Re: Union mount of block devices?

[Valerie Aurora <vaurora@redhat.com>]

On Fri, Nov 13, 2009 at 02:28:42PM +0100, Andrea Gelmini wrote:
> Hi all,
>     sorry to bother the ml but I'm not able to find the answer via Google.
>     Quick answer: is it possibile to stack block devices as union
> mount? Something like /dev/sdb1 (ro) below /dev/sdc1 (rw)?
> 
>     Long story...
>     I've got a partition with the begin of the journal over a few bad
> blocks (by the way, it's NTFS but it doesn't matter), so it is not
>     possible to fix/replay journal, and so on.
>     I can dd/dd_rescue the partition to file/partition and fix/recover
> all, but I just need to suck some files, so it would be a lot
> easier/quicker
>     to stack a good partition (rw) over the bad one (ro).

The phrase "union mount" caught my eye, but it's not what you want. :)
Union mounts merge file systems at the namespace level, not the block
or byte level of individual files.

You can try something like this to get what you describe above:

* Create a file big enough to cover the bad blocks
* Turn it into a loop device
* Create a loop device out of the good portion of your original block
  device using the "-o <offset>" argument to losetup
* Use md or whatever to concatenate the two together

There will be annoying issues with headers and alignment.  If you can
find a userspace recovery tool, you'll probably be a lot better off.

-VAL