An Example of Creating a Compressed RAM Disk

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To create a RAM disk image, you will need a spare block device to
construct it on. This can be the RAM disk device itself, or an
unused disk partition (such as an unmounted swap partition). For this
example, we will use the RAM disk device, "/dev/ram".

Note: This technique should not be done on a machine with less than 8 MB
of RAM. If using a spare disk partition instead of /dev/ram, then this
restriction does not apply.

a) Decide on the RAM disk size that you want. Say 2 MB for this example.
Create it by writing to the RAM disk device. (This step is not currently
required, but may be in the future.) It is wise to zero out the
area (esp. for disks) so that maximal compression is achieved for
the unused blocks of the image that you are about to create.

dd if=/dev/zero of=/dev/ram bs=1k count=2048

b) Make a filesystem on it. Say ext2fs for this example.

mke2fs -vm0 /dev/ram 2048

c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
and unmount it again.

d) Compress the contents of the RAM disk. The level of compression
will be approximately 50% of the space used by the files. Unused
space on the RAM disk will compress to almost nothing.

dd if=/dev/ram bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz

e) Put the kernel onto the floppy

dd if=zImage of=/dev/fd0 bs=1k

f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
that is slightly larger than the kernel, so that you can put another
(possibly larger) kernel onto the same floppy later without overlapping
the RAM disk image. An offset of 400 kB for kernels about 350 kB in
size would be reasonable. Make sure offset+size of ram_image.gz is
not larger than the total space on your floppy (usually 1440 kB).

dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400

g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
have 2^15 + 2^14 + 400 = 49552.

rdev /dev/fd0 /dev/fd0
rdev -r /dev/fd0 49552

That is it. You now have your boot/root compressed RAM disk floppy. Some
users may wish to combine steps (d) and (f) by using a pipe.