Hiroshi Honda posted on Thu, 13 Oct 2016 10:28:19 +0900 as excerpted:
> I am using Btrfs RAID5 with 10x4T disks.
> Around 2 weeks ago, one disk of it was taking long time when read and
> So, I tried replace the disk with new disk by replace command ( btrfs
> replace ... ).
> But, it failed. So, I added new disk into the array by add command and
> then deleted the bad disk by delete command.
> But, this deletion failed with error message as well. So, I deleted new
> disk from the array at the moment to return to first situation.
> But, system is showing some error message with number and the number is
> counting up.
> So, I rebooted the OS. Because, I felt this situation is something bad.
> After that, I can not mount the array...
Btrfs parity-raid currently has serious known issues that aren't likely
to be corrected in the short term, as to some extent they're in the
design. The recommendation is thus not to use btrfs raid56 in
production, and if you're already on it, to get off it ASAP, because
while it runs fine in normal operation, failure and repair modes simply
can't be guaranteed to work as one might expect at this point.
There's more about that on the list if you're interested, and others
following raid56 mode more closely than I will likely reply with further
specific details, as well, but that's the basic raid56 mode status and
recommendation in practical terms.
As for rescuing files from the array, the proper answer is that btrfs as
a whole is still stabilizing, not fully stable and mature, so the
sysadmin's rule that if you don't have at least one backup, you are by
definition of your failure to backup, defining the data as worth, at
maximum, less than the time, hassle and resources necessary to take that
backup, applies even more strongly to the not yet fully stable and mature
btrfs, than it does to properly stable and mature filesystems.
And of course btrfs raid56 mode as a feature is newer and less stabilized
than btrfs in general, so the rule that if you don't have a backup, you
really are defining the data as of throw-away value, applies even more to
anything on btrfs raid56.
So to rescue the data, restore it from backups to a preferably non-btrfs-
raid56 mode filesystem. Or if you don't have backups, don't worry about
it, because you effectively already defined the data as no more than
throw-away value by not having those backups in the first place.
That's the proper answer. In practice... all hope isn't yet lost.
There's some chance to rescue your data, but it'll take time and
patience, and likely a significant level of technical understanding along
with some help from btrfs experts that know more about the btrfs raid56
technical details than I do.
Duncan - List replies preferred. No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master." Richard Stallman
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