Re: System crash while scrubbing on 3.18.x

[Liu Bo <bo.li.liu@oracle.com>]

On Thu, Mar 05, 2015 at 05:18:21PM -0800, Cameron Berkenpas wrote:
> Hello,
> 
> Sorry for the long email...
> 
> I've found my system locks up when scrubbing with 3.18.x, but not
> with 3.17.8 across 2 systems.
> 
> I have the following BTRFS partitions on system 1:
> /  (128GiB, 49GiB used on SSD)
> /home (4.2TiB, 624GB used on HDD RAID volume)
> 
> I have the following BTRFS partitions on system 2:
> / (196GiB, 17GiB used on HDD RAID volume)
> /home (7.1TiB, 2.9TiB used on HDD RAID volume)
> 
> My OS is Netrunner 15 (which 98% Kubuntu) on system 1, and
> up-to-date debian testing on system 2.
> 
> I've never encountered a lock up while scrubbing /. Just with /home.
> 
> The systems never lock up immediately, but takes some time. VERY
> rarely I'll see the lockup when the scrub is at <100GiB completed.
> Typically it happens somewhere between 200-350GiB. A few times it's
> gone beyond 500GiB. This is probably why I've never encountered the
> issue with /, it's just not big enough on either system.
> 
> Both systems were otherwise idle while performing the scrubs that
> crashed the systems.
> 
> /home is on a partition on a RAID10 volume on a 3ware 9740-4i
> controller with 4x 3TB disks on system 1. On system 2, it's the same
> controller but with 4x 4TB disks (and / on system 2 is a partition
> on the same RAID volume rather than a separate disk). Both systems
> have 32GiB memory, and the otherwise the hardware is pretty
> different between the systems (AMD Vs. Intel, etc).
> 
> I suspect that the RAID controller probably isn't relevant. Both
> arrays and their drives are healthy.
> 
> I've also encountered the issue on a freshly formatted filesystem
> with my data copied from a backup on system 1.
> 
> I've tried tried scrubbing with btrfs-progs 3.17 (installed from the
> distribution repos on both systems), and btrfs-progs from git (using
> tag v3.18.x). Neither version made a difference.
> 
> In case this is helpful to anyone, here's how I've discovered the issue:
> I decided to test btrfs with bcache on system 1 to see if the
> stability had improved since I'd tried bcache+btrfs about a year
> ago. I backed up /home on system 1 and then freshly formatted it and
> set it to use bcache. I was running Linux 3.18.8 and encountered the
> problem that I've described above. I assumed the bcache+btrfs
> combination was still broken so I formatted the system again (this
> time still using btrfs, but without bcache) and copied all my files
> back. I encountered the same issue without bcache. Realizing the
> issue wasn't bcache related, I did ANOTHER format, this time back to
> bcache+btrfs.
> 
> From here in my testing, I found that system 2 (which has no bcache)
> also crashed when scrubbing with Linux 3.18.8. I decided to try
> 3.17.8 on system 1 (since 3.18.8 seemed to be the common denominator
> between the 2 systems), found that fixed the issue, and then
> downgraded system 2 to use 3.17.8 as well, which also fixed the
> issue there.
> 
> (Note: At one point I also tried Linux 3.18.7 and 3.18.5, however,
> those kernels are affected by the scrub/crash issue as well.)
> 
> I found something else interesting when I tested against Linux
> 3.19.0. With 3.19.0, the bcache system always crashes fairly early
> in the scrub (<100GiB), but the non-bcache system has no issues.
> This suggests my problem with 3.19.0 is a bcache+btrfs issue (or
> simply an issue with bcache).
> 
> I'm not sure if bcache is relevant to the BTRFS devs at this point,
> but I thought I'd put that there for anyone who might find that
> information useful.
> 
> To summarize:
> I've tested with 2 systems, and scrubbing caused crashes occurred on
> both with Linux 3.18.8, but not with 3.17.8 for both systems
> I've tested 1 system with and without bcache, and bcache made no
> difference between Linux 3.17.8 and 3.18.8.
> I've tested with 3.19.0, and I crash when scrubbing on the bcache
> system, but not the non-bcache system.

Better to have some stacks about the scrub crash.

Thanks,

-liubo