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Puppy is actually a couple of compressed files:
vmlinuz
, the Linux kernel
Loads into RAM when live-CD boots.
image.gz
, the basic filesystem
When the live-CD boots 'image.gz' loads into a "ramdisk" (a filesystem
that's totally in RAM). The ".gz" means that the file is compressed, so
it's uncompressed into the ramdisk and then becomes Puppy's basic
filesystem (i.e., the "/" or top directory and all the subdirectories
/bin, /sbin, /lib, /dev, /tmp, etc).
usr_cram.fs
, mounted on /usr
All the files inside the /usr directory are compressed in a file called 'usr_cram.fs'.
Most of the files in Puppy are in /usr (same for any Linux distro).
Usually there's no need to write to anything inside /usr (i.e., no need
to edit, create or delete files there). Therefore, contents of /usr
could be compressed, and that's what Puppy does. This file is not
uncompressed at bootup, it's mounted as-is on /usr (by means of what is
called a "loopback device"). All of /usr is only a single file that
contains a compressed and read-only filesystem.
pupXXX
, mounted on /root
As the above 3 files are read-only, your settings (e.g., configuration
changes, email, browser history, etc.) can't be saved and are lost at
shutdown. To deal with this we have the pupXXX file (where 'XXX' is
three numeric digits, pup001 is used for live-CD and pup100 is used for
USB installations currently).
'pup001' is a single file, but internally is a complete ext2
filesystem. After creating this file during first bootup, Puppy then
mounts it onto /root directory, by means of what is called a "loopback
device". This directory /root is your home directory -- all your
personal stuff goes in there. Thus, when you shutdown, it will be there
next time you start Puppy.
All directories in Puppy are read-only (anythings added to them will
disappear on reboot) except:
/root
/usr
/etc
/usr and /etc becomes read-write since Puppy version 1.0.1 with a
technique called UNIONFS. Any files written to /usr are actually stored
in /root/.usr and mounted to /usr. '/root/.usr' is just a directory
inside pup001. As /root/.usr is actually inside pup001 then it will be
saved at end of session and reloaded next time Puppy is booted. The
same happens to /etc. Files written to /etc are stored in /root/.etc
(which is inside pup001) and mounted to /etc.
So, vmlinuz is the Linux kernel, image.gz is a compressed file loaded
into ramdisk which then becomes the basic filesystem, usr_cram.fs is a
compressed file with everything else (e.g., softwares), while pupXXX is
a file that contains all the changes you made to Puppy.
To view the first three files, open the puppy iso file with programs like Winrar. If you have it burned to CD already, view the files just like viewing files in any CD (i.e, browse to the CDrom drive folder). The last file, pupXXX, is created when puppy boots for the first time if you've choosed to save your settings. Usually it's found in the first partition of your hard drive.
To know more see Barry's page: How Puppy Works. Note: If you are using Puppy version 1.0.X, scroll down the page and find the section "How Puppy works, take 1". To learn more about how Puppy version 2 (not released yet) works, see "How Puppy works, take 1".
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