Illumos 4976-4984 - metaslab improvements

4976 zfs should only avoid writing to a failing non-redundant top-level vdev
4978 ztest fails in get_metaslab_refcount()
4979 extend free space histogram to device and pool
4980 metaslabs should have a fragmentation metric
4981 remove fragmented ops vector from block allocator
4982 space_map object should proactively upgrade when feature is enabled
4983 need to collect metaslab information via mdb
4984 device selection should use fragmentation metric
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Adam Leventhal <[email protected]>
Reviewed by: Christopher Siden <[email protected]>
Approved by: Garrett D'Amore <[email protected]>

References:
  https://www.illumos.org/issues/4976
  https://www.illumos.org/issues/4978
  https://www.illumos.org/issues/4979
  https://www.illumos.org/issues/4980
  https://www.illumos.org/issues/4981
  https://www.illumos.org/issues/4982
  https://www.illumos.org/issues/4983
  https://www.illumos.org/issues/4984
  https://github.com/illumos/illumos-gate/commit/2e4c998

Notes:
    The "zdb -M" option has been re-tasked to display the new metaslab
    fragmentation metric and the new "zdb -I" option is used to control
    the maximum number of in-flight I/Os.

    The new fragmentation metric is derived from the space map histogram
    which has been rolled up to the vdev and pool level and is presented
    to the user via "zpool list".

    Add a number of module parameters related to the new metaslab weighting
    logic.

Ported by: Tim Chase <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #2595

7 files changed, 541 insertions, 156 deletions

diff --git a/module/zcommon/zpool_prop.c b/module/zcommon/zpool_prop.c
index 6775c09d3..6bde4f471 100644
--- a/module/zcommon/zpool_prop.c
+++ b/module/zcommon/zpool_prop.c
@@ -21,7 +21,7 @@
 /*
  * Copyright (c) 2007, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright 2011 Nexenta Systems, Inc. All rights reserved.
- * Copyright (c) 2012 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
  */
 
 #include <sys/zio.h>
@@ -87,6 +87,8 @@ zpool_prop_init(void)
 	    PROP_READONLY, ZFS_TYPE_POOL, "<size>", "ALLOC");
 	zprop_register_number(ZPOOL_PROP_EXPANDSZ, "expandsize", 0,
 	    PROP_READONLY, ZFS_TYPE_POOL, "<size>", "EXPANDSZ");
+	zprop_register_number(ZPOOL_PROP_FRAGMENTATION, "fragmentation", 0,
+	    PROP_READONLY, ZFS_TYPE_POOL, "<percent>", "FRAG");
 	zprop_register_number(ZPOOL_PROP_CAPACITY, "capacity", 0, PROP_READONLY,
 	    ZFS_TYPE_POOL, "<size>", "CAP");
 	zprop_register_number(ZPOOL_PROP_GUID, "guid", 0, PROP_READONLY,
diff --git a/module/zfs/metaslab.c b/module/zfs/metaslab.c
index 9c09837d5..68a1591c2 100644
--- a/module/zfs/metaslab.c
+++ b/module/zfs/metaslab.c
@@ -32,6 +32,7 @@
 #include <sys/vdev_impl.h>
 #include <sys/zio.h>
 #include <sys/spa_impl.h>
+#include <sys/zfeature.h>
 
 #define	WITH_DF_BLOCK_ALLOCATOR
 
@@ -66,7 +67,7 @@ int zfs_condense_pct = 200;
 /*
  * The zfs_mg_noalloc_threshold defines which metaslab groups should
  * be eligible for allocation. The value is defined as a percentage of
- * a free space. Metaslab groups that have more free space than
+ * free space. Metaslab groups that have more free space than
  * zfs_mg_noalloc_threshold are always eligible for allocations. Once
  * a metaslab group's free space is less than or equal to the
  * zfs_mg_noalloc_threshold the allocator will avoid allocating to that
@@ -79,6 +80,23 @@ int zfs_condense_pct = 200;
 int zfs_mg_noalloc_threshold = 0;
 
 /*
+ * Metaslab groups are considered eligible for allocations if their
+ * fragmenation metric (measured as a percentage) is less than or equal to
+ * zfs_mg_fragmentation_threshold. If a metaslab group exceeds this threshold
+ * then it will be skipped unless all metaslab groups within the metaslab
+ * class have also crossed this threshold.
+ */
+int zfs_mg_fragmentation_threshold = 85;
+
+/*
+ * Allow metaslabs to keep their active state as long as their fragmentation
+ * percentage is less than or equal to zfs_metaslab_fragmentation_threshold. An
+ * active metaslab that exceeds this threshold will no longer keep its active
+ * status allowing better metaslabs to be selected.
+ */
+int zfs_metaslab_fragmentation_threshold = 70;
+
+/*
  * When set will load all metaslabs when pool is first opened.
  */
 int metaslab_debug_load = 0;
@@ -123,11 +141,6 @@ int metaslab_load_pct = 50;
 int metaslab_unload_delay = TXG_SIZE * 2;
 
 /*
- * Should we be willing to write data to degraded vdevs?
- */
-boolean_t zfs_write_to_degraded = B_FALSE;
-
-/*
  * Max number of metaslabs per group to preload.
  */
 int metaslab_preload_limit = SPA_DVAS_PER_BP;
@@ -135,13 +148,24 @@ int metaslab_preload_limit = SPA_DVAS_PER_BP;
 /*
  * Enable/disable preloading of metaslab.
  */
-boolean_t metaslab_preload_enabled = B_TRUE;
+int metaslab_preload_enabled = B_TRUE;
 
 /*
- * Enable/disable additional weight factor for each metaslab.
+ * Enable/disable fragmentation weighting on metaslabs.
  */
-boolean_t metaslab_weight_factor_enable = B_FALSE;
+int metaslab_fragmentation_factor_enabled = B_TRUE;
 
+/*
+ * Enable/disable lba weighting (i.e. outer tracks are given preference).
+ */
+int metaslab_lba_weighting_enabled = B_TRUE;
+
+/*
+ * Enable/disable metaslab group biasing.
+ */
+int metaslab_bias_enabled = B_TRUE;
+
+static uint64_t metaslab_fragmentation(metaslab_t *);
 
 /*
  * ==========================================================================
@@ -236,6 +260,123 @@ metaslab_class_get_dspace(metaslab_class_t *mc)
 	return (spa_deflate(mc->mc_spa) ? mc->mc_dspace : mc->mc_space);
 }
 
