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docs/filesystem: Mention toplevels and mountpoints #823

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Oct 14, 2024
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docs/filesystem: Mention toplevels and mountpoints #823

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23 changes: 21 additions & 2 deletions docs/src/filesystem.md
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Expand Up @@ -146,7 +146,26 @@ However, for some use cases, it may be easier to allow some level of mutability.
There are two options for this, each with separate trade-offs: transient roots
and state overlays.

#### Enabling transient root
### Other toplevel directories

Creating other toplevel directories and content (e.g. `/afs`, `/arbitrarymountpoint`)
or in general further nested data is supported - just create the directory
as part of your container image build process (e.g. `RUN mkdir /arbitrarymountpoint`).
These directories will be lifecycled with the container image state,
and appear immutable by default, the same as all other directories
such as `/usr` and `/opt`.

Mounting separate filesystems there can be done by the usual mechanisms
of `/etc/fstab`, systemd `.mount` units, etc.

#### SELinux for arbitrary toplevels

Note that operating systems using SELinux may use a label such as
`default_t` for unknown toplevel directories, which may not be
accessible by some processes. In this situation you currently may
need to also ensure a label is defined for them in the file contexts.

## Enabling transient root

This feature enables a fully transient writable rootfs by default.
To do this, set the
Expand All @@ -161,7 +180,7 @@ write (transiently, i.e. until the next reboot) to all top-level directories,
including `/usr` and `/opt`, with symlinks to `/var` for content that should
persist.

#### Enabling state overlays
## Enabling state overlays

This feature enables a writable overlay on top of `/opt` (or really, any
toplevel or subdirectory baked into the image that is normally read-only).
Expand Down