. On a 32 GB server with 8 TB of ZFS, that is already 10 GB less for the machines. Since Proxmox VE 8.1, the installer limits this cache to 10% of memory by default (16 GiB at most); on an older installation, set the limit yourself.
  • A system disk that is too small. ISO images, templates and logs pile up: 128 GB is a comfortable minimum for the host.
  • Backing up to the same server. A backup stored on the server it protects disappears with it. See the 3-2-1 backup rule.
  • Hardware RAID under ZFS. ZFS must see the disks directly: connect them to the motherboard's SATA ports or to a controller in IT mode (HBA).

For raw capacity by RAID type, use the RAID and ZFS calculator. To choose the hypervisor, see Choosing your hypervisor.

Sources

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