RootFileSystemOnAReadOnlyExt2FileSystem < DULG

%SECTION0{name=RootFileSystemOnARead-OnlyExt2FileSystem}% Root File System on a Read-Only ext2 File System

When storing the root file system in on-board flash memory
it seems only natural to look for special flash filesystems
like JFFS2, or for other file system types that are designed
for such environments like cramfs.
It seems to be a bad idea to use a standard =ext2= file system
because it contains neither any type of wear leveling which
is needed for writable file systems in flash memory,
nor is it robust against unorderly shutdowns.

The situation changes if we use an =ext2= file system which we mount
*read-only*. Such a configuration can be very useful in some situations.

Advantages:
   * very fast
   * low RAM memory footprint

Disadvantages:
   * high flash memory footprint because no compression

To create an =ext2= image
that can be used as a read-only root file system
the following steps are necessary:
   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 Like with the =cramfs= root file system, we use
     ="tmpfs"= for cases where a writable file system is
     needed and 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>
     We also create the same symbolic links for device files
     that must be placed in a writable filesystem:
     %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 Like we did for the ramdisk,
     we now create an =ext2= file system image using the
     =genext2fs= tool:
     <verbatim>
$ ROOTFS_DIR=rootfs                 # directory with root file system content
$ ROOTFS_SIZE=3700                  # size of file system image
$ ROOTFS_FREE=100                   # free space wanted
$ ROOTFS_INODES=380                 # number of inodes
$ ROOTFS_DEVICES=rootfs_devices.tab # device description file
$ ROOTFS_IMAGE=ext2.img             # generated file system image

$ genext2fs -U \
        -d ${ROOTFS_DIR} \
        -D ${ROOTFS_DEVICES} \
        -b ${ROOTFS_SIZE} \
        -r ${ROOTFS_FREE} \
        -i ${ROOTFS_INODES} \
        ${ROOTFS_IMAGE}
     </verbatim>
     <verbatim>
     </verbatim>
   1 We can again use the same setup as before for the JFFS2 filesystem,
     just changing the boot argument to ="rootfstype=ext2"=
     (or simply omit it completely as this is the default anyway),
     and we must change the ="rw"= argument into ="ro"= to mount
     our root file system really read-only:
     <verbatim>
...
Linux version 2.4.25 (wd@xpert) (gcc version 3.3.3 (DENX ELDK 3.1.1 3.3.3-9)) #1 Sun Jun 12 18:32:18 MEST 2005
On node 0 totalpages: 4096
zone(0): 4096 pages.
zone(1): 0 pages.
zone(2): 0 pages.
Kernel command line: root=/dev/mtdblock6 ro rootfstype=ext2 ip=192.168.3.80:192.168.3.1::255.255.255.0:tqm860l:eth1:off panic=1
Decrementer Frequency = 187500000/60
Calibrating delay loop... 49.86 BogoMIPS
...
     </verbatim>
9.5.3. Root File System on a cramfs File System		1. Abstract		9.5.5. Root File System on a Flash Card
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