[nSLUG] Multiply-claimed blocks?

Ben Armstrong synrg at sanctuary.nslug.ns.ca
Tue Aug 30 07:49:54 ADT 2011


On 08/30/2011 02:36 AM, Mike Spencer wrote:
> My first-ever apparently unprovoked Linux crash in 12 years.
> Everything stopped working.
...
> Are multiply-claimed blocks a common occurrence. A reliable sign of
> impending disk failure?  A black swan?

I have experienced a few over the years, mostly due to using ext2
instead of a journalling filesystem like ext3 or ext4, coupled with an
unclean shutdown. I wouldn't say it was common, no, at least not since I
wholly switched to using ext3 or ext4.

Anything is possible, I suppose, impending hardware failure included.
However, cosmic rays are well known to cause bit flipping in RAM, so
even in the absence of abrupt power loss or hardware failure, you could
end up with a corrupted filesystem this way.

http://en.wikipedia.org/wiki/Soft_error#Cosmic_rays_creating_energetic_neutrons_and_protons

Here's a recent blog article about analysis and manual correction of
just such an error that I found interesting (though I doubt if I would
go through the bother of all this unless I were dealing with extremely
valuable data I could not recover in any other way):

http://blog.candysporks.org/2011/07/10/hard-drive-woes-single-bit-error/

> PS: I have no idea how anybody can recover from a crash like that or
>     even from many simpler things if they have only one computer. How
>     do you look up stuff if you can't grovel trough the docs?

Keep a live system around for just such an occasion, e.g.

http://www.debian.org/CD/live

Ben



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