[Beowulf] instances where a failed storage block is not all zero?

Mark Hahn hahn at mcmaster.ca
Wed Jul 7 16:14:42 PDT 2010

> With "modern" hardware are there currently any notable instances where a
> failed read of a hardware storage area block results in that missing
> data being filled in with something other than null bytes?   For

I'm surprised at the question: I expect failed reads to result in 
out-of-band errors, not zeros.  a failed read on a disk, for instance,
indicates that the ECC failed - considering that the ECC is quite 
strong, it would be surprising to encounter a failure which wasn't
even detected by the ECC.

on what kind of medium are you finding errors-returned-as-zero?

> a region of a DVD goes bad.  (Assuming that the software reading it can
> even go on beyond the failure, which is often not possible, for instance
> on many tapes.)

I know it's sorta possible to read raw (extended) sectors from disks,
but it's pretty deep voodoo.  I guess I would expect the contents to 
not fail-to-zero on disks (which I guess use some kind of NRZ-like encoding).
I wouldn't be surprised if damaged flash would read all-0 or all-1
(flash erase sets a block to all-1, right, and writing is basically
clearing selective zeros?)

> will fill in with null bytes, but there is a lot of other software out
> there...

I think it's a question of whether you're using an exotic interface
or not - normal kernel block/char devices aren't ever going to do this.
people in the forensics/recovery business would be the ones to ask.

-mark hahn

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