Re: [Bug-ddrescue] Improving ddrescue

[Robert Trevellyan]

Have you experimenting with --min-read-rate and --idirect?


On Fri, Nov 25, 2016 at 10:24 PM, Santiago Cassina <
address@hidden> wrote:

> Hello, I use ddrescue a lot to recover damaged disks, most of them are WD
> (green and blue), I want to share my own experience because I believe it
> can help improving your software
>
> This story starts with a big 2TB green WD disk, which started to copy at a
> rate of 32MB/sec with some (fast and very slow) peaks. It was a LOT of data
> to recover so I spent 6 days running ddrescue constantly (and checking it
> visually).  I noticed sometimes when the reading speed dropped (very very
> slow, about 1KB/sec or less) that by pressing CTRL-C and re-running
> ddrescue the reading speed was fast again (sometimes fast and sometimes
> very slow as before), so I tried modifying parameters as sector size, block
> size, dynamic access, reverse direction, but such modifications were not
> helpful, instead killing and starting ddrescue again with default
> parameters every 7 seconds helped me to improve read speed.
>
> I didn't read ddrescue sources, so I don't know if it gots stucked on a
> bottleneck when some read error is found, but the this experience made me
> to create two bash files
>
> 1)  rekill_recover.sh
>
> #!/bin/bash
> while :
> do
>     echo "Killing again..."
>     killall ddrescue
>     sleep 12
> done
> (you can try 7-12-20 seconds, the block of time you detect as the best to
> kill and rerun)
>
> 2) rerun_recover.sh
>
> #!/bin/bash
> while :
> do
>     echo "Running again..."
>     ddrescue /dev/source /rescued.img /tmp/rescued_img.log
> done
>
> I used those scripts and I rescued (almost) the entire disk faster than
> expected. I thought it was just luck
>
> Two days ago I received a very damaged HDD WD (blue this time) with same
> reading speed drops, I used those scripts again (each one in different
> terminals obviously) and voilá, I got better reading speeds again, so my
> "guess" is one of this two options
>
> 1) When I kill ddrescue, "he" forgets what was reading, ignores that block
> and I lose data (but I BELIEVE this is not happening)
> 2) When I kill ddrescue, "he" forgets he was reading slowly and starts
> again with all the power, energy and faith he can do the job
>
> I think 2) is happening, and I want it too
>
>
> I don't know if this can help you to improve your software, just sharing
> my experience.
>
>
> By the way, when I detect (visually) that the reading speed is
> considerably fast again (more than 10MB/sec) I stop killing ddrescue for a
> while, until I detect he stucks again and I start to call rekill script.
> Maybe, maybe, maybe you can apply some of this experience on the sources
> or by creating a simple bash script (I don't think can be better than mine
> ;) with the ability to do some "smart-automagic" kill-rerun-rekill-rerun
> depending on reading speeds
>
> If you need to, I can provide with a secure shell connection to a terminal
> with this second disk connected to check it for yourself. Or perhaps by
> vnc, teamviewer, anything I can do to help you improving your algorythm
>
> Best regards
>
> --
>  _______________________________
>         Santiago Cassina
>   Lic. en Análisis de Sistemas
>            M.P. 8064
>  Cel/Whatsapp: (0387) 154-752661
>
>
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