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content/os/installation/boot-process/image-preloading/_index.md

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+---
+tag: ["rancheros", "os"]
+category: "os"
+layout: list-docs
+title: Image Preloading
+weight: 152
+---
+
+## 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).