+void
+metaslab_class_histogram_verify(metaslab_class_t *mc)
+{
+	vdev_t *rvd = mc->mc_spa->spa_root_vdev;
+	uint64_t *mc_hist;
+	int i, c;
+
+	if ((zfs_flags & ZFS_DEBUG_HISTOGRAM_VERIFY) == 0)
+		return;
+
+	mc_hist = kmem_zalloc(sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE,
+	    KM_PUSHPAGE);
+
+	for (c = 0; c < rvd->vdev_children; c++) {
+		vdev_t *tvd = rvd->vdev_child[c];
+		metaslab_group_t *mg = tvd->vdev_mg;
+
+		/*
+		 * Skip any holes, uninitialized top-levels, or
+		 * vdevs that are not in this metalab class.
+		 */
+		if (tvd->vdev_ishole || tvd->vdev_ms_shift == 0 ||
+		    mg->mg_class != mc) {
+			continue;
+		}
+
+		for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
+			mc_hist[i] += mg->mg_histogram[i];
+	}
+
+	for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
+		VERIFY3U(mc_hist[i], ==, mc->mc_histogram[i]);
+
+	kmem_free(mc_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
+}
+
+/*
+ * Calculate the metaslab class's fragmentation metric. The metric
+ * is weighted based on the space contribution of each metaslab group.
+ * The return value will be a number between 0 and 100 (inclusive), or
+ * ZFS_FRAG_INVALID if the metric has not been set. See comment above the
+ * zfs_frag_table for more information about the metric.
+ */
+uint64_t
+metaslab_class_fragmentation(metaslab_class_t *mc)
+{
+	vdev_t *rvd = mc->mc_spa->spa_root_vdev;
+	uint64_t fragmentation = 0;
+	int c;
+
+	spa_config_enter(mc->mc_spa, SCL_VDEV, FTAG, RW_READER);
+
+	for (c = 0; c < rvd->vdev_children; c++) {
+		vdev_t *tvd = rvd->vdev_child[c];
+		metaslab_group_t *mg = tvd->vdev_mg;
+
+		/*
+		 * Skip any holes, uninitialized top-levels, or
+		 * vdevs that are not in this metalab class.
+		 */
+		if (tvd->vdev_ishole || tvd->vdev_ms_shift == 0 ||
+		    mg->mg_class != mc) {
+			continue;
+		}
+
+		/*
+		 * If a metaslab group does not contain a fragmentation
+		 * metric then just bail out.
+		 */
+		if (mg->mg_fragmentation == ZFS_FRAG_INVALID) {
+			spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
+			return (ZFS_FRAG_INVALID);
+		}
+
+		/*
+		 * Determine how much this metaslab_group is contributing
+		 * to the overall pool fragmentation metric.
+		 */
+		fragmentation += mg->mg_fragmentation *
+		    metaslab_group_get_space(mg);
+	}
+	fragmentation /= metaslab_class_get_space(mc);
+
+	ASSERT3U(fragmentation, <=, 100);
+	spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
+	return (fragmentation);
+}
+
+/*
+ * Calculate the amount of expandable space that is available in
+ * this metaslab class. If a device is expanded then its expandable
+ * space will be the amount of allocatable space that is currently not
+ * part of this metaslab class.
+ */
+uint64_t
+metaslab_class_expandable_space(metaslab_class_t *mc)
+{
+	vdev_t *rvd = mc->mc_spa->spa_root_vdev;
+	uint64_t space = 0;
+	int c;
+
+	spa_config_enter(mc->mc_spa, SCL_VDEV, FTAG, RW_READER);
+	for (c = 0; c < rvd->vdev_children; c++) {
+		vdev_t *tvd = rvd->vdev_child[c];
+		metaslab_group_t *mg = tvd->vdev_mg;
+
+		if (tvd->vdev_ishole || tvd->vdev_ms_shift == 0 ||
+		    mg->mg_class != mc) {
+			continue;
+		}
+
+		space += tvd->vdev_max_asize - tvd->vdev_asize;
+	}
+	spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
+	return (space);
+}
+
 /*
  * ==========================================================================
  * Metaslab groups
@@ -288,7 +429,15 @@ metaslab_group_alloc_update(metaslab_group_t *mg)
 	mg->mg_free_capacity = ((vs->vs_space - vs->vs_alloc) * 100) /
 	    (vs->vs_space + 1);
 
-	mg->mg_allocatable = (mg->mg_free_capacity > zfs_mg_noalloc_threshold);
+	/*
+	 * A metaslab group is considered allocatable if it has plenty
+	 * of free space or is not heavily fragmented. We only take
+	 * fragmentation into account if the metaslab group has a valid
+	 * fragmentation metric (i.e. a value between 0 and 100).
+	 */
+	mg->mg_allocatable = (mg->mg_free_capacity > zfs_mg_noalloc_threshold &&
+	    (mg->mg_fragmentation == ZFS_FRAG_INVALID ||
+	    mg->mg_fragmentation <= zfs_mg_fragmentation_threshold));
 
 	/*
 	 * The mc_alloc_groups maintains a count of the number of
@@ -309,6 +458,7 @@ metaslab_group_alloc_update(metaslab_group_t *mg)
 		mc->mc_alloc_groups--;
 	else if (!was_allocatable && mg->mg_allocatable)
 		mc->mc_alloc_groups++;
+
 	mutex_exit(&mg->mg_lock);
 }
 
@@ -398,6 +548,7 @@ metaslab_group_passivate(metaslab_group_t *mg)
 	}
 
 	taskq_wait(mg->mg_taskq);
+	metaslab_group_alloc_update(mg);
 
 	mgprev = mg->mg_prev;
 	mgnext = mg->mg_next;
@@ -414,20 +565,115 @@ metaslab_group_passivate(metaslab_group_t *mg)
 	mg->mg_next = NULL;
 }
 
