Re: degraded raid scribbling upon wrong device

[Adam Borowski <kilobyte@angband.pl>]

On Thu, Jul 13, 2017 at 08:40:12AM +0200, Adam Borowski wrote:
> Here's a set of test cases, two of them in some cases seem to scribble upon
> the wrong device:
> 
> * deg-mid-missing
> * deg-last-replaced (not on the innocent "re")
> * but never deg-last-missing
> 
> When all goes ok, there are no errors other than wrong generation on the
> re-added disk (expected).   When it goes bad, there's a lot of corruption.
> In all cases, though, the "Device missing:" field is wrong.

I did not explore this adequately yet, in a good part because of ENOSPC
triggering a lot of time for an unrelated reason that Omar just fixed
(thanks!).  So, here's what I know so far:

* copying in, say, 2.2GB /usr/share is a lot more likely to trigger than
  dd-ing 2.2GB of /dev/null
* no "real" degrading is needed: in the original scripts, the missing device
  is empty so all blocks are doubled anyway.  It's not about degraded chunks
  but because of a bogus device.
* bogus output of "btrfs f u" is a sure predictor that, with enough tries,
  you'll get corruption -- if it shows something when it should say
  "missing", shit is likely to happen


Meow!
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