[ale] Finding filesystems on a disk

[mark at vielle.datasys.net (Mark R. Lindsey)]

Howdy, all you filesystem fans. I'd appreciate your thoughts.

I recently witnessed the corruption of the partition table on a hard
drive. I know that, in general, it looked like this:

	hda1	Empty			No file system
	hda2	Linux root		ext2
	hda3	Linux swap		Linux swap
	hda4	Extended Partition
	hda5	Linux /usr		ext2
	hda6	Linux /var		ext2

hda is about 2 gig. I believe that Linux was originally installed using
the default settings for the drive in the BIOS, including LBA. I need to
rebuild that partition table enough to find those filesystems.

I wrote a script to make a partition between N and the end of the drive
(logical cylinder 1023), then attempt to mount a ext2 filesystem on that
partition, read only. I was fortunate that the write, sync, and ioctl()
lets the kernel reread the partition table properly, and I found one
of those filesystems using that method. 

However, it found only one, and I'm wondering what other ways I could try
to find the filesystems that are still on there. My two ideas are as follows:

 - Write a scanner that'd read through hda looking for the signature of
 ext2 filesystems (is this possible logically?)

 - Maybe the drive wasn't originally LBA, and it just happened that one
 of the partitions started on an LBA-addressable cylinder. The data on
 that partition was fine, and easily readable -- is it possible that
 using LBA you could mount a partition whose data wasn't made using LBA?

It's possible that I didn't properly implement the idea of scanning for
filesystems using fdisk and mount -- your thoughts there would be helpful.

Thanks,
Mark