+uint64_t
+metaslab_group_get_space(metaslab_group_t *mg)
+{
+	return ((1ULL << mg->mg_vd->vdev_ms_shift) * mg->mg_vd->vdev_ms_count);
+}
+
+void
+metaslab_group_histogram_verify(metaslab_group_t *mg)
+{
+	uint64_t *mg_hist;
+	vdev_t *vd = mg->mg_vd;
+	uint64_t ashift = vd->vdev_ashift;
+	int i, m;
+
+	if ((zfs_flags & ZFS_DEBUG_HISTOGRAM_VERIFY) == 0)
+		return;
+
+	mg_hist = kmem_zalloc(sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE,
+	    KM_PUSHPAGE);
+
+	ASSERT3U(RANGE_TREE_HISTOGRAM_SIZE, >=,
+	    SPACE_MAP_HISTOGRAM_SIZE + ashift);
+
+	for (m = 0; m < vd->vdev_ms_count; m++) {
+		metaslab_t *msp = vd->vdev_ms[m];
+
+		if (msp->ms_sm == NULL)
+			continue;
+
+		for (i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++)
+			mg_hist[i + ashift] +=
+			    msp->ms_sm->sm_phys->smp_histogram[i];
+	}
+
+	for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i ++)
+		VERIFY3U(mg_hist[i], ==, mg->mg_histogram[i]);
+
+	kmem_free(mg_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
+}
+
 static void
-metaslab_group_add(metaslab_group_t *mg, metaslab_t *msp)
+metaslab_group_histogram_add(metaslab_group_t *mg, metaslab_t *msp)
 {
+	metaslab_class_t *mc = mg->mg_class;
+	uint64_t ashift = mg->mg_vd->vdev_ashift;
+	int i;
+
+	ASSERT(MUTEX_HELD(&msp->ms_lock));
+	if (msp->ms_sm == NULL)
+		return;
+
 	mutex_enter(&mg->mg_lock);
+	for (i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
+		mg->mg_histogram[i + ashift] +=
+		    msp->ms_sm->sm_phys->smp_histogram[i];
+		mc->mc_histogram[i + ashift] +=
+		    msp->ms_sm->sm_phys->smp_histogram[i];
+	}
+	mutex_exit(&mg->mg_lock);
+}
+
+void
+metaslab_group_histogram_remove(metaslab_group_t *mg, metaslab_t *msp)
+{
+	metaslab_class_t *mc = mg->mg_class;
+	uint64_t ashift = mg->mg_vd->vdev_ashift;
+	int i;
+
+	ASSERT(MUTEX_HELD(&msp->ms_lock));
+	if (msp->ms_sm == NULL)
+		return;
+
+	mutex_enter(&mg->mg_lock);
+	for (i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
+		ASSERT3U(mg->mg_histogram[i + ashift], >=,
+		    msp->ms_sm->sm_phys->smp_histogram[i]);
+		ASSERT3U(mc->mc_histogram[i + ashift], >=,
+		    msp->ms_sm->sm_phys->smp_histogram[i]);
+
+		mg->mg_histogram[i + ashift] -=
+		    msp->ms_sm->sm_phys->smp_histogram[i];
+		mc->mc_histogram[i + ashift] -=
+		    msp->ms_sm->sm_phys->smp_histogram[i];
+	}
+	mutex_exit(&mg->mg_lock);
+}
+
+static void
+metaslab_group_add(metaslab_group_t *mg, metaslab_t *msp)
+{
 	ASSERT(msp->ms_group == NULL);
+	mutex_enter(&mg->mg_lock);
 	msp->ms_group = mg;
 	msp->ms_weight = 0;
 	avl_add(&mg->mg_metaslab_tree, msp);
 	mutex_exit(&mg->mg_lock);
+
+	mutex_enter(&msp->ms_lock);
+	metaslab_group_histogram_add(mg, msp);
+	mutex_exit(&msp->ms_lock);
 }
 
 static void
 metaslab_group_remove(metaslab_group_t *mg, metaslab_t *msp)
 {
+	mutex_enter(&msp->ms_lock);
+	metaslab_group_histogram_remove(mg, msp);
+	mutex_exit(&msp->ms_lock);
+
 	mutex_enter(&mg->mg_lock);
 	ASSERT(msp->ms_group == mg);
 	avl_remove(&mg->mg_metaslab_tree, msp);
@@ -440,9 +686,9 @@ metaslab_group_sort(metaslab_group_t *mg, metaslab_t *msp, uint64_t weight)
 {
 	/*
 	 * Although in principle the weight can be any value, in
-	 * practice we do not use values in the range [1, 510].
+	 * practice we do not use values in the range [1, 511].
 	 */
-	ASSERT(weight >= SPA_MINBLOCKSIZE-1 || weight == 0);
+	ASSERT(weight >= SPA_MINBLOCKSIZE || weight == 0);
 	ASSERT(MUTEX_HELD(&msp->ms_lock));
 
 	mutex_enter(&mg->mg_lock);
@@ -454,9 +700,43 @@ metaslab_group_sort(metaslab_group_t *mg, metaslab_t *msp, uint64_t weight)
 }
 
 /*
+ * Calculate the fragmentation for a given metaslab group. We can use
+ * a simple average here since all metaslabs within the group must have
+ * the same size. The return value will be a value between 0 and 100
+ * (inclusive), or ZFS_FRAG_INVALID if less than half of the metaslab in this
+ * group have a fragmentation metric.
+ */
+uint64_t
+metaslab_group_fragmentation(metaslab_group_t *mg)
+{
+	vdev_t *vd = mg->mg_vd;
+	uint64_t fragmentation = 0;
+	uint64_t valid_ms = 0;
+	int m;
+
+	for (m = 0; m < vd->vdev_ms_count; m++) {
+		metaslab_t *msp = vd->vdev_ms[m];
+
+		if (msp->ms_fragmentation == ZFS_FRAG_INVALID)
+			continue;
+
+		valid_ms++;
+		fragmentation += msp->ms_fragmentation;
+	}
+
+	if (valid_ms <= vd->vdev_ms_count / 2)
+		return (ZFS_FRAG_INVALID);
+
+	fragmentation /= valid_ms;
+	ASSERT3U(fragmentation, <=, 100);
+	return (fragmentation);
+}
+
+/*
  * Determine if a given metaslab group should skip allocations. A metaslab
- * group should avoid allocations if its used capacity has crossed the
- * zfs_mg_noalloc_threshold and there is at least one metaslab group
+ * group should avoid allocations if its free capacity is less than the
+ * zfs_mg_noalloc_threshold or its fragmentation metric is greater than
+ * zfs_mg_fragmentation_threshold and there is at least one metaslab group
  * that can still handle allocations.
  */
 static boolean_t
@@ -467,12 +747,19 @@ metaslab_group_allocatable(metaslab_group_t *mg)
 	metaslab_class_t *mc = mg->mg_class;
 
