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

[Sun Yangkai <sunk67188@gmail.com>]

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

I just come with a ... thing I found and I have no idea why it happened.

I've wrote a script to show the 10 least used block groups(used_space is just
calculated from length and used_pct, so please just ignore them) and before
periodic reclaim, I got:

Searching for the 10 least used DATA block groups...

       vaddr           length     used_pct   used_space
--------------------------------------------------------
 6353387388928       1024MiB          5%         51MiB
 6354461130752       1024MiB         30%        307MiB
 6295292084224       1024MiB         80%        819MiB
 6056900427776       1024MiB         89%        911MiB
 4620552634368       1024MiB         97%        993MiB
 6050457976832       1024MiB         98%       1003MiB
 6122398679040       1024MiB         98%       1003MiB
 6270596022272       1024MiB         98%       1003MiB
 6350166163456       1024MiB         98%       1003MiB
  383347851264       1024MiB         99%       1013MiB

And unallocated space is 3GiB so with dynamic periodic reclaim, the first two
block groups will be reclaimed:

[12月28 21:47] [  T357] BTRFS info (device sda): relocating block group
6353387388928 flags data
[  +0.262467] [  T357] BTRFS info (device sda): found 1970 extents, stage: move
data extents
[  +1.334556] [  T357] BTRFS info (device sda): found 1966 extents, stage:
update data pointers
[  +0.618457] [  T357] BTRFS info (device sda): relocating block group
6354461130752 flags data
[  +1.009694] [  T357] BTRFS info (device sda): found 166 extents, stage: move
data extents
[  +0.388070] [  T357] BTRFS info (device sda): found 166 extents, stage: update
data pointers

And after the reclaim I got:

Searching for the 10 least used DATA block groups...

       vaddr           length     used_pct   used_space
--------------------------------------------------------
 6355534872576       1024MiB          6%         61MiB
 6356608614400       1024MiB         16%        163MiB
 6295292084224       1024MiB         80%        819MiB
 4620552634368       1024MiB         97%        993MiB
 6050457976832       1024MiB         98%       1003MiB
 6270596022272       1024MiB         98%       1003MiB
 3782605471744       1024MiB         99%       1013MiB
 4549685673984       1024MiB         99%       1013MiB
 5882820034560       1024MiB         99%       1013MiB
 5909764243456       1024MiB         99%       1013MiB

These two block groups could be merged into existing chunks, but I have no idea
why that didn't happen. But when I run

btrfs balance start -dvrange=6355534872576..6355534872576 /mnt

It can be merged and free some unallocated space.

So I think the periodic reclaim has a different behavior with manually balance?

Thanks,
Sun YangKai