2018-03-28 21:52:44 +02:00
|
|
|
# Pygos Filesystem Layout
|
2018-04-15 16:59:05 +02:00
|
|
|
|
|
|
|
|
## 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](http://lists.busybox.net/pipermail/busybox/2010-December/074114.html)
|
|
|
|
|
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`).
|
|
|
|
|
|
2018-04-19 22:05:31 +02:00
|
|
|
Many systems have a `/usr/libexec` directory containing executables not intended
|
2018-04-15 16:59:05 +02:00
|
|
|
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.
|
2018-04-19 22:05:31 +02:00
|
|
|
Storing user home directories. For instance, the `/root` directory has been
|
|
|
|
|
moved to `/usr/root`.
|
2018-04-15 16:59:05 +02:00
|
|
|
|
|
|
|
|
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`.
|
|
|
|
|
|
|
|
|
|
|
2018-04-19 22:05:31 +02:00
|
|
|
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`.
|
2018-04-15 16:59:05 +02:00
|
|
|
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Of course, if the overlay setup is not needed, it can be completely disabled in
|
|
|
|
|
which case bind mounts to `/cfg/preserve` are made during system boot instead
|
|
|
|
|
of overlay mounts. The filesystem then becomes completely read only, except for
|
|
|
|
|
the tmpfs mounts which are not persisted across reboots.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
## Multiarch Directories
|
|
|
|
|
|
|
|
|
|
Some processors support executing op codes for slightly different architectures.
|
|
|
|
|
For instance, 64 bit x86 processors can be set into 32 bit mode and run
|
|
|
|
|
programs built for 32 bit x86. Such programs then require libraries also built
|
|
|
|
|
for 32 bit x86, creating the necessity for having two different versions of the
|
|
|
|
|
`/lib` directory. Shared libraries may have to be duplicated because some
|
|
|
|
|
32 bit programs need a 32 bit version and 64 bit programs need their version.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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.
|
