Re: [BUG] Btrfs scrub sometime recalculate wrong parity in raid5

[Hugo Mills <hugo@carfax.org.uk>]

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On Sun, Jun 26, 2016 at 03:33:08PM -0700, ronnie sahlberg wrote:
> On Sat, Jun 25, 2016 at 7:53 PM, Duncan <1i5t5.duncan@cox.net> wrote:
> > Could this explain why people have been reporting so many raid56 mode
> > cases of btrfs replacing a first drive appearing to succeed just fine,
> > but then they go to btrfs replace a second drive, and the array crashes
> > as if the first replace didn't work correctly after all, resulting in two
> > bad devices once the second replace gets under way, of course bringing
> > down the array?
> >
> > If so, then it looks like we have our answer as to what has been going
> > wrong that has been so hard to properly trace and thus to bugfix.
> >
> > Combine that with the raid4 dedicated parity device behavior you're
> > seeing if the writes are all exactly 128 MB, with that possibly
> > explaining the super-slow replaces, and this thread may have just given
> > us answers to both of those until-now-untraceable issues.
> >
> > Regardless, what's /very/ clear by now is that raid56 mode as it
> > currently exists is more or less fatally flawed, and a full scrap and
> > rewrite to an entirely different raid56 mode on-disk format may be
> > necessary to fix it.
> >
> > And what's even clearer is that people /really/ shouldn't be using raid56
> > mode for anything but testing with throw-away data, at this point.
> > Anything else is simply irresponsible.
> >
> > Does that mean we need to put a "raid56 mode may eat your babies" level
> > warning in the manpage and require a --force to either mkfs.btrfs or
> > balance to raid56 mode?  Because that's about where I am on it.
> 
> Agree. At this point letting ordinary users create raid56 filesystems
> is counterproductive.
> 
> 
> I would suggest:
> 
> 1, a much more strongly worded warning in the wiki. Make sure there
> are no misunderstandings
> that they really should not use raid56 right now for new filesystems.

   I beefed up the warnings in several places in the wiki a couple of
days ago.

   Hugo.

> 2, Instead of a --force flag. (Users tend to ignore ---force and
> warnings in documentation.)
> Instead ifdef out the options to create raid56 in mkfs.btrfs.
> Developers who want to test can just remove the ifdef and recompile
> the tools anyway.
> But if end-users have to recompile userspace, that really forces the
> point that "you
> really should not use this right now".
> 
> 3, reach out to the documentation and fora for the major distros and
> make sure they update their
> documentation accordingly.
> I think a lot of end-users, if they try to research something, are
> more likely to go to <their-distro> fora and wiki
> than search out an upstream fora.

-- 
Hugo Mills             | "No! My collection of rare, incurable diseases!
hugo@... carfax.org.uk | Violated!"
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                Stimpson J. Cat, The Ren & Stimpy Show

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