BTRFS lag

[Eben King]

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?