[Linux-cluster] Unformatting a GFS cluster disk

[Lon Hohberger]

On Tue, 2008-03-25 at 16:54 -0500, Bob Peterson wrote:
> On Tue, 2008-03-25 at 18:47 +0000, DRand at amnesty.org wrote:
> > 
> > Help! 
> > 
> > We have accidentally formatted a GFS cluster with this command. 
> > 
> > mkfs.gfs -J 1024 -j 4 -p lock_gulm -t aicluster:cmsgfs /dev/sda 
> > 
> > We were using the disk as a backup system. We need to unformat it..
> > Are there any tools that will let us get at the disk directory? 
> > 
> > Damon. 
> 
> Hi Damon,
> 
> The short answer is no.
> There are no tools I'm aware of to "unformat" a gfs file system.
> However:
> 
> Your ability to get back data depends on how the file system
> looked before the mkfs.  If /dev/sda was ext3 before, I don't know
> what you can do to recover it.  I do know that gfs and ext3 have
> their superblocks in different places, and I don't know ext3.

I don't know GFS, but block 0 of the partition is here the primary
superblock is on ext3.  Fortunately, ext3 stores a pile of backup
superblocks throughout the disk - but I forgot the algorithm to
determine what block #s contained them.

> The same pretty much goes for every other file system (xfs, vfat, etc.).

e2fsck -b [alternate_superblock_address] *might* help if you can figure
out an alternate superblock location.  Some of your data will be
unrecoverable, and some will be unrecognizable (e.g. stored as a
numbered file in /lost+found of the partition after e2fsck completes).

-- Lon