How To Recover From Creating >2TB ext3 Filesystem on MSDOS Partition Table?

[Jon Forrest]

Ling C. Ho wrote:
> Can u recreate your sdb1 using parted, but specifying a different end 
> size, or just use "-1" ? And maybe try changing the label to "gpt" ? 
> Then run e2fsck -n and see what it does.

I'll add this to the small collection of suggestions. I clearly
have to be very careful in what I do to restore this because
I'll probably only have one chance.

> I wonder how you were able to 
> create a 4TB ext3 filesystem with the msdos label under SL4.3. Never 
> worked for me without the labelling it gpt.

There are two mysteries in my mind - 1) how the file system was allowed
to be created, and 2) what was the exact scenario that caused
the corruption, i.e. what is it about an msdos partition table
that causes problems when a file system is >2TB.

As for #1, I didn't create the file system. This is on a cluster
that I recently took over managing. The file system was created
before I started here. However, the person who did it is quite
knowledgeable. Since it was done on a system running Scientific
Linux 4.3, which is based on a fairly old kernel and tools,
I'm wondering if the tools didn't recognize the dangerous
configuration. Ted Ts'o was surprised to hear about this himself.

Regarding #2, there are a number of places where very knowledgeable
people describe the danger in creating >2TB file systems on msdos
partition tables but I haven't seen an explanation of the fundemental
problem. I would love to learn this (I'm not doubting that it's true).

Cordially,

-- 
Jon Forrest
Unix Computing Support
College of Chemistry
173 Tan Hall
University of California Berkeley
Berkeley, CA
94720-1460
510-643-1032
jlforrest at berkeley.edu