Help, my disk array has one dead member

[Karl Auer]

On Wed, 2017-03-22 at 20:11 -0700, Bruce Ferrell wrote:
> I've done the dd thing on simple disks; the wisdom is it's ok between
> same type disks (make/model/etc).  The wisdom of raid is NO, full
> stop.

Ummmm - you may have missed the rest of the "dd thing". No-one is
suggesting copying the dd image back onto a disk of any description.

What was suggested was reading the virtual disk (not any physical disk)
block for block into a file on a different device. The result is a disk
image. That image can then be mounted on a loopback device and
inspected as if it were a real disk; files and directories can be
copied off it and the data recovered in that way. Data that was DOA is
still DOA, but as the RAID is still basically readable, dd will get you
all that can be got (barring the use of, as you rightly say, "serious
lab facilities").

> filesystems are inactive because it's read only, but the physical
> device is active and capable of change in the midst of 
> a possible dd.  tar, rsync etc are safe on the filesystem because
> it's ro but the physical volume is rw.

Yes, that's an issue. Hence my suggestion that the RAID should be
accessed read-only and from a LiveCD. While the RAID may still be
moving stuff around at the physical level (unlikely since it has
faulted itself into read-only mode anyway) the virtual disk - the thing
the OS and thus dd sees - will not be changing and can safely be read.

Happy to be corrected if my understanding here is wrong.

Regards, K.

-- 
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Karl Auer (kauer at biplane.com.au)
http://www.biplane.com.au/kauer
http://twitter.com/kauer389

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