---
effort: 010-lab-inner-loop-cost
updated: 2026-08-24
---

# Measurements

Taken 2026-08-24 on a workstation with hardware virtualisation available, an NVMe-backed ext4
root, and 300 GB free. One virtual machine, 1 GiB memory, 2 CPUs, from a cached distribution
image.

## The environment, before anything ran

| Fact | Value | Consequence |
|---|---|---|
| Hardware virtualisation | present | virtual machines run at native speed; the choice in [ADR 0016](../../02-DECISIONS/0016-a-lab-node-is-a-virtual-machine.md) is not paying an emulation penalty |
| Storage drivers the daemon offers | **`dir` only** | no copy-on-write, therefore no cheap snapshot |
| Host filesystems | ext4 throughout | nothing copy-on-write to put a pool on |
| btrfs kernel module | **available** | the kernel can do it |
| `btrfs-progs` | **not installed** | which is the entire reason the driver is absent |

The last two rows are the finding. The daemon advertises only `dir` because the userspace tool
for anything better is missing — not because the host cannot do better.

## Raising a machine

| Step | Time |
|---|---|
| launch call returns | **3.4 s** |
| machine actually usable — a command executes on it | **14.3 s** |

The gap matters for the lifecycle design: `raise` returning is not the same as the scenario
being ready, so the verb has to wait for the second number, not report the first. Reporting
the first would be the mesh's own recurring failure — transport reported as effect.

## Snapshot and restore

| Operation | Time | Disk |
|---|---|---|
| snapshot, first | **9.9 s** | **+1.6 GB** |
| snapshot, second | **> 120 s — did not complete** | — |
| restore call returns | **10.4 s** | — |
| machine usable again | **20.1 s** total | — |

Instance on disk before snapshotting: 1.5 GB. Snapshot directory afterwards: 1.6 GB. **A `dir`
snapshot is a full copy** — the storage cost equals the instance, and nothing is shared.

Implied copy throughput on the first snapshot is roughly 160 MB/s, which is far below what the
underlying NVMe can do and is consistent with a real, durable copy rather than a metadata
operation.

**The second snapshot is the more troubling number.** It exceeded two minutes and was still
running when the observation was cut off; only the first snapshot exists. Whatever the cause —
page cache exhausted by the preceding restore, writeback contention — the practical
consequence is that snapshot cost here is **not merely high, it is unpredictable**.

## What this projects to

A four-machine scenario, taking the optimistic single-machine numbers and assuming the
operations are serial:

| | one machine | four machines |
|---|---|---|
| raise, to usable | 14 s | ~57 s |
| snapshot | 10 s, 1.6 GB | ~40 s, 6.4 GB |
| restore, to usable | 20 s | ~80 s |

A reset-and-rerun cycle is therefore **around a minute and a half at best**, and unbounded at
worst, before any of the mesh's own work begins.

## The judgement

**This is too slow for an inner loop**, and the reason is not the design.

[ADR 0029](../../02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md) argues that
making the bootstrap path the inner development loop turns the least-exercised code in the
system into the most-exercised. That argument holds only while resetting is cheap. At a minute
and a half a cycle, with occasional multi-minute stalls, the loop is one a person works around
— and the path stays under-exercised for exactly the reason it always was.

Nothing about virtual machines causes this. Hardware virtualisation is present and the machines
boot in fourteen seconds. **The cost is entirely the storage driver**, and the driver is absent
because one userspace package is not installed on the host.

The mesh already has the mechanism for that: a module declares a package, and a hook makes it a
working capability — which is precisely what was just done for the virtualisation daemon
itself, and what [`04-ISSUES/007`](../../04-ISSUES/007-an-installed-package-is-not-a-capability/00-report.md)
is about.

## Not measured, and it matters

The copy-on-write comparison **was not run**, because running it would mean installing a package
by hand, which the mesh's rules forbid and which would have made the measurement unreproducible
anyway. Copy-on-write snapshots are expected to be near-instant with storage proportional to
what changed. That expectation is well founded and is still an expectation.

Until it is measured, the correct statement is: *the current configuration is too slow, and the
likely fix is known but unverified.*