Andrew,

  Facing some challenges to test this. If you have a chance to test
  it again, the following output will be interesting to observe.
    iostat -ctx -p sda 3 >  /tmp/iostat.out

  Also note your system time when this problem occurs, (iostat has time
  stamp, I wish see the waitQ and activeQ at that time, hopefully
  captured in the file /tmp/iostat.out above).

  We need more clarity on the test-case which can reproduce this issue.
  As I know you are writing into the btrfs. However is that a large
  number of small files (you are creating new files) OR you are writing
  a few large new files ?

  If there is any script to test this that will make understanding
  a lot easier.

  PS: Does anybody know Solaris lockstat(1M) equivalent in Linux ?

Thanks, Anand