Reclaim and return guest memory in a Firecracker microVM with the virtio balloon device
domain: firecracker-microvm.github.io · 4 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Configure the balloon before boot: curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/balloon -d '{"amount_mib":128,"deflate_on_oom":true,"stats_polling_interval_s":5}'
Change the target size at runtime: curl --unix-socket /tmp/firecracker.socket -X PATCH http://localhost/balloon -d '{"amount_mib":64}'
Poll the device config with GET http://localhost/balloon and read statistics via GET http://localhost/balloon/statistics when a non-zero polling interval was configured
Optionally set free_page_reporting:true at create time so the guest continually reports unused page ranges that Firecracker reclaims with MADV_DONTNEED (guest needs PAGE_REPORTING kernel config)
Known gotchas
The balloon is paravirtualized and best-effort: a compromised/misbehaving guest driver can break the memory restrictions — keep the host able to handle the guest using ALL memory it was given at boot
Only amount_mib and stats_polling_interval_s can be changed at runtime; all other device options are fixed at create time
deflate_on_oom only helps guest physical allocations; a guest with vm.overcommit_memory=1 will always deflate instead of OOM, but that guest config is not recommended
Statistics cannot be enabled after boot if the balloon was created without stats; and if stats were enabled pre-boot they cannot be switched to 0 later
free_page_reporting can only be enabled pre-boot and runs continuously with no way to stop it
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