Locking a file backed mdconfig into memory

[John Baldwin]

On Monday 31 May 2010 2:45:25 pm Jeremy Chadwick wrote:
> On Mon, May 31, 2010 at 11:25:51AM -0700, Dave Hayes wrote:
> > Jeremy Chadwick <freebsd at jdc.parodius.com> writes:
> > > Is the mfsroot file compressed (.gz extension)?  Reason I ask is that
> > > the OP states he's using RELENG_7...
> > 
> > Yes it is compressed. 
> > 
> > > http://jdc.parodius.com/freebsd/pxeboot_serial_install_7.html#step7
> > 
> > Thanks much for this. I did a simple test, I rebuilt a DVD that wasn't
> > booting to use a lower level of compression (gzip -9 to gzip -6) on
> > mfsroot without changing anything else. This caused it to boot normally.
> > 
> > I'm not sure it's conclusive evidence, but it certainly looks like a
> > weak datapoint supporting this kernel bug being the source of my
> > problem.
> > 
> > Is this problem fixed in 8.0 or by a patch?
> 
> With regards to said bug, gzip compression seems to work fine on
> RELENG_8, at least in my experiences:
> 
> http://jdc.parodius.com/freebsd/pxeboot_serial_install_8.html
> 
> I'm not sure the level of compression is what triggers the bug though;
> I haven't tested all levels (1 through 9).
> 
> CC'ing jhb@ since he last updated PR kern/120127 (which I would say is
> still a problem on RELENG_7 :-) ).  John, are you aware of any gzip
> decompression / mfsroot changes which might explain the problem on
> RELENG_7?  I haven't done a "thorough" series of tests, but on my
> testbed boxes RELENG_8 works fine with a gzip'd mfsroot.

Ok, if you are using a stock mfsroot from a release build, that should work 
fine.  If you have built a custom mfsroot that is larger, then you may need to  
increase NKPT on i386.  In very recent 7 and later you can do this by setting 
it to a new value in your kernel config.  In older versions you can do this by 
manually adding a #define to set a new value of NKPT in opt_global.h or 
hacking on the source directly.

-- 
John Baldwin