Illumos #3954, #4080, #4081

3954 metaslabs continue to load even after hitting zfs_mg_alloc_failure limit
4080 zpool clear fails to clear pool
4081 need zfs_mg_noalloc_threshold
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  https://www.illumos.org/issues/3954
  https://www.illumos.org/issues/4080
  https://www.illumos.org/issues/4081
  illumos/illumos-gate@22e30981d8

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775

include/sys/metaslab_impl.h

@@ -24,7 +24,7 @@
  */
 
 /*
- * Copyright (c) 2012 by Delphix. All rights reserved.
+ * Copyright (c) 2013 by Delphix. All rights reserved.
  */
 
 #ifndef _SYS_METASLAB_IMPL_H
@@ -45,6 +45,7 @@ struct metaslab_class {
 	metaslab_group_t	*mc_rotor;
 	space_map_ops_t		*mc_ops;
 	uint64_t		mc_aliquot;
+	uint64_t		mc_alloc_groups; /* # of allocatable groups */
 	uint64_t		mc_alloc;	/* total allocated space */
 	uint64_t		mc_deferred;	/* total deferred frees */
 	uint64_t		mc_space;	/* total space (alloc + free) */
@@ -58,6 +59,8 @@ struct metaslab_group {
 	uint64_t		mg_aliquot;
 	uint64_t		mg_bonus_area;
 	uint64_t		mg_alloc_failures;
+	boolean_t		mg_allocatable;		/* can we allocate? */
+	uint64_t		mg_free_capacity;	/* percentage free */
 	int64_t			mg_bias;
 	int64_t			mg_activation_count;
 	metaslab_class_t	*mg_class;