Re: 6TB partition, Data only 2TB - aka When you haven't hit the "usual" problem

[Hugo Mills <hugo@carfax.org.uk>]

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On Mon, Jan 11, 2016 at 03:39:43PM -0700, Chris Murphy wrote:
> On Mon, Jan 11, 2016 at 3:30 PM, Hugo Mills <hugo@carfax.org.uk> wrote:
> > On Mon, Jan 11, 2016 at 03:20:36PM -0700, Chris Murphy wrote:
> >> On Mon, Jan 11, 2016 at 3:10 PM, Hugo Mills <hugo@carfax.org.uk> wrote:
> >> > On Mon, Jan 11, 2016 at 02:31:41PM -0700, Chris Murphy wrote:
> >> >> On Mon, Jan 11, 2016 at 2:03 AM, Hugo Mills <hugo@carfax.org.uk> wrote:
> >> >> > On Sun, Jan 10, 2016 at 05:13:28PM -0700, Chris Murphy wrote:
> >> >> >> On Sat, Jan 9, 2016 at 2:04 PM, Hugo Mills <hugo@carfax.org.uk> wrote:
> >> >> >> > On Sat, Jan 09, 2016 at 09:59:29PM +0100, cheater00 . wrote:
> >> >> >> >> OK. How do we track down that bug and get it fixed?
> >> >> >> >
> >> >> >> >    I have no idea. I'm not a btrfs dev, I'm afraid.
> >> >> >> >
> >> >> >> >    It's been around for a number of years. None of the devs has, I
> >> >> >> > think, had the time to look at it. When Josef was still (publicly)
> >> >> >> > active, he had it second on his list of bugs to look at for many
> >> >> >> > months -- but it always got trumped by some new bug that could cause
> >> >> >> > data loss.
> >> >> >>
> >> >> >>
> >> >> >> Interesting. I did not know of this bug. It's pretty rare.
> >> >> >
> >> >> >    Not really. It shows up maybe on average once a week on IRC. It
> >> >> > gets reported much less on the mailing list.
> >> >>
> >> >> Is there a pattern? Does it only happen at a 2TiB threshold?
> >> >
> >> >    No, and no.
> >> >
> >> >    There is, as far as I can tell from some years of seeing reports of
> >> > this bug, no correlation with RAID level, hardware, OS, kernel
> >> > version, FS size, usage of the FS at failure, or allocation level of
> >> > either data or metadata at failure.
> >> >
> >> >    I haven't tried correlating with the phase of the moon or the
> >> > losses on Lloyds Register yet.
> >>
> >> Huh. So it's goofy cakes.
> >>
> >> This is specifically where btrfs_free_extent produces errno -28 no
> >> space left, and then the fs goes read-only?
> >
> >    The symptoms I'm using for a diagnosis of this bug are that the FS
> > runs out of (usually data) space when there's still unallocated space
> > remaining that it could use for another block group.
> >
> >    Forced RO isn't usually a symptom, although the FS can get into a
> > state where you can't modify it (as distinct from being explicitly
> > read-only).
> >
> >    Block-group level operations, like balance, device delete, device
> > add sometimes seem to have some kind of (usually small) effect on the
> > point at which the error occurs. If you hit the problem and run a
> > balance, you might end up making things worse by a couple of
> > gigabytes, or making things better by the same amount, or having no
> > effect at all.
> 
> Are there any compile time options not normally set that would help find it?
> # CONFIG_BTRFS_FS_CHECK_INTEGRITY is not set
> # CONFIG_BTRFS_FS_RUN_SANITY_TESTS is not set
> # CONFIG_BTRFS_DEBUG is not set
> # CONFIG_BTRFS_ASSERT is not set
> 
> Once it starts to happen, it sounds like it's straightforward to
> reproduce in a short amount of time. I'm kinda surprised I've never
> run into this.

   It does sometimes have a repeating nature: I'm reasonably sure
we've seen a few people get it repeatedly on different filesystems.
This might point at a particular workload needed to trigger it. (Or
just bad luck / statistical likelihood). Some people have never hit
it.

   There is (or at least, was) an ENOSPC debugging option. I think
that's a mount option. That's probably the most useful one, but the
range of usefulness of existing debug output may be very small. :)

   (Sorry for the vague nature of this reply -- it's been a very long
day).

   Hugo.

-- 
Hugo Mills             | "What are we going to do tonight?"
hugo@... carfax.org.uk | "The same thing we do every night, Pinky. Try to
http://carfax.org.uk/  | take over the world!"
PGP: E2AB1DE4          |

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