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Re: [Bug-ddrescue] When will another pass help?

From: andrew zajac
Subject: Re: [Bug-ddrescue] When will another pass help?
Date: Fri, 18 Apr 2008 12:55:04 -0400 (EDT)

Colin Berry <address@hidden> wrote:
Where does the advantage of a second pass come from? Does it presume that
the problem is elecrical or hardware related and thus not static over time?
Or would there be an advantage if the problem were purely media related
(jelly stain on a cdrom) and therefore a stable reproducible error?

That's a great question.  Although block devices are digital (ones and zeros), the media on the platters of your hard drive are analog.  That means that subsequent reads to the same block can yield different results.  The hard disk's controller deals with this and performs error correction all the time, you are just abstracted from it.

The key to rescuing the data on your drive is that you only need one good read.  That read may come after the first pass or after several dozen.

I reckon everyone who does this regularly has their own superstition about what will improve your chances or getting a good read.  Usually, keeping your drive cool is a good strategy.  I would be interested in hearing what others do...

Can we tell by looking at the logfile of the first pass if another will
help, or is it better to run multiple passes and evaluate for diminishing

Every time I juice a lemon, it seems that I can always squeeze out one last drop if I try again.

The logfile only shows the areas that are problematic, but offers no description of how hard those areas are to read.  You just have to guess.

Good luck.

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