I am looking for information about the maximum size (if there is one)
for an initrd image.

I have been trying some things with loading one and a small 4 MB image
works just fine (run as 4 MB ramdisk with virtually no free space).

The same 4 MB image contained inside an actual 40 MB ramdisk (runs as
4 MB of data with about 36 MB of free space) will also work fine.

But if I take that larger sized ramdisk and fill it with more files to
make it more like 30 MB .. so it now (should run) as the 40 MB ramdisk
with about 10 MB free space, everything falls apart. First attempt
the Kernel wouldn't even load. Later (not quite sure what I did) I
got the Kernel to load but right after it starts up I get a Kernel
message :

initrd overwritten (0xXXXXXXXX < 0xXXXXXXXX) - disabling it.

And then when it gets to the point were it tries to mount the initrd
root, it fails saying it can't mount it.

Repeating this last scenario but only filling the ramdisk to about 16 MB,
it will work OK. So somewhere between 16 and 32 MB it stops working.

So it appears that I am hitting some kind of size limit (?) for the
initrd image size. The kernel is installed with RAMDISK=42000 so
the actual ramdisk is large enough.

Seems that usually you hear of initrd images being rather small.
Is there a size limit I am hitting?

There is no limit. Of couse it can't be bigger than the amount of ram you have.

The default size of the ram disk in slack is 16,384. To use an image that is bigger than that pass ramdisk_size=? to the kernel.

In case your are curious were I got 16,384 from...

Just so you know I didn't pull it out of thin air. If you compile a new kerenl you can make this number anything you want but making it to big is not a good idea. Wastes memory because the ram disk memory once loaded can never recovered until reboot.

well.........is that so ?
from kerneldocumentation/initrd.txt
-----------
<snip>
Now, the initrd can be unmounted and the memory allocated by the RAM
disk can be freed:

# umount /initrd
# blockdev --flushbufs /dev/ram0 # /dev/rd/0 if using devfs
<snip>
---------

egag

If you don't pass the RAMDISK size, RAM will be dynamically allocated. If you want to load other stuff into RAM, create another RAMDISK for it in /dev/ram2.

Note that the when you are in the PPC archicture, your initrd has a maximum file size, but I can't remember the size right now.

Regards
Breno Leitão