# RTO — the M band, measured (2026-08-02)

The S-band figure (66 MB → 42.0 s, two independent passes agreeing to 0.6 s) had a spread tight
enough to prove **fixed work dominates**, which is exactly why it said nothing about M. This adds a
second point 327× larger, measured on the **same app, the same way**: clock from the restore request
to the app **serving the correct discriminator**, read over the path `DATABASE_URL` actually names.

## Method

`rallly`'s postgres volume grown 66 MB → **21.1 GB** by inserting 200 000 rows into a `cc_bulk` table.
`ALTER TABLE … SET STORAGE EXTERNAL` is load-bearing: without it TOAST compresses `repeat(...)` to
almost nothing and the volume never really grows, which would have produced a fake M-band number.
The `cc_proof` canary row is untouched, so the discriminator is unchanged.

`/mnt/sys_drive` was first grown 20 G → 70 G with `pct resize` — the same operation the product
performs via `SysDataGrowGB` (`felhom-agent/internal/reconcile/bringup.go:426`). **It had to be**:
see the capacity ceiling below.

## Results — 2 reps, both returning the correct discriminator

| rep | volume | backup | recovery unit | **RTO to correct data** | discriminator |
|---|---|---|---|---|---|
| 1 | 21 616 MB | 406.4 s | 41 149 MB | **624.5 s** | correct |
| 2 | 21 615 MB | 387.2 s | 41 133 MB | **591.8 s** | correct |
| **mean** | **21.1 GB** | **396.8 s** | **40.2 GB (1.90×)** | **608.1 s (10.1 min)** | **2/2** |

## The two-point model

| | 66 MB (S) | 21.1 GB (M) | ratio |
|---|---|---|---|
| data | ×1 | **×327** | |
| RTO | 42.0 s | **608.1 s** | **×14.5** |

Time grew 14.5× for 327× the data — strongly sub-linear, i.e. a large fixed cost plus a
byte-proportional term:

> **RTO ≈ 40 s + 26.9 s/GB**  ·  **backup ≈ 29 s + 17.4 s/GB**

| size | projected RTO |
|---|---|
| 1 GB | 67 s |
| 5 GB | 2.9 min |
| 10 GB | 5.2 min |
| **20 GB** | **9.6 min** (measured **10.1 min** — the model is fit through the mean) |
| 50 GB | 23.1 min |
| 100 GB | 45.5 min |

**Where the bands actually break.** The fixed ~40 s dominates below roughly **1.5 GB** — that is the S
band, and it is why the S numbers were so tightly clustered. Above it the per-GB term takes over and
RTO becomes essentially linear in data.

## The capacity ceiling — the more consequential result

**The recovery unit is 1.90× the app's data**, because for a DB-backed app it carries *both* the
volume tar and the SQL dump: 21.1 GB of data produced a **40.2 GB** unit.

The default appliance ships `/mnt/sys_drive` at **20 GB** (`mp1 … size=20G`, the golden's baked size;
`SysDataGrowGB` is a per-customer grow that was 0 for this customer). So on a default box:

> **the largest app that can hold a local Tier-1/Tier-2 recovery unit is ≈ 10 GB — and that fills the
> volume completely.** A realistic ceiling is nearer **8 GB**.

**The M band does not fit on a default box at all.** This test only reached 21 GB because
`/mnt/sys_drive` was grown to 70 G first. That is a sizing decision for the S/M/L tiers, not a defect
— but it is the constraint that actually bites, and it is invisible until an app crosses it.

## Caveats — stated, not buried

- **Two points define a line; they do not test linearity.** A third point (~5 GB) would confirm the
  model's shape. Not taken.
- **The 1.90× ratio is DB-app-specific.** It is volume-tar + SQL-dump. A file-only app (sqlite, no DB
  container) should be nearer 1.0×, but that was **not measured** and is inference.
- **Synthetic data.** `repeat(md5(...))` stored uncompressed is deliberately incompressible; real app
  data that compresses well would move both the unit size and the per-GB rate.
- One app, one shape, one box. The fixed term in particular is a property of this stack's stop/start
  and health-wait, which differs per app.
