RootFileSystemOnACramfsFileSystem < DULG

%SECTION0{name=RootFileSystemOnCramfsFileSystem}% Root File System on a cramfs File System

_cramfs_ is a compressed, read-only file system.

Advantages are:
   * file system uses compression,
     thus making efficient use of flash memory
   * Allows for quick boot times as only used files get
     loaded and uncompressed

Disadvantages are:
   * only the whole image can be replaced (not individual files)
   * additional storage needed for writable persistent data
   * =mkcramfs= tool does not support device table,
     so we need root permissions to create the required device files
   
To create a =cramfs= based root file system please proceed as follows:

   1 Create a directory tree with the content of the target root filesystem.
     We do this by unpacking our master tarball:
     <verbatim>
$ mkdir rootfs
$ cd rootfs
$ tar -zxf /tmp/rootfs.tar.gz
     </verbatim>
   1 Create the required device files. We do this here by unpacking
     a special tarball which holds only the device file entries.
     %X% Note: this requires root permissions!
     <verbatim>
# cd rootfs
# tar -zxf /tmp/devices.tar.gz
     </verbatim>
   1 Many tools require some storage place in a filesystem,
     so we must provide at least one (small) writable filesystem.
     For all data which may be lost when the system goes down,
     a ="tmpfs"= filesystem is the optimal choice.
     To create such a writable tmpfs filesystem we add the
     following lines to the =/etc/rc.sh= script:
     <verbatim>
# mount TMPFS because root-fs is readonly
/bin/mount -t tmpfs -o size=2M tmpfs /tmpfs
     </verbatim>
     Some tools require write permissions on some device nodes
     (for example, to change ownership and permissions),
     or dynamically (re-) create such files
     (for example, =/dev/log= which is usually a Unix Domain socket).
     The files are placed in a writable filesystem;
     in the root filesystem symbolic links are used
     to point to their new locations:
     %TABLE{ headeralign="left, left" tableborder="0" cellpadding="0" cellspacing="3" databg="none"}%
     | dev/ptyp0 | &rarr; | /tmpfs/dev/ptyp0 | &nbsp;&nbsp;&nbsp; | dev/ttyp0 | &rarr; | /tmpfs/dev/ttyp0 |
     | dev/ptyp1 | &rarr; | /tmpfs/dev/ptyp1 | &nbsp;&nbsp;&nbsp; | dev/ttyp1 | &rarr; | /tmpfs/dev/ttyp1 |
     | dev/ptyp2 | &rarr; | /tmpfs/dev/ptyp2 | &nbsp;&nbsp;&nbsp; | dev/ttyp2 | &rarr; | /tmpfs/dev/ttyp2 |
     | dev/ptyp3 | &rarr; | /tmpfs/dev/ptyp3 | &nbsp;&nbsp;&nbsp; | dev/ttyp3 | &rarr; | /tmpfs/dev/ttyp3 |
     | dev/ptyp4 | &rarr; | /tmpfs/dev/ptyp4 | &nbsp;&nbsp;&nbsp; | dev/ttyp4 | &rarr; | /tmpfs/dev/ttyp4 |
     | dev/ptyp5 | &rarr; | /tmpfs/dev/ptyp5 | &nbsp;&nbsp;&nbsp; | dev/ttyp5 | &rarr; | /tmpfs/dev/ttyp5 |
     | dev/ptyp6 | &rarr; | /tmpfs/dev/ptyp6 | &nbsp;&nbsp;&nbsp; | dev/ttyp6 | &rarr; | /tmpfs/dev/ttyp6 |
     | dev/ptyp7 | &rarr; | /tmpfs/dev/ptyp7 | &nbsp;&nbsp;&nbsp; | dev/ttyp7 | &rarr; | /tmpfs/dev/ttyp7 |
     | dev/ptyp8 | &rarr; | /tmpfs/dev/ptyp8 | &nbsp;&nbsp;&nbsp; | dev/ttyp8 | &rarr; | /tmpfs/dev/ttyp8 |
     | dev/ptyp9 | &rarr; | /tmpfs/dev/ptyp9 | &nbsp;&nbsp;&nbsp; | dev/ttyp9 | &rarr; | /tmpfs/dev/ttyp9 |
     | dev/ptypa | &rarr; | /tmpfs/dev/ptypa | &nbsp;&nbsp;&nbsp; | dev/ttypa | &rarr; | /tmpfs/dev/ttypa |
     | dev/ptypb | &rarr; | /tmpfs/dev/ptypb | &nbsp;&nbsp;&nbsp; | dev/ttypb | &rarr; | /tmpfs/dev/ttypb |
     | dev/ptypc | &rarr; | /tmpfs/dev/ptypc | &nbsp;&nbsp;&nbsp; | dev/ttypc | &rarr; | /tmpfs/dev/ttypc |
     | dev/ptypd | &rarr; | /tmpfs/dev/ptypd | &nbsp;&nbsp;&nbsp; | dev/ttypd | &rarr; | /tmpfs/dev/ttypd |
     | dev/ptype | &rarr; | /tmpfs/dev/ptype | &nbsp;&nbsp;&nbsp; | dev/ttype | &rarr; | /tmpfs/dev/ttype |
     | dev/ptypf | &rarr; | /tmpfs/dev/ptypf | &nbsp;&nbsp;&nbsp; | dev/ttypf | &rarr; | /tmpfs/dev/ttypf |
     | tmp | &rarr; | /tmpfs/tmp | &nbsp;&nbsp;&nbsp; | var | &rarr; | /tmpfs/var | 
     | dev/log   | &rarr; | /var/log/log | &nbsp; | | | |
     | In case you use dhclient also: |||||||
     | etc/dhclient.conf | &rarr; | /tmpfs/var/lib/dhclient.conf | &nbsp;&nbsp;&nbsp; | etc/resolv.conf | &rarr; | /tmpfs/var/lib/resolv.conf |
     %BR%
     To place the corresponding directories and device files
     in the =tmpfs= file system,
     the following code is added to the =/etc/rc.sh= script: 
     <verbatim>
mkdir -p /tmpfs/tmp /tmpfs/dev \
         /tmpfs/var/lib/dhcp /tmpfs/var/lock /tmpfs/var/run

