How to create a ramdisk image using existing FS

How to create a ramdisk image using existing FS

1. Regular method
=================

Create an empty ramdisk image
-----------------------------

Enter your working directory, excute
#dd if=/dev/zero of=./ramdisk bs=1M count=2000
This command create a image named ramdisk about 2000M

Then format it to be a ext2 FS
#mke2fs -F -v ramdisk

Now ramdisk is formatted to an ext2 ramdisk image, even there is almost nothing in it.


Mount the empty ramdisk to a dir
--------------------------------

In your working directory , create a directory,
#mkdir tmp

Then mount the ramdisk image to this directory,
#sudo mount -o loop ramdisk tmp

Now the tmp directory is your rootfs, you can copy your real rootfs to it.
For example:

cp -r /media/wr8/realfs/* tmp/


Finish your ramdisk creating
----------------------------
#sync
#umount tmp

Now the ramdisk image holds your real fs in it.
You can compresss it at this time:
gzip -v9 ramdisk

After this command, a file named ramdisk.gz is gernerated.

2. CPIO method
=================
For a directory which hold a rootfs, using the following command to
create a ramdisk:


#cd path_to_working_dir/root_dir
#sudo find . | cpio -o -H newc > ../ramdisk
#cd ..
#gzip -v9 ramdisk

Here path_to_working_dir/root_dir holds the rootfs.
The ramdisk.gz is generated in path_to_working_dir.

when you boot ramdisk generated by cpio, there are two point to emphasize:
1. If there is no "init" file in "path_to_working_dir/root_dir", you should add
   "rdinit=xxx", e.g. "rdinit=/sbin/init" to the boot command line.
2. The "root=/dev/ram0" and "ramdisk_size=xxx" in the boot command line are not necessary.


another method for creating cpio format ramdisk:
#cd working_dir
#cp linux_souce/scripts/gen_initramfs_list.sh ./
#mkdir usr
#cp linux_build/usr/gen_init_cpio ./usr/
#./gen_initramfs_list -o cpio.gz -u 0 -g 0 root_dir

Here root_dir is the directory holding the rootfs , and the cpio.gz is the generated initramfs image
in gzipped format.