4334 Improve ZFS N-way mirror read performance

*** 20,29 ****
--- 20,30 ----
   */
  
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2012 by Delphix. All rights reserved.
+  * Copyright (c) 2013 Steven Hartland. All rights reserved.
   */
  
  #ifndef _SYS_VDEV_H
  #define _SYS_VDEV_H
  
*** 116,125 ****
--- 117,129 ----
  
  extern void vdev_queue_init(vdev_t *vd);
  extern void vdev_queue_fini(vdev_t *vd);
  extern zio_t *vdev_queue_io(zio_t *zio);
  extern void vdev_queue_io_done(zio_t *zio);
+ extern int vdev_queue_length(vdev_t *vd);
+ extern uint64_t vdev_queue_lastoffset(vdev_t *vd);
+ extern void vdev_queue_register_lastoffset(vdev_t *vd, zio_t *zio);
  
  extern void vdev_config_dirty(vdev_t *vd);
  extern void vdev_config_clean(vdev_t *vd);
  extern int vdev_config_sync(vdev_t **svd, int svdcount, uint64_t txg,
      boolean_t);