ECC RAM is worth it for a Proxmox host with 5 VMs.

Yes. Periodic memtest isn’t a substitute.

A memtest only catches errors that already exist in your RAM. It doesn’t prevent new ones. The moment you boot back into Proxmox, a cosmic ray or voltage fluctuation can flip a bit, and that bit flip might silently corrupt a database, crash a VM, or brick your ZFS pool. With 5 VMs running real workloads, the odds of encountering a single-bit error over months of uptime are non-trivial.

ECC catches those flips in real time. It corrects single-bit errors and alerts you to double-bit errors. That means your VMs keep running, your data stays consistent, and you don’t wake up to a kernel panic at 3 AM.

Non-ECC + periodic memtest is fine for a desktop where a crash is an inconvenience. For a host running 5 VMs, the extra cost of ECC (usually $20-50 more per stick) is cheap insurance against unexplained downtime and silent corruption.

If your motherboard and CPU support it, just get ECC.