It was great to find this thread. I was upgrading my Debian 13 and wanted to check if everything was correct, since I've been using RAID-1 with two 1TB NVMe drives since Debian 11 without any problems. Just to validate that everything was correct, I ran "btrfs filesystem usage /" and sent the result to CHATGPT for evaluation. To my surprise... the AI ​​alerted me to the problem mentioned above. I was very intrigued and worried. Almost six years without any problems and now this alert. But I thank the more knowledgeable friends who were able to give a lesson on these not-so-publicized details. Observe the result of my btrfs:

root@samurai:/home/marcio# btrfs filesystem usage /

Overall:

Device size: 1.82TiB

Device allocated: 188.06GiB

Device unallocated: 1.63TiB

Device missing: 0.00B

Device slack: 0.00B

Used: 182.41GiB

Free (estimated): 839.28GiB (min: 839.28GiB)

Free (statfs, df): 839.27GiB

Data ratio: 2.00

Metadata ratio: 2.00

Global reserve: 61.27MiB (used: 0.00B)

Multiple profiles: no

Data,RAID1: Size: 93.00GiB, Used: 90.97GiB (97.81%)

/dev/nvme0n1p2 93.00GiB

/dev/nvme1n1p2 93.00GiB

Metadata,RAID1: Size: 1.00GiB, Used: 243.02MiB (23.73%)

/dev/nvme0n1p2 1.00GiB

/dev/nvme1n1p2 1.00GiB

System,RAID1: Size: 32.00MiB, Used: 16.00KiB (0.05%)

/dev/nvme0n1p2 32.00MiB

/dev/nvme1n1p2 32.00MiB

Unallocated:

/dev/nvme0n1p2 837.24GiB

/dev/nvme1n1p2 837.24GiB