4045 zfs write throttle & i/o scheduler performance work
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>

*** 308,318 ****
                  mutex_exit(&vc->vc_lock);
                  return (SET_ERROR(ENOMEM));
          }
  
          fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
!             ve->ve_data, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_CACHE_FILL,
              ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
  
          ve->ve_fill_io = fio;
          zio_vdev_io_bypass(zio);
          zio_add_child(zio, fio);
--- 308,318 ----
                  mutex_exit(&vc->vc_lock);
                  return (SET_ERROR(ENOMEM));
          }
  
          fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
!             ve->ve_data, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_NOW,
              ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
  
          ve->ve_fill_io = fio;
          zio_vdev_io_bypass(zio);
          zio_add_child(zio, fio);