Skip spurious resilver IO on raidz vdev

On a raidz vdev, a block that does not span all child vdevs, excluding
its skip sectors if any, may not be affected by a child vdev outage or
failure. In such cases, the block does not need to be resilvered.
However, current resilver algorithm simply resilvers all blocks on a
degraded raidz vdev. Such spurious IO is not only wasteful, but also
adds the risk of overwriting good data.

This patch eliminates such spurious IOs.

Reviewed-by: Gvozden Neskovic <neskovic@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes #5316

module/zfs/vdev.c

@@ -1819,6 +1819,21 @@ vdev_dtl_empty(vdev_t *vd, vdev_dtl_type_t t)
 	return (empty);
 }
 
+/*
+ * Returns B_TRUE if vdev determines offset needs to be resilvered.
+ */
+boolean_t
+vdev_dtl_need_resilver(vdev_t *vd, uint64_t offset, size_t psize)
+{
+	ASSERT(vd != vd->vdev_spa->spa_root_vdev);
+
+	if (vd->vdev_ops->vdev_op_need_resilver == NULL ||
+	    vd->vdev_ops->vdev_op_leaf)
+		return (B_TRUE);
+
+	return (vd->vdev_ops->vdev_op_need_resilver(vd, offset, psize));
+}
+
 /*
  * Returns the lowest txg in the DTL range.
  */