# Host capacity (/docs/guides/host-capacity)



`max_apps` is the first key in [`config.toml`](/docs/reference/config), above every section:

```toml
max_apps = 300
```

Everything that counts slots follows from it: the slot ring, the eight loopback ports each slot
reserves from 21000, the nbd minors on a `zerofs` host, the 1024 entries an app is given in the
kernel's connection-tracking table, and what the metrics page calls `nibrunner_slots{of="total"}`.
A host that is full says so by naming this key.

## What this machine holds [#what-this-machine-holds]

`install` measures three bounds, assuming an app to be 1 vCPU, 256 MiB and an 8 GiB volume:

| Bound             | What it is                                                                                                                                     |
| ----------------- | ---------------------------------------------------------------------------------------------------------------------------------------------- |
| Runs at once      | Guest memory, after the host and the storage cache, over 256 MiB.                                                                              |
| Holds on disk     | The disk under `paths.snapshot_dir`, after the storage cache and an 8 GiB reserve, over one snapshot — plus one volume on a `local-file` host. |
| Fits on its ports | 5567: each slot reserves eight ports from 21000, and the last port has to be a port.                                                           |

A host whose configuration `install` wrote gets the least of the three, with the line above the
key naming all of them. Every `nibrunnerd start` then says the three against the number set:

```
max_apps = 300: assuming an app is 1 vCPU, 256 MiB and 8 GiB to start, this host runs 29 at once, holds 320 on disk, and fits 5567 on its ports; above 29 counts on apps sleeping
```

Above what the memory runs at once is only said — that is what [sleeping](/docs/guides/sleep-and-wake)
is for. Above what the disk holds or the ports fit, `start` says the number to set instead. Both
are advice, not refusals: an app given more counts for more, and the number is yours to keep.

## Changing it [#changing-it]

1. Edit `max_apps`.
2. Run `nibrunnerd start`.

<Callout title="Good to know">
  * **Raising it on a `zerofs` host asks for a reboot.** The nbd module is loaded with
    `max_apps + 1` minors, which cannot be raised in place, and `start` says so rather than
    reloading a module with volumes attached to it.
  * **Lowering it below a slot an app holds moves that app** to a new slot on the next start:
    a new loopback port and a new guest address.
</Callout>