Re: Massive I/O usage from btrfs-cleaner after upgrading to 5.16

["François-Xavier Thomas" <fx.thomas@gmail.com>]

Hi,

More details on graph[0]:
- First patch (1-byte file) on 5.16.0 did not have a significant impact.
- Both patches on 5.16.0 did reduce a large part of the I/O but still
have a high baseline I/O compared to 5.15

Some people reported that 5.16.1 improved the situation for them, so
I'm testing that. It's too early to tell but for now the baseline I/O
still seems to be high compared to 5.15. Will update with more results
tomorrow.

François-Xavier

[0] https://i.imgur.com/agzAKGc.png

On Mon, Jan 17, 2022 at 10:37 PM François-Xavier Thomas
<fx.thomas@gmail.com> wrote:
>
> Hi Filipe,
>
> Thank you so much for the hints!
>
> I compiled 5.16 with the 1-byte file patch and have been running it
> for a couple of hours now. I/O seems to have been gradually increasing
> compared to 5.15, but I will wait for tomorrow to have a clearer view
> on the graphs, then I'll try the both patches.
>
> François-Xavier
>
> On Mon, Jan 17, 2022 at 5:59 PM Filipe Manana <fdmanana@kernel.org> wrote:
> >
> > On Mon, Jan 17, 2022 at 12:02:08PM +0000, Filipe Manana wrote:
> > > On Mon, Jan 17, 2022 at 11:06:42AM +0100, François-Xavier Thomas wrote:
> > > > Hello all,
> > > >
> > > > Just in case someone is having the same issue: Btrfs (in the
> > > > btrfs-cleaner process) is taking a large amount of disk IO after
> > > > upgrading to 5.16 on one of my volumes, and multiple other people seem
> > > > to be having the same issue, see discussion in [0].
> > > >
> > > > [1] is a close-up screenshot of disk I/O history (blue line is write
> > > > ops, going from a baseline of some 10 ops/s to around 1k ops/s). I
> > > > downgraded from 5.16 to 5.15 in the middle, which immediately restored
> > > > previous performance.
> > > >
> > > > Common options between affected people are: ssd, autodefrag. No error
> > > > in the logs, and no other issue aside from performance (the volume
> > > > works just fine for accessing data).
> > > >
> > > > One person reports that SMART stats show a massive amount of blocks
> > > > being written; unfortunately I do not have historical data for that so
> > > > I cannot confirm, but this sounds likely given what I see on what
> > > > should be a relatively new SSD.
> > > >
> > > > Any idea of what it could be related to?
> > >
> > > There was a big refactor of the defrag code that landed in 5.16.
> > >
> > > On a quick glance, when using autodefrag it seems we now can end up in an
> > > infinite loop by marking the same range for degrag (IO) over and over.
> > >
> > > Can you try the following patch? (also at https://pastebin.com/raw/QR27Jv6n)
> >
> > Actually try this one instead:
> >
> > https://pastebin.com/raw/EbEfk1tF
> >
> > Also, there's a bug with defrag running into an (almost) infinite loop when
> > attempting to defrag a 1 byte file. Someone ran into this and I've just sent
> > a fix for it:
> >
> > https://patchwork.kernel.org/project/linux-btrfs/patch/bcbfce0ff7e21bbfed2484b1457e560edf78020d.1642436805.git.fdmanana@suse.com/
> >
> > Maybe that is what you are running into when using autodefrag.
> > Firt try that fix for the 1 byte file case, and if after that you still run
> > into problems, then try with the other patch above as well (both patches
> > applied).
> >
> > Thanks.
> >
> >
> >
> > >
> > > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> > > index a5bd6926f7ff..0a9f6125a566 100644
> > > --- a/fs/btrfs/ioctl.c
> > > +++ b/fs/btrfs/ioctl.c
> > > @@ -1213,6 +1213,13 @@ static int defrag_collect_targets(struct btrfs_inode *inode,
> > >                 if (em->generation < newer_than)
> > >                         goto next;
> > >
> > > +               /*
> > > +                * Skip extents already under IO, otherwise we can end up in an
> > > +                * infinite loop when using auto defrag.
> > > +                */
> > > +               if (em->generation == (u64)-1)
> > > +                       goto next;
> > > +
> > >                 /*
> > >                  * For do_compress case, we want to compress all valid file
> > >                  * extents, thus no @extent_thresh or mergeable check.
> > >
> > >
> > > >
> > > > François-Xavier
> > > >
> > > > [0] https://www.reddit.com/r/btrfs/comments/s4nrzb/massive_performance_degradation_after_upgrading/
> > > > [1] https://imgur.com/oYhYat1