On Thu, Jan 30, 2020 at 10:20 AM David Sterba <dsterba@xxxxxxx> wrote: > > On Wed, Jan 29, 2020 at 03:55:06PM -0700, Chris Murphy wrote: > > On Wed, Jan 29, 2020 at 2:20 PM Martin Steigerwald <martin@xxxxxxxxxxxx> wrote: > > > > > > So if its just a cosmetic issue then I can wait for the patch to land in > > > linux-stable. Or does it still need testing? > > > > I'm not seeing it in linux-next. A reasonable short term work around > > is mount option 'metadata_ratio=1' and that's what needs more testing, > > because it seems decently likely mortal users will need an easy work > > around until a fix gets backported to stable. And that's gonna be a > > while, me thinks. > > We're looking into some fix that could be backported, as it affects a > long-term kernel (5.4). > > The fix > https://lore.kernel.org/linux-btrfs/20200115034128.32889-1-wqu@xxxxxxxx/ > IMHO works by accident and is not good even as a workaround, only papers > over the problem in some cases. The size of metadata over-reservation > (caused by a change in the logic that estimates the 'over-' part) adds > up to the global block reserve (that's permanent and as last resort > reserve for deletion). > > In other words "we're making this larger by number A, so let's subtract > some number B". The fix is to use A. > > > Is that mount option sufficient? Or does it take a filtered balance? > > What's the most minimal balance needed? I'm hoping -dlimit=1 > > > > I can't figure out a way to trigger this though, otherwise I'd be > > doing more testing. > > I haven't checked but I think the suggested workarounds affect statfs as > a side effect. Also as the reservations are temporary, the numbers > change again after a sync. Yeah I'm being careful to qualify to mortal users that any workarounds are temporary and uncertain. I'm not even certain what the pattern is, people with new file systems have hit it. A full balance seems to fix it, and then soon after the problem happens again. I don't do any balancing these days, for over a year now, so I wonder if that's why I'm not seeing it. But yeah a small number of people are hitting it, but it also stops any program that does a free space check (presumably using statfs). A more reliable/universal work around in the meantime is still useful; in particular if it doesn't require changing mount options, or only requires it temporarily (e.g. not added to /etc/fstab, where it can be forgotten for the life of that system). -- Chris Murphy
