Ramdisk support

[Oliver Fromme]

Aditya Godbole wrote:
 > Oliver Fromme wrote:
 > > You can also put the image of the root file system into the
 > > kernel itself, so it doesn't have to be loaded separately.
 > > The kernel option to allocate appropriate space is called
 > > MD_ROOT_SIZE.
 > 
 > Thanks. I'm investigating this option. How do I specify the filesystem
 > image that should be put into the kernel?

You don't.  It's not necessary.  MD_ROOT_SIZE only reserves
space in the kernel image.  Then you can use strings(1) to
find the offset in the kernel image, and then write your
image into the reserved space with standard shell tools.

For example, suposse your root FS image is 5 MB:
# ls -l rootfs.img
-rw-------  1 root  wheel  5242880 Nov  9 15:34 rootfs.img

Then add these lines to your kernel configuration:
options  MD_ROOT
options  MD_ROOT_SIZE=5120	# number of KBytes

Build the kernel as usual.  It will now contain 5 MB of
free space, and you can find the offset with strings(1):
# strings -td kernel | grep "MFS Filesystem"
4505376 MFS Filesystem goes here
9748256 MFS Filesystem had better STOP here

The first number is the start offset, the second is the
end offset.  The difference should be exactly 5 MB in this
case (5252880 bytes), which is the size of our root FS
image.

Now construct a new kernel binary that contains the root FS
image at the right place.  You can use dd(1), but it is
probably easier to use head+cat+tail:
# head -c 4505376 kernel > kernel.new
# cat rootfs.img >> kernel.new
# tail -c +9748257 kernel >> kernel.new

Note that the tail(1) number must be one more than the end
offset, because tail(1) starts numbering bytes with 1, not
with 0.  Verify that the resulting "kernel.new" file has
exactly the same size as the original kernel.  Of course,
you can automate the whole process easily with a shell
script.

That's all.  If you're going to mount the root FS read-only,
you can also compress the image with mkuzip(8) (requires
geom_uzip(4) to be present in the kernel), to save space.

 > > Then where are you booting from?  At least your kernel has
 > > to come from somewhere, i.e. you need either networking or
 > > some kind of media (disk, USB stick or similar).
 > 
 > Currently I'm using u-boot to get the kernel image from the network
 > and boot it. Since I'm using u-boot, I proabably need to do something
 > in my kernel bootstrap code that the loader would do otherwise.

I don't know u-boot.  What is that?

Best regards
   Oliver

-- 
Oliver Fromme,  secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
Dienstleistungen mit Schwerpunkt FreeBSD: http://www.secnetix.de/bsd
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and may not necessarily reflect the opinions of secnetix in any way.

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