Add or remove memory to a running Firecracker microVM dynamically with the virtio-mem device (memory hotplug)
domain: firecracker-microvm.github.io · 6 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Configure the virtio-mem device once during VM setup (before InstanceStart) via PUT /hotplug/memory with total_size_mib (required, multiple of slot), block_size_mib (default 2), slot_size_mib (default 128)
curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/hotplug/memory -d '{"total_size_mib":1024,"block_size_mib":2,"slot_size_mib":128}' - on boot the whole hotpluggable region starts unplugged (requested_size 0)
To grow memory at runtime, PATCH /hotplug/memory with a requested_size_mib multiple of block_size_mib up to total_size_mib; the guest driver asynchronously plugs blocks until plugged_size_mib == requested_size_mib (monitor via GET /hotplug/memory)
To shrink, PATCH a lower requested_size_mib; when all blocks in a slot are unplugged Firecracker protects the slot from guest access
This hotpluggable pool is SEPARATE from boot memory configured in machine-config - only hotplug memory can be resized dynamically
Guest requires CONFIG_VIRTIO_MEM and a recent kernel (x86_64 >= 5.16, aarch64 >= 5.18)
Known gotchas
Only configurable before InstanceStart; snapshot-restored VMs reuse the size saved in the snapshot
Set guest boot param memhp_default_state=online_movable for reliable hot-removal, or memory_hotplug.memmap_on_memory=1 memhp_default_state=online when hot-removal is not needed and the hotplug region is much larger than boot memory
Hotplugging is asynchronous - poll GET /hotplug/memory until plugged_size_mib matches requested_size_mib
Keep default block/slot sizes unless strict memory protection is required; setting block_size_mib == slot_size_mib makes it harder for the guest to hot-remove
Official doc: https://raw.githubusercontent.com/firecracker-microvm/firecracker/main/docs/memory-hotplug.md
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