[CentOS] Using CentOS 7 to attempt recovery of failed disk

Sun Sep 27 12:21:15 UTC 2020
Tony Mountifield <tony at softins.co.uk>

In article <E02FA554-9D6D-4E7D-8A78-5FBDE1DE939D at kicp.uchicago.edu>,
Valeri Galtsev <galtsev at kicp.uchicago.edu> wrote:
> > On Sep 26, 2020, at 8:05 AM, Jerry Geis <jerry.geis at gmail.com> wrote:
> > 
> > I have a disk that is flagging errors, attempting to rescue the data.
> > 
> > I tried dd first - if gets about 117G of 320G disk and stops incrementing
> > the save image any more.
> did you try
> dd conv=noerror …
> this flag makes dd not stop on input error. Whatever is irrecoverable is irrecoverable, but this way you will get stuff
> beyond failure point.

You need conv=noerror,sync so that unreadable sectors get replaced by zeros instead of not being written out at all.
Without sync, the filesystem geometry on the destination image will be wrong after the first error.

You also need bs=4096 so that ONLY the bad sector(s) get zeroed, and not the surrounding ones. If you have, say,
bs=1M, then you will get a megabyte of zeros if any block within that megabyte is bad.

I'm speaking from recent experience!


Tony Mountifield
Work: tony at softins.co.uk - http://www.softins.co.uk
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