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Re: [Bug-ddrescue] On the reading of slow areas (option introduced in 1.

From: Gábor KATONA
Subject: Re: [Bug-ddrescue] On the reading of slow areas (option introduced in 1.15pre2)
Date: Fri, 14 Oct 2011 09:12:57 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:7.0.1) Gecko/20110929 Thunderbird/7.0.1

  Hello Antonio,

I start from the end of your letter.

> I guess you mean "marking differently blocks skipped after finding a
> slow area from those skipped after finding an unreadable area". This
> could be useful, but is a major change to the current algorithm. I'll
> think about it. Feedback is welcome. :-)
Yes, this is what I mean. Is it really a major change? Marking these
areas differently seems to me an easy change. If ddrescue would take as
non-tried not only the areas with '?', but also areas with the new
'slow' mark, it would result exactly in the desired functionality. What
am I missing?

> Not exactly. Slow areas are indeed read correctly, so they are marked
> as '+' (finished). If the read rate of such a correctly read area
> falls below the --min-read-rate, ddrescue will then skip ahead a
> variable amount depending on rate and error history. This skipped area
> will be marked as '*' (non-trimmed).
I also thought it works like this, maybe I was not so clear. The key is
to treat these areas independently, and allow a second (or more) run
with -n for these.

> The areas marked as non-trimmed are tried before splitting. So they
> are tried in a run with the -n option. The problem is you can run
> ddrescue only once with the "-n" or "--min-read-rate" options. The
> second time you use the "-n" option, ddrescue does nothing because
> only non-split and bad-block areas are left. The second time you use
> the "--min-read-rate" option, it has no effect because no more
> non-tried areas are left.
Basically I would like to run ddrescue  more than once with the -n
option, to try again slow areas, but leave out bad areas.

In my case I have choosen to run ddrescue first with a higher speed
threshold, then change all '*' area to '?', and read again with a lower
speed threshold. This way I was able to first recover the fast areas,
then the slower ones. The price was to try bad areas twice. As I have
several bad areas with different sizes, this increased second run
significantly, but at least I was sure, the a big part is saved in a few
hours during the first run.



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