RootFileSystemOnARamdisk < DULG

%SECTION0{name=RootFileSystemOnARamdisk}% Root File System on a Ramdisk


Ram disks are used very often to hold the root file system of embedded systems. They have several advantages:
   * well-known
   * well-supported by the Linux kernel
   * simple to build
   * simple to use - you can even combine the ramdisk
     with the Linux kernel into a single image file
   * RAM based, thus pretty fast
   * writable file system
   * original state of file system after each reboot = easy
     recovery from accidental or malicious data corruption etc.
On the other hand, there are several disadvantages, too:
   * big memory footprint:
     you always have to load the complete filesystem into RAM,
     even if only small parts of are actually used
   * slow boot time: you have to load (and uncompress) the whole
     image before the first application process can start
   * only the whole image can be replaced (not individual files)
   * additional storage needed for writable persistent data
Actually there are only very few situations where a ramdisk image is the
optimal solution. But because they are so easy to build and use we will discuss them here anyway.

In almost all cases you will use an =ext2= file system in your ramdisk
image. The following steps are needed to create it:

   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 We use the =genext2fs= tool to create the ramdisk image
     as this allows to use a simple
     text file to describe which devices shall be created in the
     generated file system image.
     That means that no =root= permissions are required at all.
     We use the following device table
     =rootfs_devices.tab=:
     <verbatim>
#<name>    <type> <mode> <uid> <gid> <major> <minor> <start>  <inc>  <count>
/dev            d  755  0       0        -      -       -       -       -
/dev/console    c  640  0       0        5      1       -       -       -
/dev/fb0        c  640  0       0       29      0       -       -       -
/dev/full       c  640  0       0        1      7       -       -       -
/dev/hda        b  640  0       0        3      0       -       -       -
/dev/hda        b  640  0       0        3      1       1       1       16
/dev/kmem       c  640  0       0        1      2       -       -       -
/dev/mem        c  640  0       0        1      1       -       -       -
/dev/mtd        c  640  0       0       90      0       0       2       16
/dev/mtdblock   b  640  0       0       31      0       0       1       16
/dev/mtdr       c  640  0       0       90      1       0       2       16
/dev/nftla      b  640  0       0       93      0       -       -       -
/dev/nftla      b  640  0       0       93      1       1       1       8
/dev/nftlb      b  640  0       0       93      16      -       -       -
/dev/nftlb      b  640  0       0       93      17      1       1       8
/dev/null       c  640  0       0        1      3       -       -       -
/dev/ptyp       c  640  0       0        2      0       0       1       10
/dev/ptypa      c  640  0       0        2      10      -       -       -
/dev/ptypb      c  640  0       0        2      11      -       -       -
/dev/ptypc      c  640  0       0        2      12      -       -       -
/dev/ptypd      c  640  0       0        2      13      -       -       -
/dev/ptype      c  640  0       0        2      14      -       -       -
/dev/ptypf      c  640  0       0        2      15      -       -       -
/dev/ram        b  640  0       0        1      0       0       1       2
/dev/ram        b  640  0       0        1      1       -       -       -
/dev/rtc        c  640  0       0       10      135     -       -       -
/dev/tty        c  640  0       0        4      0       0       1       4
/dev/tty        c  640  0       0        5      0       -       -       -
/dev/ttyS       c  640  0       0        4      64      0       1       8
/dev/ttyp       c  640  0       0        3      0       0       1       10
/dev/ttypa      c  640  0       0        3      10      -       -       -
/dev/ttypb      c  640  0       0        3      11      -       -       -
/dev/ttypc      c  640  0       0        3      12      -       -       -
/dev/ttypd      c  640  0       0        3      13      -       -       -
/dev/ttype      c  640  0       0        3      14      -       -       -
/dev/ttypf      c  640  0       0        3      15      -       -       -
/dev/zero       c  640  0       0        1      5       -       -       -
     </verbatim>
     A description of the format of this table is part of
     the manual page for the =genext2fs= tool, _genext2fs(8)_.
   1 We can 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=ramdisk.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>
   1 Compress the file system image:
     <verbatim>
$ gzip -v9 ramdisk.img
rootfs.img:      55.6% -- replaced with ramdisk.img.gz
     </verbatim>
   1 Create an U-Boot image file from it:
     <verbatim>
$ mkimage -T ramdisk -C gzip -n 'Test Ramdisk Image' \
> -d ramdisk.img.gz uRamdisk
Image Name:   Test Ramdisk Image
Created:      Sun Jun 12 16:58:06 2005
Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
Data Size:    1618547 Bytes = 1580.61 kB = 1.54 MB
Load Address: 0x00000000
Entry Point:  0x00000000
     </verbatim>

We now have a root file system image =uRamdisk= that can be
used with U-Boot.
9.5. Root File System: Design and Building		1. Abstract		9.5.2. Root File System on a JFFS2 File System
Prev		Home		Next