---
status: active
initiated: 2026-08-24
touches:
  - 03-DESIGN/01-to-be/03-scenario-lifecycle.md
  - 02-DECISIONS/0016-a-lab-node-is-a-virtual-machine.md
  - 02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md
became: []
---

# 010 — What the lab's inner loop actually costs

## What is being investigated

[The lifecycle design](../../03-DESIGN/01-to-be/03-scenario-lifecycle.md) closes on an open
question that is measurable rather than arguable:

> **What a snapshot costs.** Whole-scenario snapshots of several machines are the operation the
> inner loop repeats most, so their cost sets the loop's speed. If restoring is slow, the loop
> is slow, and everything above is theory.

Measured on a workstation, 2026-08-24. Numbers in [`measurements.md`](measurements.md).

## Why it matters

[ADR 0029](../../02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md) makes the
bootstrap scenario the inner development loop for tiers 0 and 1 — the argument being that
raising a node from nothing stops being the least-exercised path and becomes the most-exercised
one. **That argument is only true if raising and resetting are cheap.** A loop that costs
minutes is a loop people avoid, and the least-exercised path stays least-exercised.

## Status

Measured, and the finding is a blocker rather than a data point.

**A snapshot on the current host is a full copy of the machine's disk.** 1.6 GB and ten seconds
for one small virtual machine at best — and over two minutes when observed a second time. Cost
scales with the number of machines and the size of their disks, not with what changed.

The cause is not virtual machines and not incus. It is that the host offers incus exactly one
storage driver, `dir`, which has no copy-on-write and therefore no cheap snapshot. The kernel
supports btrfs; the userspace tool that would let incus use it is simply not installed.

So the question *"is the lab's inner loop fast enough"* currently answers itself the wrong way,
for a reason that is one declared package away from being fixed — and declaring packages is
something the mesh already does.

## Open questions

| Question | Why it matters |
|---|---|
| How much does a copy-on-write pool actually improve it? Expected to be near-instant snapshots and delta-sized storage, but **expected is not measured**. | The whole inner-loop argument rests on the answer. |
| Why was the second snapshot more than twelve times slower than the first? | If snapshot cost is unpredictable rather than merely high, that is worse — a loop with a variable multi-minute step is one nobody trusts. |
| Does a scenario snapshot need the machines stopped? | Stateless snapshots of a running virtual machine capture the disk but not memory. Whether a mesh restored that way is coherent is not established. |
| What is the cost at scenario scale — four machines rather than one? | Only single-machine numbers were taken. If the operation is serial, four machines is four times the wait. |