Lighthouse Pup Architecture: The Layered File System

The way to understand the diagram is to view each of those layers as a complete filesystem, that is, a complete directory hierarchy from "/" down. These layers are laid one on top of the other, which is achieved by the aufs/unionfs filesystem. The way it works is very simple: say that the red (core system) layer has a file /usr/lib/libcairo.so. This file will be visible at the top layer. If a blue (extra SFS) layer has the same file, it will not be visible, as it is overlaid by the same file on a higher layer.

SFS (Squash File System) add-ons are typically compressed software files, (a 'filesystem in a file') that are layered into the LHP file system during boot up from LiveCD / pfix=ram, or a Frugal Install and initially take up no space in the save file. Each SFS is a separate read-only layer, (blue in the diagram above.) They often include many apps, all in one 'combo-pack'. Lighthouse supports up to 40 concurrent SFS add-ons. Several popular ones are included in the Lighthouse Mariner ISO. Those and many more can be found here.

How to install an SFS

With a Frugal Install, place them in /mnt/home e.g., /mnt/home/LibreOffice.sfs This is the top level directory of the partition where your save file is. (/mnt/home is actually a symlink to the root of that partition.) As of LHP 5.00 G, if the save file is in a subdirectory, SFS files can be placed in either the top directory or in that subdirectory, e.g., /mnt/home/lhp503/LibreOffice.sfs

Then, using BootManager in the System menu, move desired SFS's to the right-hand pane, click OK and reboot.

If the SFS file name ends with _nnn it must match the LHP version you have, e.g., Devx-LHP_503.sfs for LHP 5.03.

What is unique about the way Lighthouse uses an SFS?

SFS add-ons can be selected and configured at first or any boot, prior to mounting the layered file system.

These are layered into the main file system, each SFS in a separate layer, the same way BootManager does it, but at any boot, including LiveCD. After the first boot, a standard Frugal Install is recommended for proper layering of SFS files.
If sufficient free RAM is detected, all core files and SFS add-ons can be copied to RAM. This means the Lighthouse Mariner ISO is a true LiveCD with the Mariner add-ons & LibreOffice running in RAM at first boot. To select the SFS at first boot from the LiveCD, it must be present in the top-level of the CD, which is the case in the Mariner ISO.
If any .sfs add-ons are found, before they are loaded, a brief prompt during boot-up is offered to select and configure them, (Press Enter within 5 secs...) The SFS configuration allows you to choose which add-ons you'd like to use that session, and whether or not to copy them to RAM. Skip the prompt and use all SFS selected in prior configuration / BootManager / default selections from LiveCD.
Available free RAM is recalculated after each copy2ram. If RAM is low, only SFS files that will safely fit in RAM will be copied and the rest will mount in place. This way all selected add-ons will be loaded, making maximum use of available RAM.

Lighthouse may or may not include support for loading and unloading SFS add-ons from the desktop without rebooting, and not at all with a full install. Shinobar has written an experimental utility called SFS_Load-on-the-fly and other methods are discussed here and here.

Personal preference is often the deciding factor. That said, SFS files are ideal for large apps e.g., LibreOffice or combo-packs of many apps, e.g., Mariner or KDE, because they reside compressed, typically at /mnt/home and use little or no space in the LHPsave file. PETS on the other hand, are easily installed without rebooting and thus are ideal for smaller packages or frequent updates, e.g., Sylpheed, printer drivers, Flash and browsers. PETS are a bit easier to build and often use pinstall.sh and puninstall.sh scripts ideal for handling setup and uninstall tasks.

ram disk Temporary Storage	This is the tmpfs filesystem running in RAM, with new and changed files. It is the top layer at first boot from LiveCD with puppy pfix=ram entered at the boot menu.
LHPsave.4fs 'pupsave'	This is the persistent storage, where all your data, settings, email, installed packages, etc., get saved permanently. The ".4fs" means that the file contains a ext4 filesystem. If the host partition is not formatted with ext4, or the LHPsave is encrypted, then it would be .3fs or .2fs. When saving your session to an LHPsave file this becomes the new top layer instead of the ram disk.
LHP_nnn.sfs	This is Lighthouse. The kernel, built-in applications, window manager, scripts, everything. The ".sfs" means the file contains a squashfs compressed filesystem. The "nnn" is the Lighthouse version number without the dots, for example "503".
.sfs or _nnn.sfs	These are additional squashfs files. The "*" can be anything, as long as its name does not begin with a 'z' or distro-specific prefixes, like LHP, pup, wary, luci etc. For example, Devx-LHP_503.sfs is the complete environment for compiling C/C++ applications. Again, the "_nnn", if present, is the Lighthouse version number.