Locking a file backed mdconfig into memory

[Jeremy Chadwick]

On Fri, May 28, 2010 at 12:26:12PM -0700, Dave Hayes wrote:
> Jeremy Chadwick <freebsd at jdc.parodius.com> writes:
> > And I think you meant this for variable names, in addition to what
> > vfs.root.mountfrom should be (specific to RELENG_8):
> > mfsroot_load="YES"
> > mfsroot_type="mfs_root"
> > mfsroot_name="/some/path/mfsroot"
> 
> I'm using RELENG_7, but it seems rootfs_* works just like mfsroot_* ...
> is the former deprecated? 

I spent a small bit of time looking through both RELENG_7 and RELENG_8
source (/usr/src) as well as /boot (for the Forth portion) and I was
unable to find any reference to "rootfs" aside from actual variable
names (not loader variables) in C code or include files.  None were
strings.  There's a strong possibility I'm not looking the right way
(grep -ri ...).

I can assure you that mfsroot_xxx works on both platforms because I
wrote the below documentation:

http://jdc.parodius.com/freebsd/pxeboot_serial_install.html

> > vfs.root.mountfrom="ufs:/dev/md0"
> 
> Hm, 'ufs:md0' currently works. What trouble can be had from using
> the abbreviated device name?

I have no idea; probably none.  This is just the first time I've seen
the variable set to something "shorthand".  I would have to review
the parser code to see how it works.

> > If using RELENG_7 and the mfsroot was made on RELENG_7, replace
> > "/dev/md0" with "/dev/md0c".
> 
> Is there a reason for doing this? 

Yes, the "c" slice is gone with present-day GEOM.  I'm fairly certain
this is also the reason for the "geometry does not match label (255h,63s
!= 16h,63s)" message seen when running RELENG_8 with a disk which was
labelled using an older GEOM (e.g. RELENG_7).

This also applies to mfsroot images because all they are is raw disk
images that are literally loaded into memory.

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |