Writing to superblock?

Writing to superblock?

[BandiPat]

Quickly getting the devs of Zenwalk convinced to move over to Grub2 it 
seems.  They are now busy working on a script that will help the user 
set up Grub2 when installing Zenwalk!  Let me just say I'm very pleased 
with their enthusiasm over converting from Lilo to Grub2.

Anyway, the writer of the script ask me a question I have not been able 
to find anything about, so I thought I would come straight to you guys! 
  I already know that Grub2 will work with XFS and will write to the 
MBR, but he is asking about Superblocks also when using XFS.  Will it? 
Any special instructions needed to do that?  I hope I am asking the 
right question.  Just let me know if you need more clarification.

TIA,
Pat


-- 
        ---Zenwalk v6.0--Linux 2.6.28---
         Registered Linux User #225206
"Ever tried Zen computing?"  http://www.zenwalk.org

Re: Writing to superblock?

[Niels Böhm]

On Friday 20 February 2009, BandiPat wrote:
>
> Anyway, the writer of the script ask me a question I have not been able
> to find anything about, so I thought I would come straight to you guys!
>   I already know that Grub2 will work with XFS and will write to the
> MBR, but he is asking about Superblocks also when using XFS.  Will it?

If I understand you correctly, you talk about writing GRUB to a root sector of 
a partition that contains a XFS filesystem (which happens to coincide with 
the start of the superblock of XFS' first allocation group), right?

I mean the traditional way to execute raw boot code from a partition is to 
chainload its first sector and jump to its first byte, which means there has 
to be boot code at this place or at the very least a jump instruction that 
points to the real boot code.

But I don't think this is possible with XFS since it has its magic number in 
the first 4 bytes. It looks like this:

00000000  58 46 53 42 00 00 10 00  00 00 00 00 00 20 00 00  |XFSB......... ..|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020  19 c5 80 51 80 34 4a c9  b6 aa c5 44 0b 16 c7 7b  |...Q.4J....D...{|
etc.

For this, also have a peek at 
http://xfs.org/index.php/XFS_FAQ#Q:_Does_LILO_work_with_XFS.3F


The alternative would be to find another unused cave with stable location in 
XFS' metadata, although I doubt there is one. But you may want to check more 
deeply at http://oss.sgi.com/projects/xfs/papers/xfs_filesystem_structure.pdf


Regards,
Niels Böhm

Re: Writing to superblock?

[Robert Millan]

On Fri, Feb 20, 2009 at 02:30:58PM -0500, BandiPat wrote:
>
> Anyway, the writer of the script ask me a question I have not been able  
> to find anything about, so I thought I would come straight to you guys!  
> I already know that Grub2 will work with XFS and will write to the MBR, 
> but he is asking about Superblocks also when using XFS.  Will it? Any 
> special instructions needed to do that?  I hope I am asking the right 
> question.  Just let me know if you need more clarification.

This is highly discouraged and won't work at all in some cases (I think XFS
is one of those).  Just install to MBR instead.

-- 
Robert Millan

  The DRM opt-in fallacy: "Your data belongs to us. We will decide when (and
  how) you may access your data; but nobody's threatening your freedom: we
  still allow you to remove your data and not access it at all."