Re: [PATCH] btrfs: make periodic dynamic reclaim the default for data

[Boris Burkov <boris@bur.io>]

On Fri, Dec 26, 2025 at 11:07:28AM +0800, Sun Yangkai wrote:
> Hi Boris,

First off, sorry for not replying promptly. I've been in and out of the
office around the holidays.

> 
> Thank you for bring such a feature for btrfs. I love it a lot and try to enable
> it on my machine.

I really appreciate your kind words and your interest in the feature.
Thank you!

> 
> But I've get into some unexpected behavior when periodic dynamic reclaim is
> enabled and the filesystem is nearly full.

Oops! Let's debug it :)

> 
> [12月26 10:41] [T20373] BTRFS info (device sda): relocating block group
> 5214541578240 flags data
> [  +0.012446] [T20373] BTRFS error (device sda): error relocating chunk
> 5214541578240
> [  +0.000033] [T20373] BTRFS info (device sda): relocating block group
> 4540021997568 flags data
> [  +0.008927] [T20373] BTRFS error (device sda): error relocating chunk
> 4540021997568
> [  +0.000025] [T20373] BTRFS info (device sda): relocating block group
> 5606746750976 flags data
> [12月26 10:42] [T20373] BTRFS error (device sda): error relocating chunk
> 5606746750976
> [12月26 10:47] [T12072] BTRFS info (device sda): relocating block group
> 5606746750976 flags data
> [  +3.960400] [T12072] BTRFS error (device sda): error relocating chunk
> 5606746750976
> [12月26 10:52] [ T7643] BTRFS info (device sda): relocating block group
> 5606746750976 flags data
> [  +3.960314] [ T7643] BTRFS error (device sda): error relocating chunk
> 5606746750976
> [12月26 10:57] [T20373] BTRFS info (device sda): relocating block group
> 5606746750976 flags data
> [  +3.954485] [T20373] BTRFS error (device sda): error relocating chunk
> 5606746750976
> [12月26 11:02] [ T7701] BTRFS info (device sda): relocating block group
> 5606746750976 flags data
> [  +4.561796] [ T7701] BTRFS error (device sda): error relocating chunk
> 5606746750976
> 
> I guess the condition of when the periodic reclaim should happen is unpolished.

Yeah, it looks like it is triggering too frequently in conditions where
it isn't likely to succeed. Hopefully we can tune up the heuristics (or
just fix the bug you found) and it works better.

It seems to be triggering every 5 minutes or so, right? Is that the
interval of the cleaner thread running on your system? Or am I
misinterpreting the time stamps? I would normally expect the default of
30s.

> 
> I'm still digging further into it.

Were you able to confirm whether that negative reclaimable_bytes bug was
the root cause here?

If you aren't able to reproduce but it is still happening on one of your
systems, we can try to instrument the periodic reclaim lifecycle with
bpftrace to catch calls to the various important functions setting it
reclaimable, etc.

Please let me know if I can assist you with that, or if you do have a
reproducer I could also look at.

Thanks,
Boris

> 
> Thanks,
> Sun YangKai