On Fri, Sep 28, 2007 at 03:11:00PM +0200, Erik Mouw wrote:
> There are however ways to confuse it: if you reformat an ext3
> filesystem to reiserfs (version 3), mounting that filesystem without
> "-t reiserfs" will trick mount(8) into mounting it as an ext3
> filesystem (which will usually fail). This is because the ext3
> superblocks lives at offset 0x400, and the reiserfs superblock at
> 0x8000. When you format a partition as reiserfs, it will not erase old
> ext3 superblocks. Before looking for a reiserfs superblock, mount(8)
> first looks for an ext3 superblock. The old ext3 superblock wasn't
> erased, but usually most of the other ext3 structures are and so
> mount(8) will fail to mount the filesystem. Don't know if this
> particular "bug" is still there, but it has bitten me in the past.
This is easy to fix, though. Quoting mkfs.xfs:
/*
* Zero out the beginning of the device, to obliterate any old
* filesystem signatures out there. This should take care of
* swap (somewhere around the page size), jfs (32k),
* ext[2,3] and reiserfs (64k) - and hopefully all else.
*/
buf = libxfs_getbuf(xi.ddev, 0, BTOBB(WHACK_SIZE));
bzero(XFS_BUF_PTR(buf), WHACK_SIZE);
libxfs_writebuf(buf, LIBXFS_EXIT_ON_FAILURE);
libxfs_purgebuf(buf);
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