increase number of outstanding async writes when sync task is waiting
zfs - Zettabyte File System
When a sync task is waiting for a txg to complete, we should hurry it along by increasing the number of outstanding async writes (i.e. make vdev_queue_max_async_writes() return a larger number). Initially we might just have a tunable for "minimum async writes while a synctask is waiting" and set it to 3.