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[Bug-ddrescue] On the reading of slow areas (option introduced in 1.15pr

From: Gábor KATONA
Subject: [Bug-ddrescue] On the reading of slow areas (option introduced in 1.15pre2)
Date: Sun, 09 Oct 2011 10:10:45 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:6.0.2) Gecko/20110907 Thunderbird/6.0.2

  Dear Antonio,

Unfortunately I had to try your excellent program. I still need lot of work to finish, but it really seems to work, thank you very much.

My drive is damaged in a way, that is has several very slow areas surrounded by less slow areas, surrounded by even less slow areas embedded in the normal area, so I'm using 1.15pre2 with the --min_read_rate option.

If I noticed it correctly, than all areas which fall below the given threshold gets marked with *, meaning failed block non-trimmed. If I understand the algorithm correctly, these blocks will not be read on a subsequent run with a -n option, but only when we turn to splitting. My experience also suggests this behaviour. The problem with this strategy is the following. If I first want to rescue the most healthy parts, I would run

ddrescue -n --min-read-rate=5M device image log

When it finishes I want to give a chance to slower parts, but not to very slow ones, so I would issue

ddrescue -n --min-read-rate=100k device image log

As I understood and experienced this will not work as I expect, because blocks that were skipped during the first run are not read in this run.

If this is correct, I would suggest to have a new logfile status character meaning "slow block", and have a new option meaning "try again slow blocks". This way slow blocks would not be marked as bad blocks, and they could be saved before turning to the most difficult parts, but only after the most healthy blocks are saved. In cases similar to mine it would increase the number of saved blocks by several percents (I guess in my case about 10%) during the first, let's say, day. Saving all slow blocks can require several weeks, and the drive could totally die during this.

Thank you again for ddrescue,

Gábor Katona

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