Neil Brown [off-list ref] writes:

On Monday May 15, jdc@uwo.ca wrote:

quoted

I accidentally ran mkswap on an md raid1 device which had a mounted
ext3 filesystem on it.  I also did a swapon, but I don't think
anything was written to swap before I noticed the mistake.  How much
of the partition is toast, and is it something e2fsck might fix?

I think (and an strace seems to confirm) that mkswap only writes in
the first 4k of the device.  This will have held the superblock, but
there is always at least one backup - I think it is as block 8193.
But 'fsck -n' should help you out, though you might need
'fsck.ext2 -n' as 'fsck' might think it is a swap device...

Thanks for the quick response!  I tried e2fsck, and it found the
backup superblock on its own.  I answered yes to dozens of questions,
and the filesystem is now intact.

Ofcourse, if the filesystem is mounted, then unmounting the filesystem
should write the superblock, which might fix any corruption you
caused..

I decided to do a Alt-Sysrq-u Alt-Sysrq-b reboot to minimize what
was written to disk when things were already messed up.

Adrian:  You seem to be the MAINTAINER of mkswap.. any chance of 
opening for O_EXCL as well as O_RDWR.  That would make it a lot safer.

That would have saved me lots of worry!

Thanks again,

Dan