 	/*
-	 * A metaslab group is considered allocatable if its free capacity
-	 * is greater than the set value of zfs_mg_noalloc_threshold, it's
-	 * associated with a slog, or there are no other metaslab groups
-	 * with free capacity greater than zfs_mg_noalloc_threshold.
+	 * We use two key metrics to determine if a metaslab group is
+	 * considered allocatable -- free space and fragmentation. If
+	 * the free space is greater than the free space threshold and
+	 * the fragmentation is less than the fragmentation threshold then
+	 * consider the group allocatable. There are two case when we will
+	 * not consider these key metrics. The first is if the group is
+	 * associated with a slog device and the second is if all groups
+	 * in this metaslab class have already been consider ineligible
+	 * for allocations.
 	 */
-	return (mg->mg_free_capacity > zfs_mg_noalloc_threshold ||
+	return ((mg->mg_free_capacity > zfs_mg_noalloc_threshold &&
+	    (mg->mg_fragmentation == ZFS_FRAG_INVALID ||
+	    mg->mg_fragmentation <= zfs_mg_fragmentation_threshold)) ||
 	    mc != spa_normal_class(spa) || mc->mc_alloc_groups == 0);
 }
 
@@ -701,16 +988,8 @@ metaslab_ff_alloc(metaslab_t *msp, uint64_t size)
 	return (metaslab_block_picker(t, cursor, size, align));
 }
 
-/* ARGSUSED */
-static boolean_t
-metaslab_ff_fragmented(metaslab_t *msp)
-{
-	return (B_TRUE);
-}
-
 static metaslab_ops_t metaslab_ff_ops = {
-	metaslab_ff_alloc,
-	metaslab_ff_fragmented
+	metaslab_ff_alloc
 };
 
 metaslab_ops_t *zfs_metaslab_ops = &metaslab_ff_ops;
@@ -761,24 +1040,8 @@ metaslab_df_alloc(metaslab_t *msp, uint64_t size)
 	return (metaslab_block_picker(t, cursor, size, 1ULL));
 }
 
-static boolean_t
-metaslab_df_fragmented(metaslab_t *msp)
-{
-	range_tree_t *rt = msp->ms_tree;
-	uint64_t max_size = metaslab_block_maxsize(msp);
-	int free_pct = range_tree_space(rt) * 100 / msp->ms_size;
-
-	if (max_size >= metaslab_df_alloc_threshold &&
-	    free_pct >= metaslab_df_free_pct)
-		return (B_FALSE);
-
-
-	return (B_TRUE);
-}
-
 static metaslab_ops_t metaslab_df_ops = {
-	metaslab_df_alloc,
-	metaslab_df_fragmented
+	metaslab_df_alloc
 };
 
 metaslab_ops_t *zfs_metaslab_ops = &metaslab_df_ops;
@@ -825,15 +1088,8 @@ metaslab_cf_alloc(metaslab_t *msp, uint64_t size)
 	return (offset);
 }
 
-static boolean_t
-metaslab_cf_fragmented(metaslab_t *msp)
-{
-	return (metaslab_block_maxsize(msp) < metaslab_min_alloc_size);
-}
-
 static metaslab_ops_t metaslab_cf_ops = {
-	metaslab_cf_alloc,
-	metaslab_cf_fragmented
+	metaslab_cf_alloc
 };
 
 metaslab_ops_t *zfs_metaslab_ops = &metaslab_cf_ops;
@@ -894,16 +1150,8 @@ metaslab_ndf_alloc(metaslab_t *msp, uint64_t size)
 	return (-1ULL);
 }
 
-static boolean_t
-metaslab_ndf_fragmented(metaslab_t *msp)
-{
-	return (metaslab_block_maxsize(msp) <=
-	    (metaslab_min_alloc_size << metaslab_ndf_clump_shift));
-}
-
 static metaslab_ops_t metaslab_ndf_ops = {
-	metaslab_ndf_alloc,
-	metaslab_ndf_fragmented
+	metaslab_ndf_alloc
 };
 
 metaslab_ops_t *zfs_metaslab_ops = &metaslab_ndf_ops;
@@ -1008,6 +1256,7 @@ metaslab_init(metaslab_group_t *mg, uint64_t id, uint64_t object, uint64_t txg)
 	msp->ms_tree = range_tree_create(&metaslab_rt_ops, msp, &msp->ms_lock);
 	metaslab_group_add(mg, msp);
 
+	msp->ms_fragmentation = metaslab_fragmentation(msp);
 	msp->ms_ops = mg->mg_class->mc_ops;
 
 	/*
@@ -1075,69 +1324,114 @@ metaslab_fini(metaslab_t *msp)
 	kmem_free(msp, sizeof (metaslab_t));
 }
 
+#define	FRAGMENTATION_TABLE_SIZE	17
+
 /*
- * Apply a weighting factor based on the histogram information for this
- * metaslab. The current weighting factor is somewhat arbitrary and requires
- * additional investigation. The implementation provides a measure of
- * "weighted" free space and gives a higher weighting for larger contiguous
- * regions. The weighting factor is determined by counting the number of
- * sm_shift sectors that exist in each region represented by the histogram.
- * That value is then multiplied by the power of 2 exponent and the sm_shift
- * value.
+ * This table defines a segment size based fragmentation metric that will
+ * allow each metaslab to derive its own fragmentation value. This is done
+ * by calculating the space in each bucket of the spacemap histogram and
+ * multiplying that by the fragmetation metric in this table. Doing
+ * this for all buckets and dividing it by the total amount of free
+ * space in this metaslab (i.e. the total free space in all buckets) gives
+ * us the fragmentation metric. This means that a high fragmentation metric
+ * equates to most of the free space being comprised of small segments.
+ * Conversely, if the metric is low, then most of the free space is in
+ * large segments. A 10% change in fragmentation equates to approximately
+ * double the number of segments.
  *
- * For example, assume the 2^21 histogram bucket has 4 2MB regions and the
- * metaslab has an sm_shift value of 9 (512B):
- *
- * 1) calculate the number of sm_shift sectors in the region:
- *	2^21 / 2^9 = 2^12 = 4096 * 4 (number of regions) = 16384
- * 2) multiply by the power of 2 exponent and the sm_shift value:
- *	16384 * 21 * 9 = 3096576
- * This value will be added to the weighting of the metaslab.
+ * This table defines 0% fragmented space using 16MB segments. Testing has
+ * shown that segments that are greater than or equal to 16MB do not suffer
+ * from drastic performance problems. Using this value, we derive the rest
+ * of the table. Since the fragmentation value is never stored on disk, it
+ * is possible to change these calculations in the future.
+ */
+int zfs_frag_table[FRAGMENTATION_TABLE_SIZE] = {
+	100,	/* 512B	*/
+	100,	/* 1K	*/
+	98,	/* 2K	*/
+	95,	/* 4K	*/
+	90,	/* 8K	*/
+	80,	/* 16K	*/
+	70,	/* 32K	*/
+	60,	/* 64K	*/
+	50,	/* 128K	*/
+	40,	/* 256K	*/
+	30,	/* 512K	*/
+	20,	/* 1M	*/
+	15,	/* 2M	*/
+	10,	/* 4M	*/
+	5,	/* 8M	*/
+	0	/* 16M	*/
+};
+
+/*
+ * Calclate the metaslab's fragmentation metric. A return value
+ * of ZFS_FRAG_INVALID means that the metaslab has not been upgraded and does
+ * not support this metric. Otherwise, the return value should be in the
+ * range [0, 100].
  */
 static uint64_t
-metaslab_weight_factor(metaslab_t *msp)
+metaslab_fragmentation(metaslab_t *msp)
 {
-	uint64_t factor = 0;
-	uint64_t sectors;
+	spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
+	uint64_t fragmentation = 0;
+	uint64_t total = 0;
+	boolean_t feature_enabled = spa_feature_is_enabled(spa,
+	    SPA_FEATURE_SPACEMAP_HISTOGRAM);
 	int i;
 
+	if (!feature_enabled)
+		return (ZFS_FRAG_INVALID);
+
 	/*
-	 * A null space map means that the entire metaslab is free,
-	 * calculate a weight factor that spans the entire size of the
-	 * metaslab.
+	 * A null space map means that the entire metaslab is free
+	 * and thus is not fragmented.
 	 */
-	if (msp->ms_sm == NULL) {
+	if (msp->ms_sm == NULL)
+		return (0);
+
+	/*
+	 * If this metaslab's space_map has not been upgraded, flag it
+	 * so that we upgrade next time we encounter it.
+	 */
+	if (msp->ms_sm->sm_dbuf->db_size != sizeof (space_map_phys_t)) {
+		uint64_t txg = spa_syncing_txg(spa);
 		vdev_t *vd = msp->ms_group->mg_vd;
 
-		i = highbit64(msp->ms_size) - 1;
-		sectors = msp->ms_size >> vd->vdev_ashift;
-		return (sectors * i * vd->vdev_ashift);
+		msp->ms_condense_wanted = B_TRUE;
+		vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
+		spa_dbgmsg(spa, "txg %llu, requesting force condense: "
+		    "msp %p, vd %p", txg, msp, vd);
+		return (ZFS_FRAG_INVALID);
 	}
 
-	if (msp->ms_sm->sm_dbuf->db_size != sizeof (space_map_phys_t))
-		return (0);
+	for (i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
+		uint64_t space = 0;
+		uint8_t shift = msp->ms_sm->sm_shift;
+		int idx = MIN(shift - SPA_MINBLOCKSHIFT + i,
+		    FRAGMENTATION_TABLE_SIZE - 1);
 
-	for (i = 0; i < SPACE_MAP_HISTOGRAM_SIZE(msp->ms_sm); i++) {
 		if (msp->ms_sm->sm_phys->smp_histogram[i] == 0)
 			continue;
 
-		/*
-		 * Determine the number of sm_shift sectors in the region
-		 * indicated by the histogram. For example, given an
-		 * sm_shift value of 9 (512 bytes) and i = 4 then we know
-		 * that we're looking at an 8K region in the histogram
-		 * (i.e. 9 + 4 = 13, 2^13 = 8192). To figure out the
-		 * number of sm_shift sectors (512 bytes in this example),
-		 * we would take 8192 / 512 = 16. Since the histogram
-		 * is offset by sm_shift we can simply use the value of
-		 * of i to calculate this (i.e. 2^i = 16 where i = 4).
-		 */
-		sectors = msp->ms_sm->sm_phys->smp_histogram[i] << i;
-		factor += (i + msp->ms_sm->sm_shift) * sectors;
+		space = msp->ms_sm->sm_phys->smp_histogram[i] << (i + shift);
+		total += space;
+
+		ASSERT3U(idx, <, FRAGMENTATION_TABLE_SIZE);
+		fragmentation += space * zfs_frag_table[idx];
 	}
-	return (factor * msp->ms_sm->sm_shift);
+
+	if (total > 0)
+		fragmentation /= total;
+	ASSERT3U(fragmentation, <=, 100);
+	return (fragmentation);
 }
 
+/*
+ * Compute a weight -- a selection preference value -- for the given metaslab.
+ * This is based on the amount of free space, the level of fragmentation,
+ * the LBA range, and whether the metaslab is loaded.
+ */
 static uint64_t
 metaslab_weight(metaslab_t *msp)
 {
@@ -1161,6 +1455,29 @@ metaslab_weight(metaslab_t *msp)
 	 * The baseline weight is the metaslab's free space.
 	 */
 	space = msp->ms_size - space_map_allocated(msp->ms_sm);
+
+	msp->ms_fragmentation = metaslab_fragmentation(msp);
+	if (metaslab_fragmentation_factor_enabled &&
+	    msp->ms_fragmentation != ZFS_FRAG_INVALID) {
+		/*
+		 * Use the fragmentation information to inversely scale
+		 * down the baseline weight. We need to ensure that we
+		 * don't exclude this metaslab completely when it's 100%
+		 * fragmented. To avoid this we reduce the fragmented value
+		 * by 1.
+		 */
+		space = (space * (100 - (msp->ms_fragmentation - 1))) / 100;
+
+		/*
+		 * If space < SPA_MINBLOCKSIZE, then we will not allocate from
+		 * this metaslab again. The fragmentation metric may have
+		 * decreased the space to something smaller than
+		 * SPA_MINBLOCKSIZE, so reset the space to SPA_MINBLOCKSIZE
+		 * so that we can consume any remaining space.
+		 */
+		if (space > 0 && space < SPA_MINBLOCKSIZE)
+			space = SPA_MINBLOCKSIZE;
+	}
 	weight = space;
 
