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Add writeup on filesystem layout

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Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
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David Oberhollenzer 2018-04-15 16:59:05 +02:00
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# 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](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`).
Many systems have a `/usr/libexec` directory containing executable 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.
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.
Depending on the system configuration, the `/usr` directory may contain read
only user home directories, may be used as a mount point for a dedicated
partition or network share with home directories, or may be used for an overlay
setup as outlined below.
## 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 `/root` directories.
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.
## The srv and opt directories
A `/srv` directory is included in the filesystem to store static service files
such as web sites served by a web server. The intention is to have those files
backed into the squashfs image.
If custom applications are built and bundled into the filesystem, it is
strongly encouraged to use the standard locations for those. However, if it is
desired for some reason, a `/opt` directory is also added by default. The
contents of the `PATH` variable and the configuration file of the loader may
have to be adjusted.