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>

*** 870,880 ****
                      ZIO_FLAG_CANFAIL);
          }
          if (lwb->lwb_zio == NULL) {
                  lwb->lwb_zio = zio_rewrite(zilog->zl_root_zio, zilog->zl_spa,
                      0, &lwb->lwb_blk, lwb->lwb_buf, BP_GET_LSIZE(&lwb->lwb_blk),
!                     zil_lwb_write_done, lwb, ZIO_PRIORITY_LOG_WRITE,
                      ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE, &zb);
          }
  }
  
  /*
--- 870,880 ----
                      ZIO_FLAG_CANFAIL);
          }
          if (lwb->lwb_zio == NULL) {
                  lwb->lwb_zio = zio_rewrite(zilog->zl_root_zio, zilog->zl_spa,
                      0, &lwb->lwb_blk, lwb->lwb_buf, BP_GET_LSIZE(&lwb->lwb_blk),
!                     zil_lwb_write_done, lwb, ZIO_PRIORITY_SYNC_WRITE,
                      ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE, &zb);
          }
  }
  
  /*