Re: [ext2resize] Ext2resize for big-endian (Ultrasparc)

[Andreas D. <ad...@tu...>]

Dave Wapstra writes:
> I recently installed LVM on a Sun E450 and was experimenting with
> ext2resize.  After extending the logical volume, I extended my ext2
> filesystem, at least that was they idea.
> 
> After messing up the filesystem a couple of times, I finally noticed
> the little TODO list for ext2resize:
> 
> misc todo
> ---------
> make it work on big-endian systems.

I'm surprised you were even able to mess up the filesystem!!!  It should
have failed when checking the EXT2_MAGIC in the superblock, because of
endian issues (at least that's what I assumed).  I will have to ensure
that we refuse to do anything on big-endian machines until this works
properly.

> I would like to work on making ext2resize work on big-endian (specifically
> (ultra)sparc), but although I can hack a little C code, I have no idea
> where to start on making ext2resize big-endian compliant.

There are two ways to do this, and I'm not sure which is best:

1) leave the superblock and all on-disk data in little-endian format,
   and convert to machine endianness as needed.  This is done in the kernel,
   but it may be because they are directly mapping on-disk data and have
   no control over when it is written to disk.

2) Convert the superblock and all on-disk data to machine endianness when
   it is read from disk.  This would require new routines for read_super,
   read_GDT, read_inode, etc.  This is what's done in the e2fsprogs
   libext2fs.  There are a few cases (with data that isn't repeatedly
   accessed, like directory entries) where it keeps the data little-endian.

It is worthwhile to hear from Andrew Clausen (parted author and user of
a divergent libext2resize) to see what he has done in this regard.  We
could merge the endianness changes back into libext2resize.

Cheers, Andreas
-- 
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert