On Thu, Aug 11, 2011 at 05:40:36PM -0700, Simon Kirby wrote:

By chance, does "vmstat 1" or top show a high system cpu% while this is
happening,

Absolutely, usually around 80-90%

and if so, does cat /proc/<pid>/stack of the PIDs in R state
sometimes show things under btrfs_run_delayed_refs like
setup_cluster_no_bitmap() and rb_next()?

No, I am not seeing what you describe.  The stack traces are usually
showing what I originally quoted.  I don't know if I've seen
setup_cluster_no_bitmap or rb_* at all.

-- 
Bruce Guenter [off-list ref]                http://untroubled.org/