ostree-prepare-root − Change the view of a mounted root filesystem to an ostree deployment
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ostree prepare−root {TARGET} |
At its core, ostree operates on an existing mounted filesystem. Tooling such as ostree admin deploy will create a new directory that can be used as a bootable target. This tool is designed to run in an initramfs and set up "remapping" mounts as a view into that filesystem.
As of more recently, this tool also has optional support for composefs, which creates a distinct mount point layered on top of the underlying filesystem.
The most common pattern today is to use systemd in an initramfs. The systemd unit shipped upstream is ordered in this way: After=sysroot.mount and Before=initrd−root−fs.target
When it runs, the mounted filesystem at the provided TARGET (usually /sysroot) will be changed such that what appears at /sysroot is actually the "deployment root" − i.e. a particular versioned subdirectory. What was formerly the "physical root" i.e. the real root of the filesystem will appear as /sysroot/sysroot.
For /var, by default a bind mount is created from the deployment root to /sysroot/var.
A read−only bind mount is created over /sysroot/usr. The immutable bit is set on the deployment root, so this provides basic protection for filesystem mutation. If the sysroot.readonly option is enabled, instead a writable bind mount for /sysroot/etc, and everything else is mounted read−only.
Finally, when higher level tooling such as systemd performs a switch−root operation, what was /sysroot becomes / and after the transition into the real root, the system will be booted into the "deployment", which is a versioned immutable filesystem tree. The ostree tooling running in the real root thereafter performs further changes by operating on /sysroot which is now the "physical root".
The /usr/lib/ostree/prepare−root.conf (or /etc/ostree/prepare−root.conf) config file is parsed by ostree−prepare−root. This file must be present in the initramfs. The default dracut module will copy it from the real root if present.
sysroot.readonly
A boolean value; the default is false. If this is set to true, then the /sysroot mount point is mounted read−only.
etc.transient
A boolean value; the default is false. If this is set to true, then the /etc mount point is mounted transiently i.e. a non−persistent location.
composefs.enabled
This can be yes, no. maybe or signed. The default is maybe. If set to yes or signed, then composefs is always used, and the boot fails if it is not available. Additionally if set to signed, boot will fail if the image cannot be validated by a public key. If set to maybe, then composefs is used if supported.
composefs.keypath
Path to a file with Ed25519 public keys in the initramfs, used if composefs.enabled is set to signed. The default value for this is /etc/ostree/initramfs−root−binding.key. For a valid signed boot the target OSTree commit must be signed by at least one public key in this file, and the commitfs digest listed in the commit must match the target composefs image.
As mentioned above, this tool comes with a systemd unit file ostree−prepare−root.service and it is primarily expected to be invoked this way.
The default for ostree is to create a plain hardlinked filesystem tree. composefs support is currently experimental; see the upstream doc/composefs.md for more information on using it.