9.5.3. 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:

   $ mkdir rootfs
   $ cd rootfs
   $ tar -zxf /tmp/rootfs.tar.gz
   

2. Create the required device files. We do this here by unpacking a special tarball which holds only the device file entries. Note: this requires root permissions!

   # cd rootfs
   # tar -zxf /tmp/devices.tar.gz
   

3. 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:

   # mount TMPFS because root-fs is readonly
   /bin/mount -t tmpfs -o size=2M tmpfs /tmpfs
   

   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:

   dev/ptyp0	→	/tmpfs/dev/ptyp0		dev/ttyp0	→	/tmpfs/dev/ttyp0
   dev/ptyp1	→	/tmpfs/dev/ptyp1		dev/ttyp1	→	/tmpfs/dev/ttyp1
   dev/ptyp2	→	/tmpfs/dev/ptyp2		dev/ttyp2	→	/tmpfs/dev/ttyp2
   dev/ptyp3	→	/tmpfs/dev/ptyp3		dev/ttyp3	→	/tmpfs/dev/ttyp3
   dev/ptyp4	→	/tmpfs/dev/ptyp4		dev/ttyp4	→	/tmpfs/dev/ttyp4
   dev/ptyp5	→	/tmpfs/dev/ptyp5		dev/ttyp5	→	/tmpfs/dev/ttyp5
   dev/ptyp6	→	/tmpfs/dev/ptyp6		dev/ttyp6	→	/tmpfs/dev/ttyp6
   dev/ptyp7	→	/tmpfs/dev/ptyp7		dev/ttyp7	→	/tmpfs/dev/ttyp7
   dev/ptyp8	→	/tmpfs/dev/ptyp8		dev/ttyp8	→	/tmpfs/dev/ttyp8
   dev/ptyp9	→	/tmpfs/dev/ptyp9		dev/ttyp9	→	/tmpfs/dev/ttyp9
   dev/ptypa	→	/tmpfs/dev/ptypa		dev/ttypa	→	/tmpfs/dev/ttypa
   dev/ptypb	→	/tmpfs/dev/ptypb		dev/ttypb	→	/tmpfs/dev/ttypb
   dev/ptypc	→	/tmpfs/dev/ptypc		dev/ttypc	→	/tmpfs/dev/ttypc
   dev/ptypd	→	/tmpfs/dev/ptypd		dev/ttypd	→	/tmpfs/dev/ttypd
   dev/ptype	→	/tmpfs/dev/ptype		dev/ttype	→	/tmpfs/dev/ttype
   dev/ptypf	→	/tmpfs/dev/ptypf		dev/ttypf	→	/tmpfs/dev/ttypf
   tmp	→	/tmpfs/tmp		var	→	/tmpfs/var
   dev/log	→	/var/log/log
   In case you use dhclient also:
   etc/dhclient.conf	→	/tmpfs/var/lib/dhclient.conf		etc/resolv.conf	→	/tmpfs/var/lib/resolv.conf

   
   To place the corresponding directories and device files in the tmpfs file system, the following code is added to the /etc/rc.sh script:

   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/*
   

4. We can now create a cramfs file system image using the mkcramfs tool:

   $ 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.)
   

5. We can use the same setup as before for the JFFS2 filesystem, just changing the bootargument to "rootfstype=cramfs"