build/docs/filesystem.md

Pygos Filesystem Layout

Sub Hierarchy Merge

The filesystem hierarchy has received some cleanups and simplifications that typical GNU/Linux distributions don't have. First of all, there is no /usr sub hierarchy split and there is no /bin vs /sbin split.

The /usr split and the /bin vs /sbin split are historical artifacts. Of course, over the years, people started to pretend that it was a deliberate design choice and invented lots of interesting reasoning to justify why this is the way to go, despite the original technical reasons being obsolete for a really long time now. Please refer to this thread on the BusyBox mailing list for further discussion.

The directories typically found in /usr have been merged back into the filesystem root (/usr/bin and /usr/sbin to /bin and /usr/lib to /lib).

The /usr/share directory containing application data has also been moved to the filesystem root (i.e. there is a /share).

Many systems have a /usr/libexec directory containing executables not intended to be run by people but by programs. This has been moved to /lib/libexec.

The /usr directory is still present but now serves its original purpose again. Storing user home directories. For instance, the /root directory has been moved to /usr/root.

Since build tools, source code and headers are typically not installed, there is currently no need to think about where to put /usr/include or /usr/src.

Basic Filesystem Mount Points

A compressed, read only squashfs image is mounted to the root node of the virtual filesystem hierarchy.

The directories /dev, /proc and /sys do not contain any files and are used as mount points for devtmpfs, procfs and sysfs respectively.

The directories /tmp, /run and /var are used as mount points for tmpfs mounts. The sub hierarchy in /var is constructed during system boot.

See below on how changes to /var/lib are persisted and preserved across reboots.

The directories /boot and /mnt are used as mount points for temporary mounts. The former specifically for mounting the boot medium containing the kernel and squasfs image in order to apply updates, the later for other temporary mounts.

Persistent Configuration Changes

A directory /cfg was added to implement an overlay mount setup. The directory /cfg/preserve contains the original versions of files built into the compressed, read only root filesystem.

For instance, the original contents of /etc are actually stored in /cfg/preserve/etc.

The directory /cfg/overlay is used as a mount point for a read/write partition that is used to override and persist changes to the base configuration.

During system boot, an overlay filesystem is mounted to /etc with the lower directory set to /cfg/preserve/etc and the upper directory set to /cfg/overlay/etc.

As a result, the /etc directory initially contains files stored in the squashfs image, but changes can be made. The altered files are stored on the dedicated partition or device mounted to /cfg/overlay.

A similar setup is used for the /var/lib and the /usr directories. The /var/lib directory combines /cfg/preserve/var_lib with overlays from /cfg/overlay/var_lib.

This setup allows for simple management of site local configuration changes, simple backups and a simple "factory reset" (i.e. wiping the overlay partition or device).

On systems where this is possible, BTRFS is used for the overlay partition. BTRFS sub volumes can be used to make snapshots of the changed configuration and if something should break, allows for a simple revert to the last known good state.

The overlay setup can also be disabled (resulting in bind mounts to /cfg/preserve) or configured to use a tmpfs as backing store.

Multiarch Directories

Some processors support executing op codes for slightly different architectures. For instance, many 64 bit processors can be set into 32 bit mode and run 32 bit programs. Such programs then require additional 32 bit versions of shared libraries, already built for the 64 bit system, creating the necessity for having two different versions of the /lib directory.

For the time being, it has been decided to not include multiarch support. All packages are built for a single target architecture. This simplifies both the build process and the final system as well as reducing the memory footprint of the system image. A proposal exists for creating a separate /system32 sub hierarchy on 64 bit targets that require 32 bit binaries.