Locking a file backed mdconfig into memory

[Charles Sprickman]

On Tue, 1 Jun 2010, John Baldwin wrote:

> 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.

This is the original post I found confirming all this:

http://unix.derkeiler.com/Mailing-Lists/FreeBSD/hackers/2004-02/0241.html

I can also confirm your changes that allow it to be a kernel config option 
works as well. :)

It's nice to essentially have a full livefs as your mfsroot for 
non-sysinstall installs.

Charles

> -- 
> John Baldwin
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