Message-ID: <46A31F55.2000009@gmail.com>
Date:	Sun, 22 Jul 2007 11:11:49 +0200
From:	Rene Herman <rene.herman@...il.com>
To:	Theodore Tso <tytso@....edu>, Rene Herman <rene.herman@...il.com>,
	Al Boldi <a1426z@...ab.com>, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFH] Partition table recovery

On 07/22/2007 03:11 AM, Theodore Tso wrote:

>> This is a problem. Today the CHS fields in the partition entries don't
>> mean much of anything anymore and Linux happily ignores them but DOS
>> and (hence) Windows 9x do not. From time to time I still have the
>> Windows 98 install that's sitting in a corner of my disk throw a fit
>> just by having set the BIOS from LBA to Large (meaning the geometry the
>> BIOS pretends the disk has changes) for example. Old DOS installs that
>> I keep around for the purpose of hardware testing with the originally
>> supplied drivers make for even more of a "don't touch, don't touch!"
>> thing -- various version of DOS throw fits for various reasons.
> 
> This is true, but that's due to the fundamentally broken nature of CHS.
> You need them to boot, and that's about it.  I will say up front that I
> don't particularly care about legacy operating system such as DOS,
> Windows 98, or Minix 3.  So the idea of simply having the number of heads
> and sectors in the partition header is that we can reconstruct CHS fields
> such that it is likely with modern hardware you will get it right.

Well, I still don't believe this all to be a great idea but it was sort of 
fun so the attached does largely what you want -- build a list of all data 
partitions.

The heads/sectors fields it for now just gets from the HDIO_GETGEO call. A 
better source would be guessing the values from the partition table itself 
but that _also_ doesn't make too much sense. If you're reconstructing a 
sanitized version of the table anyway, it makes better sense to reconstruct 
it with the values HDIO_GETGEO returns at restoration time.

I kept your suggested format, but in fact, the 64-bit "start" value seems 
not very useful if we're getting the value from a 32-bit field in the old 
partition tables anyway. With that shrunk down to 32-bit again, there would 
be enough room for the complete partition table entry...

> For ancient systems that do all sorts of weird things such as ECHS,
> etc., yeah, you're pretty much doomed, and the bigger danger comes
> from futzing with BIOS settings, et. al.  But it's 2007, gosh darn it!
> "Here's a quarter, kid, buy yourself a real computer".  :-)

Thanks, but real computers won't host my ISA cards...

> Yes, I'm very aware of the extended partitioning scheme mess.  What I
> was proposing to back up here is only the real partitions, not the
> fake extended partitions.  The idea is to store *just* enough
> information so that a partition table manager can recover the
> partition tables in such a way that the original filesystem
> information can be recovered.

This should do I guess. It enters all data partitions into the list, in the 
order in which they are encountered and sets a flag to signify that a 
partition was a logical rather than primary. Another option would be to just 
reserve the first 4 entries for the primaries and the rest for the logicals 
but this saves entries if there are fewer than 4 primaries and was in fact 
easier...

The program enters partitions in what should be the same order as Linux 
itself does. Primaries from slot 0 to 3 as normal (but not backed up to 
entry 0 to 3 as said -- the LOGICAL flag indentifies them), extended 
partitions in the MBR in the order as encountered, with the logicals in the 
second-level table as encountered, and following only the first extented in 
the second-level table.

Made it into a generic C program -- didn't look at e2fsprogs sources yet.

Need to be off now and haven't yet stared at this as long as I'd like so 
don't slap me if I've left a few bugs in (although it seems to work nicely). 
The program dumps the backup sector to stdout -- it's ofcourse easy to 
change it to print the entries out so they're easy to compare against, say, 
"fdisk -l -us".

Oh, and once you've looked at it, please throw it away. As said, I still 
don't think it's a great idea ;-)

Rene.


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