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

[Sun Yangkai <sunk67188@gmail.com>]

在 2025/12/30 08:00, Boris Burkov 写道:
> 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.

Yes, my system has commit=300. It was set years ago when I knew almost nothing
about btrfs and not changed since then.

>>
>> I'm still digging further into it.
> 
> Were you able to confirm whether that negative reclaimable_bytes bug was
> the root cause here?

Yes. After changing chunk_sz to s64, this will not triggered anymore. However,
periodic also does not work properly.

> 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.

Thank you for your advice. That's what I've done and how I find the unexpected
behavior. It's really a good tool to know what's happening in kernel.

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

I've redesigned the logic and iterated some versions. I'll cleanup my code and
send the patches later. Maybe later today or tomorrow. It's not perfect, but I
hope it will be better than what we have now.

Thanks,
Sun YangKai