fork: share template mem-file via symlink for firecracker fan-out#214
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This was referenced May 8, 2026
When a Firecracker fork descends from a Template source, skip copying the snapshot mem-file and hardlink it to the source's instead. Firecracker mmaps the mem-file MAP_PRIVATE on restore, so all forks COW from the same backing inode — no per-fork copy required. Hardlink rather than symlink: firecracker's restore path temporarily aliases the source data dir to the fork data dir while loading the snapshot (withSnapshotSourceDirAlias). A symlink whose target traverses the source dir would resolve back into the fork dir during that window and trip ELOOP; a hardlink resolves by inode so the alias has no effect on it. Hardlinks require both paths on the same filesystem, which holds for our standard data-dir layout. Gated to Firecracker only because other hypervisors (cloud-hypervisor, qemu, vz) don't share MAP_PRIVATE semantics on their snapshot layouts. Restricted to Template sources because they are explicitly promoted as fork-only and can never be restored — sharing the mem-file with a non-Template source would let a later RestoreInstance mutate the file out from under live forks. Stacked on hypeship/template-as-state so the Template state both gates "this snapshot is safe to fan out from" and lets fork counts be derived at read time.
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Summary
Stacked on
ForkCountkeeps the shared mem-file alive while any fork still references it.Test plan
go test ./lib/forkvm/...—CopyOptions.SkipRelPathsskips the named relpathgo test -run 'TestInstallForkSharedMemFile' ./lib/instances/...— symlink installed at the fork's snapshot pathgo test -run 'TestForkFirecrackerSharesMemFile_FromStandby' ./lib/instances/...— end-to-end: standby Firecracker source → fork mem-file is a symlink, not a copygo test -run 'TestForkFirecrackerRunningSourceDoesNotShareMemFile' ./lib/instances/...— running-fork carve-out: mem-file is a regular file