reference console kvm ram ballooning 20260715T110517

Console (Proxmox VM 103 on Finn) runs without a balloon driver, so its KVM hypervisor process holds onto every guest page it has ever touched and never gives memory back to the host, even when the guest reports low internal usage. Observed 2026-07-15: Console's allocation is 80G (memory: 81920 in qm config 103), guest-internal usage was only 22G, but the KVM process on Finn was still resident at ~75G because a compile storm the day before had touched ~73G (93%) at peak.

Why: This makes 'Console looks idle' a misleading signal for host capacity planning on Finn. Finn's true committed memory is Console's allocation (80G) + other guests/LXCs, not Console's current internal usage.

How to apply: When reasoning about whether to raise Console's RAM allocation or about Finn's headroom, always add Console's full allocation (not its live free/used) to the other guests' footprint. Don't propose raising Console's ceiling based on current low internal usage; a burst can and will drive it back to the high-water mark. The lever to reclaim idle Console pages under host pressure is qm set 103 --balloon <min>, but it trades reclaim for slower builds during Console's bursty compile workloads, so it was deliberately left off as of 2026-07-15. Separately: Console's core count changes (qm config 103 cores) only take effect after a full qm shutdown 103 && qm start 103 from Finn; an in-guest reboot does not apply them, and a VM restart also resets the KVM high-water mark, temporarily freeing Finn RAM until the next big build re-touches it.

[auto-memory session 60d132de-4659-485c-8044-427af0ccce7f, confidence 0.75, mode staged]