[Bug-ddrescue] Reverse mode - yeah, it really is important

[Jay R. Ashworth]

And here's why:

The most common "failing drive" mode I've had to work with in the last 5 years,
indeed, my only recent failure mode that can't be described as "drive stone
cold dead", is one where, when you hit a certain spot in sequential-read-forward
mode, the drive adapter has an aneurysm, and will no longer respond properly 
to *any commands at all*, even reads of still-good sectors...

*until you powercycle the drive*.  At which point, it goes back to being 
perfectly content to return data from the good sectors.

The assumption ddrescue makes is that a sector that returns a bad read will
never be good again... which this failure mode makes into an invalid assumption.

All the drives that have done this to me have been Seagates; perhaps it's 
specific to them.

In any event, not being able to copy starting at the back of the drive and 
working forwards means that one is forced to binary search the drive by hand:

-i500G
-i250G
-i125G - oops; it died.
-i200G - well, that's ok
-i150G - nope; borken again
-i175G - this is ok

*with a power cycle after each failure*.

This is *directly* a result of ddrescue not being able to be *instructed
by the user* to start at the back and run in reverse -- if it could do that, you
could recover any drive with only one contiguous such bad area in only 2 passes
(using -e0 to die instantly on contact with the edge), and therefore, only 1 
power
cycle.

I'm willing to put some time into possibly assembling a patch to add such a 
switch,
if you'd accept it, but I suspect that you can probably do it in quite a bit 
less 
time than I could, since you're already in bed with the code base -- assuming 
I've
made a convincing argument here for why it would in fact be a helpful feature to
have.  :-)

That said, even without this, the tool is a remarkably useful addition to my 
kit, 
and thanks for the work you've put into it already.

Cheers,
-- jra

-- 
Jay R. Ashworth                   Baylink                      address@hidden
Designer                     The Things I Think                       RFC 2100
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