Profile conversion - unexpected large allocation on target profile during conversion

[Pedro Macedo <pmacedo@pmacedo.com>]

Hi folks,

I'm in the process of converting a few btrfs arrays from single to raid 
6 and noticed one behavior that seems unexpected: according to btrfs 
filesystem usage, the total value is extremely large compared to the 
used value during the conversion.

For example, on one filesystem this is the reported total/used while 
conversion is running - notice the 2TB allocation for ~370GB of data:

Data,single: Size:40032.00GiB, Used:39982.40GiB (99.88%)
Data,RAID6: Size:2021.09GiB, Used:376.60GiB (18.63%)
Metadata,RAID1C4: Size:84.00GiB, Used:83.47GiB (99.37%)
System,RAID1C4: Size:0.03GiB, Used:0.00GiB (14.06%)

If I cancel the conversion and give it a few seconds then the vast 
majority of the reported space for raid6 is reclaimed 
(bg_reclaim_threshold is set to a very high 75%, but I don't see the 
usual log messages about reclaiming used blocks):

Data,single: Size:40023.00GiB, Used:39973.42GiB (99.88%)
Data,RAID6: Size:406.25GiB, Used:384.58GiB (94.67%)
Metadata,RAID1C4: Size:84.00GiB, Used:83.47GiB (99.37%)
System,RAID1C4: Size:0.03GiB, Used:0.00GiB (14.01%)

Is this over-allocation during conversion expected or known? This is on 
kernel 6.8.9; I only really noticed this because one of the filesystems 
failed the conversion with ENOSPC even though there should be plenty of 
space. For now I'm working around the ENOSPC issue on the smaller array 
by using a loop with dconvert=raid6,limit=100 followed by a 30s sleep.


Thanks,

Pedro Macedo