4334 Improve ZFS N-way mirror read performance

*** 19,28 ****
--- 19,29 ----
   * CDDL HEADER END
   */
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2013 by Delphix. All rights reserved.
+  * Copyright (c) 2013 Steven Hartland. All rights reserved.
   */
  
  #ifndef _SYS_VDEV_IMPL_H
  #define _SYS_VDEV_IMPL_H
  
*** 114,123 ****
--- 115,125 ----
          vdev_queue_class_t vq_class[ZIO_PRIORITY_NUM_QUEUEABLE];
          avl_tree_t      vq_active_tree;
          uint64_t        vq_last_offset;
          hrtime_t        vq_io_complete_ts; /* time last i/o completed */
          kmutex_t        vq_lock;
+         uint64_t        vq_last_queued_offset;
  };
  
  /*
   * Virtual device descriptor
   */
*** 206,215 ****
--- 208,220 ----
          vdev_queue_t    vdev_queue;     /* I/O deadline schedule queue  */
          vdev_cache_t    vdev_cache;     /* physical block cache         */
          spa_aux_vdev_t  *vdev_aux;      /* for l2cache vdevs            */
          zio_t           *vdev_probe_zio; /* root of current probe       */
          vdev_aux_t      vdev_label_aux; /* on-disk aux state            */
+         uint16_t        vdev_rotation_rate; /* rotational rate of the media */
+ #define VDEV_RATE_UNKNOWN       0
+ #define VDEV_RATE_NON_ROTATING  1
  
          /*
           * For DTrace to work in userland (libzpool) context, these fields must
           * remain at the end of the structure.  DTrace will use the kernel's
           * CTF definition for 'struct vdev', and since the size of a kmutex_t is