mirror of
https://github.com/linuxkit/linuxkit.git
synced 2025-10-23 12:34:21 +00:00
4eb60514c9
Seceral YAML files used alpine:3:11. Update them to 3.13 Signed-off-by: Rolf Neugebauer <rn@rneugeba.io>
62 lines
2.6 KiB
Markdown
62 lines
2.6 KiB
Markdown
# Image Caching
|
|
|
|
linuxkit builds each runtime OS image from a combination of Docker images.
|
|
These images are pulled from a registry and cached locally.
|
|
|
|
linuxkit does not use the docker image cache to store these images. This is
|
|
for two key reasons.
|
|
|
|
First, docker does not provide support for different architecture versions. For
|
|
example, if you want to pull down `docker.io/library/alpine:3.13` by manifest,
|
|
with its signature, but get the `arm64` version while you are on an `amd64` device,
|
|
it is not supported.
|
|
|
|
Second, and more importantly, this requires a running docker daemon. Since the
|
|
very essence of linuxkit is removing daemons and operating systems where unnecessary,
|
|
just laying down bits in a file, removing docker from the image build process
|
|
is valuable. It also simplifies many use cases, like CI, where a docker daemon
|
|
may be unavailable.
|
|
|
|
## How LinuxKit Caches Images
|
|
|
|
LinuxKit pulls images down from a registry and stores them in a local cache.
|
|
It stores the root manifest or index of the image, the manifest, and all of the layers
|
|
for the requested architecture. It does not pull down layers, manifest or config
|
|
for all available architectures, only the requested one. If none is requested, it
|
|
defaults to the architecture on which you are running.
|
|
|
|
By default, LinuxKit caches images in `~/.linuxkit/cache/`. It can be changed
|
|
via a command-line option. The structure of the cache directory matches the
|
|
[OCI spec for image layout](http://github.com/opencontainers/image-spec/blob/master/image-layout.md).
|
|
|
|
Image names are kept in `index.json` in the [annotation](https://github.com/opencontainers/image-spec/blob/master/annotations.md) `org.opencontainers.image.ref.name`. For example"
|
|
|
|
```json
|
|
{
|
|
"schemaVersion": 2,
|
|
"manifests": [
|
|
{
|
|
"mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
|
|
"size": 1638,
|
|
"digest": "sha256:9a839e63dad54c3a6d1834e29692c8492d93f90c59c978c1ed79109ea4fb9a54",
|
|
"annotations": {
|
|
"org.opencontainers.image.ref.name": "docker.io/library/alpine:3.13"
|
|
}
|
|
}
|
|
]
|
|
}
|
|
```
|
|
|
|
## How LinuxKit Uses the Cache and Registry
|
|
|
|
For each image that linuxkit needs to read, it does the following. Note that if the `--pull` option
|
|
is provided, it always will pull, independent of what is in the cache.
|
|
|
|
1. Check in the cache for the image name in the cache `index.json`. If it does not find it, pull it down and store it in cache.
|
|
1. Read the root hash from `index.json`.
|
|
1. Find the root blob in the `blobs/` directory via the hash and read it.
|
|
1. Proceed to read the manifest, config and layers.
|
|
|
|
The read process is smart enough to check each blob in the local cache before downloading
|
|
it from a registry.
|