I'm using Ubuntu's 2.6.32-21-generic kernel and I created a 48GB btrfs filesystem to hold approx. 2M files. The filesystem is at 85% full and returning ENOSPC when I try to create anything new on it. I got a spew of messages in the kernel message buffer which I don't know are related, but I thought I would provide them anyway: 19:12:44 kernel [ 721.973362] space_info has 8142848 free, is full 19:12:44 kernel [ 721.973389] space_info total=3221225472, pinned=8335360, delalloc=0, may_use=0, used=3203059712, root=2875392, super=65536, reserved=1622016 19:12:44 kernel [ 721.976174] space_info has 8142848 free, is full 19:12:44 kernel [ 721.976195] space_info total=3221225472, pinned=8335360, delalloc=0, may_use=0, used=3203059712, root=2875392, super=65536, reserved=1622016 19:12:44 kernel [ 721.976690] space_info has 8142848 free, is full 19:12:44 kernel [ 721.976711] space_info total=3221225472, pinned=8335360, delalloc=0, may_use=0, used=3203059712, root=2875392, super=65536, reserved=1622016 19:12:44 kernel [ 721.977454] space_info has 8142848 free, is full 19:12:44 kernel [ 721.977479] space_info total=3221225472, pinned=8335360, delalloc=0, may_use=0, used=3203059712, root=2875392, super=65536, reserved=1622016 19:12:44 kernel [ 721.978034] space_info has 8142848 free, is full 19:12:44 kernel [ 721.978055] space_info total=3221225472, pinned=8335360, delalloc=0, may_use=0, used=3203059712, root=2875392, super=65536, reserved=1622016 19:12:44 kernel [ 721.978549] space_info has 8142848 free, is full 19:12:44 kernel [ 721.978570] space_info total=3221225472, pinned=8335360, delalloc=0, may_use=0, used=3203059712, root=2875392, super=65536, reserved=1622016 19:12:44 kernel [ 721.979076] space_info has 8142848 free, is full
Later kernels are much more effective at filling the disk, especially the 2.6.34 kernel with the latest commits from the btrfs-unstable tree. This is something that we are actively working on.
Closing, if this is still affecting you on a newer kernel please reopen.