---
title: Image Preloading
layout: os-default
redirect_from:
  - os/configuration/prepacking-docker-images/

---

## Image Preloading
---

On boot, RancherOS scans `/var/lib/rancher/preload/docker` and `/var/lib/rancher/preload/system-docker` directories and tries to load container image archives it finds there, with `docker load` and `system-docker load`.

The archives are `.tar` files, optionally compressed with `xz` or `gzip`. These can be produced by `docker save` command, e.g.:

```
$ docker save my-image1 my-image2 some-other/image3 | xz > my-images.tar.xz
```

The resulting files should be placed into `/var/lib/rancher/preload/docker` or `/var/lib/rancher/preload/system-docker` (depending on whether you want it preloaded into Docker or System Docker).

Pre-loading process only reads each new archive once, so it won't take time on subsequent boots (`<archive>.done` files are created to mark the read archives). If you update the archive (place a newer archive with the same name) it'll get read on the next boot as well.
 
Pre-packing docker images is handy when you're customizing your RancherOS distribution (perhaps, building cloud VM images for your infrastructure).