while read name minor
do      
        mknod /tmpfs/dev/ptyp$name c 2 $minor
        mknod /tmpfs/dev/ttyp$name c 3 $minor
done <<__EOD__
0  0    
1  1    
2  2    
3  3    
4  4    
5  5    
6  6    
7  7    
8  8    
9  9    
a 10    
b 11    
c 12    
d 13    
e 14    
f 15    
__EOD__ 
chmod 0666 /tmpfs/dev/*
     </verbatim>
   1 We can now create a =cramfs= file system image using the
     =mkcramfs= tool:
     <verbatim>
$ ROOTFS_DIR=rootfs                 # directory with root file system content
$ ROOTFS_ENDIAN="-r"                # target system has reversed (big) endianess
$ ROOTFS_IMAGE=cramfs.img           # generated file system image

PATH=/opt/eldk/usr/bin:$PATH
mkcramfs ${ROOTFS_ENDIAN} ${DEVICES} ${ROOTFS_DIR} ${ROOTFS_IMAGE}
Swapping filesystem endian-ness
  bin
  dev
  etc
...
-48.78% (-86348 bytes)  in.ftpd
-46.02% (-16280 bytes)  in.telnetd
-45.31% (-74444 bytes)  xinetd
Everything: 1864 kilobytes
Super block: 76 bytes
CRC: c166be6d
warning: gids truncated to 8 bits.  (This may be a security concern.)
     </verbatim>
   1 We can use the same setup as before for the JFFS2 filesystem,
     just changing the bootargument to ="rootfstype=cramfs"=
9.5.2. Root File System on a JFFS2 File System		1. Abstract		9.5.4. Root File System on a Read-Only ext2 File System
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