On Sun, Jun 26, 2016 at 03:33:08PM -0700, ronnie sahlberg wrote: > On Sat, Jun 25, 2016 at 7:53 PM, Duncan <1i5t5.dun...@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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