docs: CoW snapshot-fork backend design (Tier 3)#17
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This was referenced Jun 11, 2026
Design-only proposal: a second snapshot-capable backend (Firecracker/CH) behind the existing backend trait for high-fan-out agent sandboxes, since libkrun has no snapshot. Maps forkd (snapshot+UFFD+diff-chaining) and clone (shadow-clone+KSM) onto a3s-box, reusing the overlay rootfs CoW, warm pool, and — the key advantage — the existing guest vsock exec server as the post-fork re-injection hook. Phased plan: P0 KSM/boot-trim (done, #16) → P1 per-image template pool + re-inject handshake (doable on libkrun, de-risks) → P2 real snapshot-fork backend → P3 diff-chaining + balloon.
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Design-only proposal (no implementation) for the high-fan-out agent-sandbox story, completing the CoW optimization plan started in #16.
Core decision: libkrun has no snapshot/restore (verified: 0 of ~75
krun_*are snapshot; guest RAM is anonymous-private;krun_start_entertakes over the process). So rather than fork libkrun's memory VMM, add a second snapshot-capable backend (Firecracker / Cloud Hypervisor) behind the existing backend trait — libkrun stays default; the fork backend serves fan-out.Maps forkd (template snapshot + UFFD CoW restore + diff-snapshot chaining) and clone (shadow-clone + KSM) onto a3s-box, reusing what already exists: overlay rootfs CoW, the warm pool, and — the key advantage — the guest vsock exec server as the post-fork re-injection hook (reidentify + exec-main handshake, since a forked child needs hostname/IP/box_id/command re-stamped).
Phased plan:
Recommendation: build P1 next (cheap, libkrun-native, validates demand); treat P2 as a funded epic.