[linux-lvm] resize of LV causing superblock corruption ( Bad magic number ). how to prevent/fix?

[darx at sent.com]

Bryn,

On Mon, Jul 22, 2013, at 02:58 AM, Bryn M. Reeves wrote:
> I.e. your fdisk program moved the start of the partition. That'll pretty
> much break any file system that has fixed superblock locations.

I completely missed that, as I've never seen it happen before.  Thanks
for catching that.

I suspect my memory's of a procedure that used an older fdisk ... I'll
have to check exactly when the default/minimum became the 'modern' value
== 2048.

> Either use an fdisk that matches the defaults used when the partition
> was first created or ensure you are using sector display units (looks
> like you are - 2048 == 1MiB which is the default used by modern fdisks
> from util-linux) and carefully match the start location when
> deleting/re-adding.

Oddly, when I use 'my' fdisk to 1st create a new partition, it does
default to 2048 minimum sectors -- which, as you point out, is the value
used in my setup.

After the resizing, however, fdisk on the resized volume offers only a
minimum of 4096 sectors.  I.e., I'm unable to *match* the sector start
== 2048.

I've fixed the problem by cloning my source to a set of partitions that
start @4096.  Now, with fdisk on resize able to match the sector start,
my extended partition fsck's just find -- NO superblock issues.

I've no clue yet why fdisk changes its default from 2048->4096 before &
after the resizing.  But, for now I"m back in business.

Thanks for the help!

darx