Hotplug a virtio-block device onto a running Firecracker microVM (PCI) and rescan the guest bus to make it appear without rebooting
domain: firecracker-microvm.github.io · 5 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Firecracker supports hotplugging virtio-block and virtio-net onto a RUNNING VM, but only with PCI transport - add pci=true to the VM config; hotplug is not supported with MMIO transport
Guest kernel must have PCI support and the virtio-pci driver; Firecracker does NOT deliver a guest notification, so after the API call you must trigger a PCI bus rescan inside the guest (e.g. echo 1 > /sys/bus/pci/rescan) so the device enumerates
Attach the device on the running VM via PUT /drives/block1 with drive_id, path_on_host, is_root_device, is_read_only (device is configured through /drives/{id})
In the guest, after rescan you should see /dev/vdX; mount it, e.g. mount /dev/vdXN /mnt/block1
To remove, unmount in the guest first (umount /mnt/block1) then DELETE the drive via HTTP DELETE /drives/block1
Known gotchas
Hotplug requires the guest PCI bus to be rescanned manually - there is no automatic hotplug event delivered to the guest
Ensure the guest kernel exposes the PCI sysfs rescan interface and has virtio_blk as a module or built-in
Only virtio-block and virtio-net are hotpluggable; other devices (balloon, vsock, entropy) are not
Official doc: https://raw.githubusercontent.com/firecracker-microvm/firecracker/main/docs/device-hotplug.md
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