 	/*
@@ -1172,19 +1489,19 @@ metaslab_weight(metaslab_t *msp)
 	 * In effect, this means that we'll select the metaslab with the most
 	 * free bandwidth rather than simply the one with the most free space.
 	 */
-	weight = 2 * weight - (msp->ms_id * weight) / vd->vdev_ms_count;
-	ASSERT(weight >= space && weight <= 2 * space);
-
-	msp->ms_factor = metaslab_weight_factor(msp);
-	if (metaslab_weight_factor_enable)
-		weight += msp->ms_factor;
+	if (metaslab_lba_weighting_enabled) {
+		weight = 2 * weight - (msp->ms_id * weight) / vd->vdev_ms_count;
+		ASSERT(weight >= space && weight <= 2 * space);
+	}
 
-	if (msp->ms_loaded && !msp->ms_ops->msop_fragmented(msp)) {
-		/*
-		 * If this metaslab is one we're actively using, adjust its
-		 * weight to make it preferable to any inactive metaslab so
-		 * we'll polish it off.
-		 */
+	/*
+	 * If this metaslab is one we're actively using, adjust its
+	 * weight to make it preferable to any inactive metaslab so
+	 * we'll polish it off. If the fragmentation on this metaslab
+	 * has exceed our threshold, then don't mark it active.
+	 */
+	if (msp->ms_loaded && msp->ms_fragmentation != ZFS_FRAG_INVALID &&
+	    msp->ms_fragmentation <= zfs_metaslab_fragmentation_threshold) {
 		weight |= (msp->ms_weight & METASLAB_ACTIVE_MASK);
 	}
 
@@ -1269,9 +1586,16 @@ metaslab_group_preload(metaslab_group_t *mg)
 	while (msp != NULL) {
 		metaslab_t *msp_next = AVL_NEXT(t, msp);
 
-		/* If we have reached our preload limit then we're done */
-		if (++m > metaslab_preload_limit)
-			break;
+		/*
+		 * We preload only the maximum number of metaslabs specified
+		 * by metaslab_preload_limit. If a metaslab is being forced
+		 * to condense then we preload it too. This will ensure
+		 * that force condensing happens in the next txg.
+		 */
+		if (++m > metaslab_preload_limit && !msp->ms_condense_wanted) {
+			msp = msp_next;
+			continue;
+		}
 
 		/*
 		 * We must drop the metaslab group lock here to preserve
@@ -1329,11 +1653,12 @@ metaslab_should_condense(metaslab_t *msp)
 
 	/*
 	 * Use the ms_size_tree range tree, which is ordered by size, to
-	 * obtain the largest segment in the free tree. If the tree is empty
-	 * then we should condense the map.
+	 * obtain the largest segment in the free tree. We always condense
+	 * metaslabs that are empty and metaslabs for which a condense
+	 * request has been made.
 	 */
 	rs = avl_last(&msp->ms_size_tree);
-	if (rs == NULL)
+	if (rs == NULL || msp->ms_condense_wanted)
 		return (B_TRUE);
 
 	/*
@@ -1369,9 +1694,14 @@ metaslab_condense(metaslab_t *msp, uint64_t txg, dmu_tx_t *tx)
 	ASSERT3U(spa_sync_pass(spa), ==, 1);
 	ASSERT(msp->ms_loaded);
 
+
 	spa_dbgmsg(spa, "condensing: txg %llu, msp[%llu] %p, "
-	    "smp size %llu, segments %lu", txg, msp->ms_id, msp,
-	    space_map_length(msp->ms_sm), avl_numnodes(&msp->ms_tree->rt_root));
+	    "smp size %llu, segments %lu, forcing condense=%s", txg,
+	    msp->ms_id, msp, space_map_length(msp->ms_sm),
+	    avl_numnodes(&msp->ms_tree->rt_root),
+	    msp->ms_condense_wanted ? "TRUE" : "FALSE");
+
+	msp->ms_condense_wanted = B_FALSE;
 
 	/*
 	 * Create an range tree that is 100% allocated. We remove segments
@@ -1464,8 +1794,14 @@ metaslab_sync(metaslab_t *msp, uint64_t txg)
 	ASSERT3P(*freetree, !=, NULL);
 	ASSERT3P(*freed_tree, !=, NULL);
 
+	/*
+	 * Normally, we don't want to process a metaslab if there
+	 * are no allocations or frees to perform. However, if the metaslab
+	 * is being forced to condense we need to let it through.
+	 */
 	if (range_tree_space(alloctree) == 0 &&
-	    range_tree_space(*freetree) == 0)
+	    range_tree_space(*freetree) == 0 &&
+	    !msp->ms_condense_wanted)
 		return;
 
 	/*
@@ -1502,8 +1838,9 @@ metaslab_sync(metaslab_t *msp, uint64_t txg)
 		space_map_write(msp->ms_sm, *freetree, SM_FREE, tx);
 	}
 
-	range_tree_vacate(alloctree, NULL, NULL);
-
+	metaslab_group_histogram_verify(mg);
+	metaslab_class_histogram_verify(mg->mg_class);
+	metaslab_group_histogram_remove(mg, msp);
 	if (msp->ms_loaded) {
 		/*
 		 * When the space map is loaded, we have an accruate
@@ -1523,6 +1860,9 @@ metaslab_sync(metaslab_t *msp, uint64_t txg)
 		 */
 		space_map_histogram_add(msp->ms_sm, *freetree, tx);
 	}
+	metaslab_group_histogram_add(mg, msp);
+	metaslab_group_histogram_verify(mg);
+	metaslab_class_histogram_verify(mg->mg_class);
 
 	/*
 	 * For sync pass 1, we avoid traversing this txg's free range tree
@@ -1535,6 +1875,7 @@ metaslab_sync(metaslab_t *msp, uint64_t txg)
 	} else {
 		range_tree_vacate(*freetree, range_tree_add, *freed_tree);
 	}
+	range_tree_vacate(alloctree, NULL, NULL);
 
 	ASSERT0(range_tree_space(msp->ms_alloctree[txg & TXG_MASK]));
 	ASSERT0(range_tree_space(msp->ms_freetree[txg & TXG_MASK]));
@@ -1646,13 +1987,13 @@ metaslab_sync_done(metaslab_t *msp, uint64_t txg)
 
 	metaslab_group_sort(mg, msp, metaslab_weight(msp));
 	mutex_exit(&msp->ms_lock);
-
 }
 
 void
 metaslab_sync_reassess(metaslab_group_t *mg)
 {
 	metaslab_group_alloc_update(mg);
+	mg->mg_fragmentation = metaslab_group_fragmentation(mg);
 
