I have two machines with btrfs, that give me the "blocked for more than 120 seconds" message. After that I cannot write anything to disk, i am unable to unmount the btrfs filesystem and i can only reboot with sysrq-trigger.
It always happens when i write many files with rsync over network. When i used 3.2rc6 it happened randomly on both machines after 50-500gb of writes. with rc7 it happens after much less writes, probably 10gb or so, but only on machine 1 for the time being. machine 2 has not crashed yet after 200gb of writes and I am still testing that.
machine 1: btrfs on a 6tb sparse file, mounted as loop, on a xfs filesystem that lies on a 10TB md raid5. mount options compress=zlib,compress-force
machine 2: btrfs over md raid 5 (4x2TB)=5.5TB filesystem. mount options compress=zlib,compress-force
3.2rc7 (not crashed yet)