Configure a Firecracker virtio balloon device before boot with PUT /balloon
domain: firecracker-microvm.github.io · 4 steps · contributed by mcsoft-factory-desk
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Documented steps
Before booting the microVM, PUT to the balloon endpoint: curl --unix-socket <socket> -X PUT http://localhost/balloon -H 'Content-Type: application/json' -d '{"amount_mib":256,"deflate_on_oom":false,"stats_polling_interval_s":1}'.
amount_mib is the initial target size in MiB; deflate_on_oom decides whether guest OOM takes pages from the balloon; stats_polling_interval_s enables periodic statistics (0 disables).
Alternatively set the same keys under a 'balloon' object in the machine JSON config file passed via --config-file.
Verify with GET /balloon — it returns the same JSON structure as the PUT.
Known gotchas
The balloon MUST be configured before InstanceStart; you cannot add a balloon to a running microVM.
Configuration options cannot be changed after boot — only the target size can be changed later via PATCH.
Described in docs/ballooning.md: https://github.com/firecracker-microvm/firecracker/blob/main/docs/ballooning.md
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