 	/*
 	 * Preload the next potential metaslabs
@@ -1926,9 +2267,7 @@ top:
 		 */
 		if ((vd->vdev_stat.vs_write_errors > 0 ||
 		    vd->vdev_state < VDEV_STATE_HEALTHY) &&
-		    d == 0 && dshift == 3 &&
-		    !(zfs_write_to_degraded && vd->vdev_state ==
-		    VDEV_STATE_DEGRADED)) {
+		    d == 0 && dshift == 3 && vd->vdev_children == 0) {
 			all_zero = B_FALSE;
 			goto next;
 		}
@@ -1953,7 +2292,7 @@ top:
 			 * over- or under-used relative to the pool,
 			 * and set an allocation bias to even it out.
 			 */
-			if (mc->mc_aliquot == 0) {
+			if (mc->mc_aliquot == 0 && metaslab_bias_enabled) {
 				vdev_stat_t *vs = &vd->vdev_stat;
 				int64_t vu, cu;
 
@@ -1975,6 +2314,8 @@ top:
 				 */
 				mg->mg_bias = ((cu - vu) *
 				    (int64_t)mg->mg_aliquot) / 100;
+			} else if (!metaslab_bias_enabled) {
+				mg->mg_bias = 0;
 			}
 
 			if ((flags & METASLAB_FASTWRITE) ||
@@ -2305,12 +2646,32 @@ metaslab_check_free(spa_t *spa, const blkptr_t *bp)
 #if defined(_KERNEL) && defined(HAVE_SPL)
 module_param(metaslab_debug_load, int, 0644);
 module_param(metaslab_debug_unload, int, 0644);
+module_param(metaslab_preload_enabled, int, 0644);
+module_param(zfs_mg_noalloc_threshold, int, 0644);
+module_param(zfs_mg_fragmentation_threshold, int, 0644);
+module_param(zfs_metaslab_fragmentation_threshold, int, 0644);
+module_param(metaslab_fragmentation_factor_enabled, int, 0644);
+module_param(metaslab_lba_weighting_enabled, int, 0644);
+module_param(metaslab_bias_enabled, int, 0644);
+
 MODULE_PARM_DESC(metaslab_debug_load,
 	"load all metaslabs when pool is first opened");
 MODULE_PARM_DESC(metaslab_debug_unload,
 	"prevent metaslabs from being unloaded");
+MODULE_PARM_DESC(metaslab_preload_enabled,
+	"preload potential metaslabs during reassessment");
 
-module_param(zfs_mg_noalloc_threshold, int, 0644);
 MODULE_PARM_DESC(zfs_mg_noalloc_threshold,
 	"percentage of free space for metaslab group to allow allocation");
+MODULE_PARM_DESC(zfs_mg_fragmentation_threshold,
+	"fragmentation for metaslab group to allow allocation");
+
+MODULE_PARM_DESC(zfs_metaslab_fragmentation_threshold,
+	"fragmentation for metaslab to allow allocation");
+MODULE_PARM_DESC(metaslab_fragmentation_factor_enabled,
+	"use the fragmentation metric to prefer less fragmented metaslabs");
+MODULE_PARM_DESC(metaslab_lba_weighting_enabled,
+	"prefer metaslabs with lower LBAs");
+MODULE_PARM_DESC(metaslab_bias_enabled,
+	"enable metaslab group biasing");
 #endif /* _KERNEL && HAVE_SPL */
diff --git a/module/zfs/range_tree.c b/module/zfs/range_tree.c
index f35d20c08..4643d2632 100644
--- a/module/zfs/range_tree.c
+++ b/module/zfs/range_tree.c
@@ -81,6 +81,7 @@ range_tree_stat_incr(range_tree_t *rt, range_seg_t *rs)
 	uint64_t size = rs->rs_end - rs->rs_start;
 	int idx = highbit64(size) - 1;
 
+	ASSERT(size != 0);
 	ASSERT3U(idx, <,
 	    sizeof (rt->rt_histogram) / sizeof (*rt->rt_histogram));
 
@@ -95,6 +96,7 @@ range_tree_stat_decr(range_tree_t *rt, range_seg_t *rs)
 	uint64_t size = rs->rs_end - rs->rs_start;
 	int idx = highbit64(size) - 1;
 
+	ASSERT(size != 0);
 	ASSERT3U(idx, <,
 	    sizeof (rt->rt_histogram) / sizeof (*rt->rt_histogram));
 
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index 7bd254aa5..0824f9c7f 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -190,13 +190,10 @@ spa_prop_get_config(spa_t *spa, nvlist_t **nvp)
 {
 	vdev_t *rvd = spa->spa_root_vdev;
 	dsl_pool_t *pool = spa->spa_dsl_pool;
-	uint64_t size;
-	uint64_t alloc;
-	uint64_t space;
-	uint64_t cap, version;
+	uint64_t size, alloc, cap, version;
 	zprop_source_t src = ZPROP_SRC_NONE;
 	spa_config_dirent_t *dp;
-	int c;
+	metaslab_class_t *mc = spa_normal_class(spa);
 
 	ASSERT(MUTEX_HELD(&spa->spa_props_lock));
 
@@ -209,14 +206,10 @@ spa_prop_get_config(spa_t *spa, nvlist_t **nvp)
 		spa_prop_add_list(*nvp, ZPOOL_PROP_FREE, NULL,
 		    size - alloc, src);
 
