Overview

• Ext2 Ramdisks

• Cramfs

• Ramfs

Working with Ext2 Ramdisks

First, when we talk about a ramdisk, we're usually talking about an image of an ext2 filesystem and not necessarily the ramdisk device driver. We often use the ramdisk device driver, copying in the ramdisk image, mounting the filesystem, etc., but we could also use the flash block device driver or the loopback driver. These instructions tell how to use the loopback device to access an ext2 image, in which case ramdisk is not really involved at all.

The filesystem image files found on ftp.handheld.org are compressed. Before you can modify them, you must uncompress them using gunzip.

Given an uncompressed filesystem image, you can mount it either through a ramdisk device or through the loopback filesystem type. Via ramdisk:

        mkdir mnt
        cat init-ramdisk-2-35 > /dev/ram1
        mount /dev/ram1 mnt

        mount -o loop init-ramdisk-2-35 mnt

In both of these cases, we omitted the '-t fstype' option from mount because it will read the superblock to determine the filesystem type. I've found that I often need to supply the type when mounting fat filesystems, but maybe that's just me.

Finally, we almost always compress the image before making copies of it, and we install the compressed filesystem image for the initial ramdisk, because the initrd driver knows how to recognize and unzip compressed filesystem images. We use the highest level of compression to minimize xmodem download times.

A small note: if you did some work (copied, moved, deleted files), the ramdisk will have some dirty blocks. Those blocks are not in use by the ext2 filesystem, but they still have the original file contents, and hence aren't that easy compressable as empty blocks (i.e. blocks only containing zeros). To get a smaller compressed image, you can copy the contents of the ramdisk to a clean ramdisk. Create a second ramdisk (or loopback device) and mount it. Suppose we have the original ramdisk mounted on /tmp/orig and the new one on /tmp/new, we just copy the contents:

  cd /tmp/orig
  tar cf - . | ( cd /tmp/new ; tar xpf - )

• Modifying an existing ext2 filesystem image

  • gunzip ramdisk-file.gz
  • mkdir temp
  • mount -o loop ramdisk-file temp
  • cd temp
  • (Do what you need to do.)
  • cd ..
  • umount temp
  • gzip -9 ramdisk-file

• Creating a new ext2 filesystem image

  • dd if=/dev/zero of=init-ramdisk.img bs=1k count=8k
  • mke2fs -i 1024 -b 1024 -m 3 -F -v init-ramdisk.img
  • mkdir temp
  • mount -o loop init-ramdisk.img temp
  • cd temp
  • (Do what you need to do.)
  • cd ..
  • umount temp
  • gzip -9 init-ramdisk.img

• Creating an ext2 filesystem on a device

      mkfs -t ext2 /specialfile
      

  where specialfile is something like /dev/ram1 (for a ramdisk), /dev/flash3 (for a flash device), /dev/fd0 (for a floppy), or init-ramdisk.img (for an image of a filesystem stored in a pre-existing file). The example above makes a filesystem onto an 8MByte file. This is the equivalent of formatting the drive. Instead of ext2, we could use another filesystem type, such as reiserfs, fat, etc.

  
  

• Going from a device to a file

      dd if=specialfile of=imagefile

  
  will read the entire special file into a plain file, e.g.:

      dd if=/dev/ram1 of=init-ramdisk-2-35

  
  But remember to unmount it before you extract it, or it can be out-of-sync (i.e.: the kernel hasn't flushed all data to disk yet). And of course the "mounted" bit is set if it is still mounted, so the next time you mount it you'll get a warning about dirty filesystems.

Working With Cramfs

Accessing cramfs is similar to ext2 ramdisk, except that cramfs is a readonly filesystem. It's new to 2.4.x so most 2.2.x-based development machines do not support it. Since cramfs is readonly, you cannot 'mkfs -t cramfs' and then populate. The mkcramfs utility creates an image of a cramfs filesystem from a directory tree.

• Linux 2.4.x Desktop Computer

  • mkdir mntpoint
  • mkdir temp
  • mount -t cramfs init-2-40.cramfs mntpoint -o loop
  • cp -a mntpoint temp
  • (do what you need to do)
  • cd ..
  • mkcramfs temp init-2-41.cramfs
  • Get into the boot loader and load the file where it belongs.

• Linux 2.2.x Desktop Computer

  Since 2.2 does not support cramfs filesystems, you cannot mount the cramfs filesystem. You will need the 'src' tar ball, e.g. root-src-2-40.tar.gz
  • tar xfz root-src-2-40.tar.gz
  • cd root
  • (do what you need to do)
  • cd ..
  • mkcramfs root root-src-2-41.cramfs
  • Get into the boot loader and load the file where it belongs

Working With Ramfs

Ramfs is new to 2.4.x. It is a filesystem which keeps all files in RAM. It allows read and write access. Ramfs grows and shrinks to accomodate the files it contains, in contrast to a 'ext2 ramdisk', which gets allocated a fixed amount of RAM. To use ramfs, type:

• mkdir /mnt/mntpoint
• mount -t ramfs ramfs /mnt/mntpoint

Questions?

Please post any questions to handhelds@handhelds.org. Thank you.