System Layout

From OESF

Revision as of 15:35, 20 September 2007

Overview

This document describes the filesystem and directory layout of the Sharp Zaurus 5500, to help everyone understand better what happens when new software is installed, creating user accounts, and so on. It's not really a HOWTO in the sense that it doesn't explain "how to" do anything beyond understand the location of files and directories, and how these correspond to the ROM and RAM on the Z, and why.

Robert P. J. Day <rpjday at mindspring.com>

The memory layout

The memory on the 5500 is divided into ROM: 16M RAM: 64M, further divided into:

memory

32M for processes/buffers/cache

storage: 32M for additional file/directory storage

You can see the current usage of RAM by clicking on "Applications/System Info".

In addition to the above, their might also have additional memory on either a CF card or SD card, whose size and usage would also be displayed by clicking the same icon.

Superficially, all of the above storage/memory corresponds to the following locations or mount points by default:

/ ROM

/home RAM

/mnt/cf CF card

/mnt/card SD card

and in a terminal shell it will correspond with either of:

1. mount
2. df

While this is a fair approximation of how memory and directories are associated, it's actually quite a bit more complicated.

The directory structure

Again, simplistically, think of the two major mounts points as:

/ ROM

/home RAM

Initially, this suggests that all of the system directories like /bin, /usr, /etc and so on are read-only, but clearly this is impossible since there would be no way to do basic administration like adding a new user, installing new software and so on.

In fact, modifying system files is supported by the fact that there are about a gazillion links from the alleged system directories and files into corresponding directories and files under /home, which makes them changeable. For example,

/etc -> /home/etc

/tmp -> /home/tmp

/var -> /home/system/var

which makes everything in /etc, /tmp and /var read-write. So far, so good.

In addition, command and software management is also supported by carefully chosen symbolic links. Consider, as an example, just the /usr/bin directory:

/usr/bin -> /home/root/usr/bin

This implies that new software being installed under /usr/bin is actually going into the writable RAM, which is a good thing. But it gets more interesting.

If the contents of /home/root/usr/bin are examined, most of the contents are symbolic links back to files in /usr/bin.rom. This means that both the standard, built-in ROM commands appear to be in /home/root/usr/bin, even though they're really back in the read-only directory /usr/bin.rom. And any new software will go in /home/root/usr/bin. The same situation holds for the other directories /usr/sbin, /usr/lib, /usr/local, /usr/share and /usr/etc, which means there is access the pre-loaded ROM files while still being able to add new files.

This same technique is used for additional installation directories:

/opt/!QtPalmtop -> /home/!QtPalmtop

/opt/Qtopia -> /home/!QtPalmtop

Given the above, it's easy to see that the main software install directories are going to be:

/usr/bin

/usr/local/bin

/opt/{!QtPalmtop,Qtopia}

all of which, due to symbolic links, actually reside under /home somewhere.

The initial system directories

One final point. The initial contents of a number of the critical directories can be found as hidden tar files in the ROM-based /root directory:

1. cd /root
2. ls -a

.dev_default.tar

.home_default.tar

.var_default.tar

These tarballs are the initial contents of /dev, /home, and /var (which is really a symlink to /home/system/var). Doing a hard reset reloads all of /dev, /home and /var from these tar files.

When building a custom ROM the initial contents of any of these directories can be changed, by (as root on your Linux box) untar the appropriate tarball, make the changes, then tar it back up. You have to do it as root so that root is the owner of the files in the tarball. (fakeroot should work here, if you're familiar with that.)