4334 Improve ZFS N-way mirror read performance

*** 23,32 ****
--- 23,33 ----
   * Use is subject to license terms.
   */
  
  /*
   * Copyright (c) 2013 by Delphix. All rights reserved.
+  * Copyright (c) 2013 Steven Hartland. All rights reserved.
   */
  
  #include <sys/zfs_context.h>
  #include <sys/vdev_impl.h>
  #include <sys/spa_impl.h>
*** 227,236 ****
--- 228,239 ----
                  avl_create(&vq->vq_class[p].vqc_queued_tree,
                      fifo ? vdev_queue_timestamp_compare :
                      vdev_queue_offset_compare,
                      sizeof (zio_t), offsetof(struct zio, io_queue_node));
          }
+ 
+         vq->vq_last_queued_offset = 0;
  }
  
  void
  vdev_queue_fini(vdev_t *vd)
  {
*** 723,727 ****
--- 726,760 ----
                  mutex_enter(&vq->vq_lock);
          }
  
          mutex_exit(&vq->vq_lock);
  }
+ 
+ /*
+  * As these three methods are only used for load calculations we're not
+  * concerned if we get an incorrect value on 32bit platforms due to lack of
+  * vq_lock mutex use here, instead we prefer to keep it lock free for
+  * performance.
+  */
+ int
+ vdev_queue_length(vdev_t *vd)
+ {
+         vdev_queue_t *vq = &vd->vdev_queue;
+         int len = 0;
+ 
+         for (zio_priority_t p = 0; p < ZIO_PRIORITY_NUM_QUEUEABLE; p++)
+                 len += avl_numnodes(&vq->vq_class[p].vqc_queued_tree);
+ 
+         return (len);
+ }
+ 
+ uint64_t
+ vdev_queue_last_queued_offset(vdev_t *vd)
+ {
+         return (vd->vdev_queue.vq_last_queued_offset);
+ }
+ 
+ void
+ vdev_queue_register_last_queued_offset(vdev_t *vd, zio_t *zio)
+ {
+         vd->vdev_queue.vq_last_queued_offset = zio->io_offset + zio->io_size;
+ }