DB Recovery

[Christopher Browne]

On 9/21/05, Lennart Sorensen <lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org> wrote:
> On Wed, Sep 21, 2005 at 03:58:37PM -0400, James Golick wrote:
> > both are ext3...
> >
> > i tried dumping the inode from debugfs, but it seems that
> > at least some part of hte data has been overwritten
>
> I have recovered files from ext2 in the past.  I have never managed on
> ext3 (or any of the other filesystems).
>
> I have recovered from filesystem corruption, but never from rm.  rm does
> it's job very well. :)

And extracting data from a binary database structure isn't likely to
turn out terribly well either.

Databases tend to be "hives" of heavily interlinking systems of
pointers, very difficult to recover data from in a systematic way. 
They start "systematic," but break pretty badly :-(.

Getting fragments oughn't be too difficult, but doing it
systematically will be really difficult.

The one "saving grace" is that you generally have TWO sorts of data:

 1.  The data itself, which tends to be pretty "dense", where you have
       files chock full of tuples;

 2.  Indexes, very likely to be completely destroyed by any kind of damage,
      which are purely a re-encoding of the real data.  If an index breaks,
      it is normally fairly easy to regenerate it.

But really, the only way you ever want to recover this stuff is from a backup.

Being forced to find what fragments you can reconstruct out of the
"shrapnel" left after the explosion is a real expensive way to learn
just how important backups are.

I'd thought people learned a lot of these lessons out of the
cautionary results from:
 a) The ice storm of 1999
 b) The destruction of data centres that took place 2001-09-11
 c) The TLUG presentation on how *little* is recoverable from busted
      disk drives <http://gtalug.org/wiki/Meetings:2003-07>.  Which was a
      VERY valuable presentation!

But I guess what we continue to learn from history is that people
don't learn from history :-(.
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