Re: BTRFS raid6 unmountable after a couple of days of usage.

[Chris Murphy <lists@colorremedies.com>]

On Wed, Jul 15, 2015 at 5:07 AM, Austin S Hemmelgarn
<ahferroin7@gmail.com> wrote:
> I've managed to get the other filesystems I was having issues with mounted
> again with the device= options and clear_cache after running btrfs dev scan
> a couple of times.  It seems to me (at least from what I'm seeing) that
> there is some metadata that isn't synchronized properly between the disks.

OK see if this logic follows without mistakes:

The fs metadata is raid6, and therefore is broken up across all
drives. Since you successfully captured an image of the file system
with btrfs-image, clearly user space tool is finding a minimum of n-2
drives. If it didn't complain of missing drives, it found n drives.

And yet the kernel is not finding n drives. And even with degraded it
still won't mount, therefore it's not finding n-2 drives.

By "drives" I mean either the physical device, or more likely whatever
minimal metadata is necessary for "assembling" all devices into a
volume. I don't know what that nugget of information is that's on each
physical device, separate from the superblocks (which I think is
distributed at logical addresses and therefore not on every physical
drive), and if we have any tools to extract just that and debug it.



-- 
Chris Murphy