-		space = 0;
-		for (c = 0; c < rvd->vdev_children; c++) {
-			vdev_t *tvd = rvd->vdev_child[c];
-			space += tvd->vdev_max_asize - tvd->vdev_asize;
-		}
-		spa_prop_add_list(*nvp, ZPOOL_PROP_EXPANDSZ, NULL, space,
-		    src);
-
+		spa_prop_add_list(*nvp, ZPOOL_PROP_FRAGMENTATION, NULL,
+		    metaslab_class_fragmentation(mc), src);
+		spa_prop_add_list(*nvp, ZPOOL_PROP_EXPANDSZ, NULL,
+		    metaslab_class_expandable_space(mc), src);
 		spa_prop_add_list(*nvp, ZPOOL_PROP_READONLY, NULL,
 		    (spa_mode(spa) == FREAD), src);
 
diff --git a/module/zfs/space_map.c b/module/zfs/space_map.c
index c7a2312b4..a5bb9a617 100644
--- a/module/zfs/space_map.c
+++ b/module/zfs/space_map.c
@@ -205,10 +205,10 @@ space_map_histogram_add(space_map_t *sm, range_tree_t *rt, dmu_tx_t *tx)
 		 * reached the maximum bucket size. Accumulate all ranges
 		 * larger than the max bucket size into the last bucket.
 		 */
-		if (idx < SPACE_MAP_HISTOGRAM_SIZE(sm) - 1) {
+		if (idx < SPACE_MAP_HISTOGRAM_SIZE - 1) {
 			ASSERT3U(idx + sm->sm_shift, ==, i);
 			idx++;
-			ASSERT3U(idx, <, SPACE_MAP_HISTOGRAM_SIZE(sm));
+			ASSERT3U(idx, <, SPACE_MAP_HISTOGRAM_SIZE);
 		}
 	}
 }
diff --git a/module/zfs/txg.c b/module/zfs/txg.c
index ff25c9c1b..a39732bb9 100644
--- a/module/zfs/txg.c
+++ b/module/zfs/txg.c
@@ -539,7 +539,9 @@ txg_sync_thread(dsl_pool_t *dp)
 			txg_thread_exit(tx, &cpr, &tx->tx_sync_thread);
 		}
 
+		spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
 		vdev_get_stats(spa->spa_root_vdev, vs1);
+		spa_config_exit(spa, SCL_ALL, FTAG);
 
 		/*
 		 * Consume the quiesced txg which has been handed off to
@@ -575,7 +577,9 @@ txg_sync_thread(dsl_pool_t *dp)
 		 */
 		txg_dispatch_callbacks(dp, txg);
 
+		spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
 		vdev_get_stats(spa->spa_root_vdev, vs2);
+		spa_config_exit(spa, SCL_ALL, FTAG);
 		spa_txg_history_set_io(spa, txg,
 		    vs2->vs_bytes[ZIO_TYPE_READ]-vs1->vs_bytes[ZIO_TYPE_READ],
 		    vs2->vs_bytes[ZIO_TYPE_WRITE]-vs1->vs_bytes[ZIO_TYPE_WRITE],
diff --git a/module/zfs/vdev.c b/module/zfs/vdev.c
index 89df3674e..0303edada 100644
--- a/module/zfs/vdev.c
+++ b/module/zfs/vdev.c
@@ -2151,11 +2151,16 @@ vdev_remove(vdev_t *vd, uint64_t txg)
 	spa_t *spa = vd->vdev_spa;
 	objset_t *mos = spa->spa_meta_objset;
 	dmu_tx_t *tx;
-	int m;
+	int m, i;
 
 	tx = dmu_tx_create_assigned(spa_get_dsl(spa), txg);
 
 	if (vd->vdev_ms != NULL) {
+		metaslab_group_t *mg = vd->vdev_mg;
+
+		metaslab_group_histogram_verify(mg);
+		metaslab_class_histogram_verify(mg->mg_class);
+
 		for (m = 0; m < vd->vdev_ms_count; m++) {
 			metaslab_t *msp = vd->vdev_ms[m];
 
@@ -2163,12 +2168,27 @@ vdev_remove(vdev_t *vd, uint64_t txg)
 				continue;
 
 			mutex_enter(&msp->ms_lock);
+			/*
+			 * If the metaslab was not loaded when the vdev
+			 * was removed then the histogram accounting may
+			 * not be accurate. Update the histogram information
+			 * here so that we ensure that the metaslab group
+			 * and metaslab class are up-to-date.
+			 */
+			metaslab_group_histogram_remove(mg, msp);
+
 			VERIFY0(space_map_allocated(msp->ms_sm));
 			space_map_free(msp->ms_sm, tx);
 			space_map_close(msp->ms_sm);
 			msp->ms_sm = NULL;
 			mutex_exit(&msp->ms_lock);
 		}
+
+		metaslab_group_histogram_verify(mg);
+		metaslab_class_histogram_verify(mg->mg_class);
+		for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
+			ASSERT0(mg->mg_histogram[i]);
+
 	}
 
 	if (vd->vdev_ms_array) {
@@ -2621,9 +2641,12 @@ vdev_accessible(vdev_t *vd, zio_t *zio)
 void
 vdev_get_stats(vdev_t *vd, vdev_stat_t *vs)
 {
-	vdev_t *rvd = vd->vdev_spa->spa_root_vdev;
+	spa_t *spa = vd->vdev_spa;
+	vdev_t *rvd = spa->spa_root_vdev;
 	int c, t;
 
+	ASSERT(spa_config_held(spa, SCL_ALL, RW_READER) != 0);
+
 	mutex_enter(&vd->vdev_stat_lock);
 	bcopy(&vd->vdev_stat, vs, sizeof (*vs));
 	vs->vs_timestamp = gethrtime() - vs->vs_timestamp;
@@ -2632,7 +2655,8 @@ vdev_get_stats(vdev_t *vd, vdev_stat_t *vs)
 	if (vd->vdev_ops->vdev_op_leaf)
 		vs->vs_rsize += VDEV_LABEL_START_SIZE + VDEV_LABEL_END_SIZE;
 	vs->vs_esize = vd->vdev_max_asize - vd->vdev_asize;
-	mutex_exit(&vd->vdev_stat_lock);
+	if (vd->vdev_aux == NULL && vd == vd->vdev_top)
+		vs->vs_fragmentation = vd->vdev_mg->mg_fragmentation;
 
 	/*
 	 * If we're getting stats on the root vdev, aggregate the I/O counts
@@ -2643,15 +2667,14 @@ vdev_get_stats(vdev_t *vd, vdev_stat_t *vs)
 			vdev_t *cvd = rvd->vdev_child[c];
 			vdev_stat_t *cvs = &cvd->vdev_stat;
 
-			mutex_enter(&vd->vdev_stat_lock);
 			for (t = 0; t < ZIO_TYPES; t++) {
 				vs->vs_ops[t] += cvs->vs_ops[t];
 				vs->vs_bytes[t] += cvs->vs_bytes[t];
 			}
 			cvs->vs_scan_removing = cvd->vdev_removing;
-			mutex_exit(&vd->vdev_stat_lock);
 		}
 	}
+	mutex_exit(&vd->vdev_stat_lock);
 }
 
 void