Re: Adventures in btrfs raid5 disk recovery

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

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On Mon, Jun 20, 2016 at 11:13:51PM +0500, Roman Mamedov wrote:
> On Sun, 19 Jun 2016 23:44:27 -0400
> Zygo Blaxell <ce3g8jdj@umail.furryterror.org> wrote:
> From a practical standpoint, [aside from not using Btrfs RAID5], you'd be
> better off shutting down the system, booting a rescue OS, copying the content
> of the failing disk to the replacement one using 'ddrescue', then removing the
> bad disk, and after boot up your main system wouldn't notice anything has ever
> happened, aside from a few recoverable CRC errors in the "holes" on the areas
> which ddrescue failed to copy.

I'm aware of ddrescue and myrescue, but in this case the disk has failed,
past tense.  At this point the remaining choices are to make btrfs native
raid5 recovery work, or to restore from backups.

> But in general it's commendable that you're experimenting with doing things
> "the native way", as this is provides feedback to the developers and could help
> make the RAID implementation better. I guess that's the whole point of the
> exercise and the report, and hope this ends up being useful for everyone.

The intent was both to provide a cautionary tale for anyone considering
deploying a btrfs raid5 system today, and to possibly engage some
developers to help solve the problems.

The underlying causes seem to be somewhat removed from where the symptoms
are appearing, and at the moment I don't understand this code well enough
to know where to look for them.  Any assistance would be greatly appreciated.


> -- 
> With respect,
> Roman



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