Stuck btrfs and thread_pool

So I built a Linux box for backup jobs recently that runs off a flash drive, and saves its data to a btrfs filesystem on a RAID1 array. Kernel 2.6.39.x , using iSCSI-SCST to provide several targets. Oddly enough, there would be some btrfs worker process using 20% of CPU, and top would consistently show load of 2.5 to 10. I experimented with some btrfs mount options: adding space_cache,thread_pool=32 to /etc/fstab (and remounting with those options) seems to have dropped the load down to zero!