ext3 + fs > 2Tbyte

[Andreas Dilger]

On Nov 02, 2005  00:37 +1100, Vincent.McIntyre at csiro.au wrote:
> I tried to make a copy of the first part of the filesystem with dd;
> 
>   # dd if=/dev/sdb1 of=/tmp/sdb1.dd bs=1 count=16384 \
>       conv=noerror,sync,notrunc
> 
> This returned a file supposedly 16384 bytes long , but it didn't make
> much sense - looking at it with 'od' or 'hexdump' I get only 17 lines
> of output, not the roughly 178 I get for the same exercise with a good
> ext3 filesystem. (The /tmp filesystem has 128-byte inodes.)

"od" will compress lines that are identical (usually all-zero) as "*".
If you want all the output, use -v.

> The output appears to be just the EFI GPT partition label.

The EFI GPT label can be restored from the backup (which is located
at the end of the device) so that might have happened.

> I'm starting to suspect something in the raid device is in a strange
> state. Or that the whole filesystem has just totally disappeared. :(

od -Ax -tx4 /dev/sdb1 | grep "^[0-9a-f]30 [0-9a-f]* [0-9a-f]* 000[1-3]ef53 "

should locate the ext2 superblock magic number(s) eventually.  There is
also a utility in e2fsprogs source (misc/findsuper) that is not installed
that you could build that does this more efficiently.

If those don't appear anywhere, then something dramatically bad has
happened to your filesystem.  Aliasing would only damage at most (if
you did "dd if=/dev/zero" into a file at the end of the filesystem)
the first 300GB of your device, and there _should_ be a backup super
somewhere beyond that (haven't done math to confirm).

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.