How to Create/Modify an RAM disk Image

How to Create/Modify an RAM disk Image

Glomation GESBC-93XX series single board computers use RAM disk as the root file

system in it’s default configuration. Most likely one would want to create/change the

RAM disk to have user applications installed. This Document describes how to mount

and edit files in a RAM disk image, unmount the image and recompress it.

System Requirement

In order to mount a RAM disk image, your development system must have MTD support

compiled in, and have the Loopback Interface available. Most Linux distribution have

these configured in the kernel.

To check the availability of MTD and loopback interface, issue the following command

on the development system

# cat /proc/mtd

or

# ls -l /dev/mtd*

If any information is shown with /proc/mtd, and if you see any files named

mtdblock, mtd, etc that means the system has MTD support.

If not please rebuild the Linux kernel with MTD support. The follow instruction shows

the basic steps to do that.

Go to the kernel source tree directory on your development system and issue...

# make menuconfig

The following options must be enabled to use MTD.

Memory Technology Devices (MTD) --->

<*> Memory Technology Device (MTD) support

[*] Debugging

(0) Debugging verbosity (0 = quiet, 3 = noisy)

[*] MTD partitioning support

<*> Direct char device access to MTD devices

<*> Caching block device access to MTD devices

RAM/ROM/Flash chip drivers --->

<*> Detect flash chips by Common Flash Interface (CFI)

probe

<*> Support for Intel/Sharp flash chips

<*> Support for AMD/Fujitsu flash chips

Self-contained MTD device drivers --->

<*> MTD emulation using block device

GESBC-9315 User’s Manual

Version 0.2 Page 2 of 3 19-Apr-06

As well as....

Block devices --->

<*> RAM disk support

(32768) Default RAM disk size (kbytes)

[*] Initial RAM disk (initrd) support

After the kernel has been reconfigured, re-build the kernel, install the new kernel and

reboot your development system.

Once you have MTD and RAM disk support on your development system, you are ready

to create/modify the RAM disk image.

Create New RAM Disk Image

The Ext2 file system can be created and mounted onto an embedded device using the

concept of RAM disk.

mke2fs -vm0 /dev/ram 16384

mount -t ext2 /dev/ram /mnt

cd /mnt

cp /bin, /sbin, /etc, /dev ... files in mnt

cd ../

umount /mnt

dd if=/dev/ram bs=1k count=16384 of=ramdisk

gzip -9 ramdisk

mke2fs is the utility used to create an ext2 file system -- creating the super block, inodes,

inode table, and etc -- on any device.

In the above usage, /dev/ram is the device on which an ext2 file system of 16384 blocks

is built. Then the device (/dev/ram) is mounted on a temporary directory named /mnt and

all the necessary files are copied. Once these files are copied, the file system is

unmounted and the contents of the device (/dev/ram) is dumped into a file (ramdisk),

which is the required RAM disk (the Ext2 file system). The RAM disk file is then

compressed and ready to be loaded by Redboot.

The above sequence creates a RAM disk of 16 MB and fills it with the necessary file

utilities.

Some of the important directories to include in RAM disk are:

/bin -- Holds most of the binaries like init, busybox, shell, file management utilities, and

etc.

/dev -- Contains all the device nodes used in the device

GESBC-9315 User’s Manual

Version 0.2 Page 3 of 3 19-Apr-06

/etc -- Contains all the configuration files for the system

/lib -- Contains all the necessary libraries like libc, libdl, and so on

Modify Existing RAM Disk Image

1. First uncompress the ramdisk.gz file.

# gunzip ramdisk.gz

2. Make a directory to mount the ramdisk image.

# mkdir ramdisk_dir

3. Mount the ramdisk image as a loop back device

# mount -o loop ramdisk ramdisk_dir

You can then issue the command 'df' to see the new RAM disk partition on you

development system.

4. Edit Files

You can now edit files inside the RAM disk. Depending on how the original RAM disk

is created, you may or may not have enough space on the RAM disk to install your

application. Refer to Create New RAM Disk Image section on how to create a new RAM

disk with more storage space.

5. Unmount the ramdisk and compress it.

# umount ramdisk_dir

# gzip -9 ramdisk

You will then have a new compressed root file system that you can now load with

RedBoot