On Mon, Apr 21, 2025 at 9:32 PM Eben King <e...@gmx.us> wrote: > > Hi. I have a 4-disk 5.4 TiB (raw) BTRFS array. It is used pretty much > exclusively for backups. Today I backed up the NAS to it. Since there > had been a lot of changes, the total size of the backup was some 300 > GiB. During the backup, the "estimated free space" as reported by > "btrfs filesystem usage" dropped as expected, but the "allocated space" > didn't change until I'd written maybe 200 GiB to the array. Is this > expected behavior, or do I need to do something?
Just a quick preliminary guess: The filesystem have previously been large enough to use those 200 GB, and even though you then removed some data it doesn't release allocated blocks immediately when they become unused, but keep it for future use. // Anders
