[nSLUG] Re: Multiply-claimed blocks?

Mike Spencer mspencer at tallships.ca
Wed Aug 31 01:55:35 ADT 2011


Ben Armstrong <synrg at sanctuary.nslug.ns.ca> wrote:

> I have experienced a few [multiply-caimed blocks] over the years,
> mostly due to using ext2 instead of a journalling filesystem like
> ext3 or ext4, coupled with an unclean shutdown.

I'll likely go with ext3 on the next installation. In this case, the
system failed while running, presumably when something tried to access
libc.

> However, cosmic rays are well known to cause bit flipping in RAM,...

I'd heard that, of course. In this case, the corruption seems to have
been in a lib file on HD that (AFAIK) is written at install time and never
re-written unless upgrading or the like.  I had not heard that cosmic
rays (energetic neutrons according to the article you cited) could flip
a bit on a HD.  But, as you say, "Anything is possible...."

> Here's a recent blog article about analysis and manual correction of
> just such an error....
>
> http://blog.candysporks.org/2011/07/10/hard-drive-woes-single-bit-error/

Nice.

> ...(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)

Right.  

Soon after got my first computer, I got a take-home job to translate a
game from BASIC into C. I had spent many hours with 132 col printouts,
colored markers etc. tracing the logic of the BASIC and keeping track
of it all with a spreadsheet on a 5.25 floppy.  And, of course, just
as I was about ready to start writing C code, the floppy was
corrupted. So my first run at writing a "real" C program switched from
the game to writing a low level utility to recover every bit of
uncorrupted data from a floppy.  It was entertaining, so much so that
I never got back to the guy's game.  Ah, but things were easier with
CP/M. :-)

>> 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. 
>
> Keep a live system around for just such an occasion, e.g.
>
> http://www.debian.org/CD/live

I have (and just used for this) the Slackware distro DVD. 
It boots into a working system with (AIUI) everything on a
ramdisk and leaves the real HD alone until you mount or fsck it. But
it lacks many of the tools that I use all the time.  more instead of
less, vi instead of emacs [1], console only and no working support for
USB or LAN.

That Debian disk sounds like a good thing.  The fly in the ointment
would be that the packages it holds for re-installing corrupted bits
would be configured for Debian.  I have no idea what problems, if any,
I might encounter in that account.  I wonder if someone has made a
similar CD/DVD for Slack.  I shall have to look around.

After my very first Linux install (Caldera) I carefully kept rescue
floppies on hand.  But, you know, I never had a problem.  Guess I
became a slacker. ;-)

Thanks for the pointers,
- Mike


[1] Not to re-ignite the flame war: one of the features of Emacs I use
    all the time is to run a shell in an Emacs buffer.  It's a term of
    type "dumb" so, of course, many things don't run there.  But it
    *does* allow you to accumulate all the output from a command and
    the scroll back in it and work with it. I'm pathetically dependent
    on this and a couple of other features, especially when fumbling
    around with a problem I don't really understand.

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^



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