[Soekris] Smallest possible MFS /dev on OpenBSD

[Jean-Yves Boisiaud]

 > I've got OpenBSD 4.2 on a net4801 and am mounting /dev in RAM.
 > I'd like
 > to reduce the amount of memory used even further.
 > Currently only %4 is
 > used:
 > 	Filesystem     Size    Used   Avail Capacity  Mounted on
 > 	mfs:21800      853K   34.0K    777K     4%    /dev
 >
 > What should I be aware of in /etc/fstab to minimize the memory
 > footprint of /dev?  Currently, I have:
 > 	swap /dev mfs rw,-P/dev.base,-i1,-s2300,-c264,-m5,nosuid 0 0

I think that ramfs is dynamic, ie physical memory is not reserved during 
mount_mfs execution.

To reduce the /dev memory fs space :
- you can reduce the inodes density with -i. Default is one inode for 
8196 bytes.
- delete unused devices.

Here is the command I use on my Soekris targets :
# mount_mfs -i 256 -s 592 -P /var/run/dev swap /dev

And the result is :
# df -h /dev
Filesystem     Size    Used   Avail Capacity  Mounted on
mfs:31196     23.0K    6.0K   16.0K    27%    /dev

Number of files in /dev:
# ls /dev | wc -l
307

 >
 > Random changes to mfs' parameters aren't getting me anywhere.  There
 > should be a rational way of arriving at optimal settings for mfs:
 >
 >     -b block-size
 >     -c fragments-per-cylinder-group
 >     -f frag-size
 >     -i bytes
 >     -m free-space
 >     -s size