Re: Repair broken btrfs raid6?

[Ed Tomlinson <edt@aei.ca>]

On Tuesday, February 10, 2015 2:17:43 AM EST, Kai Krakow wrote:
> Tobias Holst <tobby@tobby.eu> schrieb:
>
>> and "btrfs scrub status /[device]" gives me the following output:
>>> "scrub status for [UUID]
>>> scrub started at Mon Feb  9 18:16:38 2015 and was aborted after 2008
>>> seconds total bytes scrubbed: 113.04GiB with 0 errors"
>
> Does not look very correct to me:
>
> Why should a scrub in a six-drivers btrfs array which is probably multi-
> terabytes big (as you state a restore from backup would take 
> days) take only 
> ~2000 seconds? And scrub only ~120 GB worth of data. Either your 6 devices 
> are really small (then why RAID-6), or your data is very sparse (then way 
> does it take so long), or scrub prematurely aborts and never checks the 
> complete devices (I guess this is it).
>
> And that's what it actually says: "aborted after 2008" seconds. I'd expect 
> "finished after XXXX seconds" if I remember my scrub runs 
> correctly (which I 
> currently don't do regularly because it takes long and IO performance sucks 
> during running it).

IO perfermance does suffer during a scrub.  I use the following:

ionice -c 3 btrfs scrub start -Bd -n 19 /<target>

The combo of -n19 and ionice makes it workable here.  

Tobias why do you think btrfsck does not work on raid6?  It runs fine
here on raid5.