MFV r254747:

[FreeBSD/FreeBSD.git] / sys / cddl / contrib / opensolaris / uts / common / fs / zfs / metaslab.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * 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 by Saso Kiselkov. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/dmu.h>
#include <sys/dmu_tx.h>
#include <sys/space_map.h>
#include <sys/metaslab_impl.h>
#include <sys/vdev_impl.h>
#include <sys/zio.h>

/*
 * Allow allocations to switch to gang blocks quickly. We do this to
 * avoid having to load lots of space_maps in a given txg. There are,
 * however, some cases where we want to avoid "fast" ganging and instead
 * we want to do an exhaustive search of all metaslabs on this device.
 * Currently we don't allow any gang, zil, or dump device related allocations
 * to "fast" gang.
 */
#define CAN_FASTGANG(flags) \
        (!((flags) & (METASLAB_GANG_CHILD | METASLAB_GANG_HEADER | \
        METASLAB_GANG_AVOID)))

uint64_t metaslab_aliquot = 512ULL << 10;
uint64_t metaslab_gang_bang = SPA_MAXBLOCKSIZE + 1;     /* force gang blocks */

/*
 * The in-core space map representation is more compact than its on-disk form.
 * The zfs_condense_pct determines how much more compact the in-core
 * space_map representation must be before we compact it on-disk.
 * Values should be greater than or equal to 100.
 */
int zfs_condense_pct = 200;

/*
 * This value defines the number of allowed allocation failures per vdev.
 * If a device reaches this threshold in a given txg then we consider skipping
 * allocations on that device.
 */
int zfs_mg_alloc_failures = 0;

SYSCTL_DECL(_vfs_zfs);
SYSCTL_INT(_vfs_zfs, OID_AUTO, mg_alloc_failures, CTLFLAG_RDTUN,
    &zfs_mg_alloc_failures, 0,
    "Number of allowed allocation failures per vdev");
TUNABLE_INT("vfs.zfs.mg_alloc_failures", &zfs_mg_alloc_failures);

/*
 * Metaslab debugging: when set, keeps all space maps in core to verify frees.
 */
static int metaslab_debug = 0;

/*
 * Minimum size which forces the dynamic allocator to change
 * it's allocation strategy.  Once the space map cannot satisfy
 * an allocation of this size then it switches to using more
 * aggressive strategy (i.e search by size rather than offset).
 */
uint64_t metaslab_df_alloc_threshold = SPA_MAXBLOCKSIZE;

/*
 * The minimum free space, in percent, which must be available
 * in a space map to continue allocations in a first-fit fashion.
 * Once the space_map's free space drops below this level we dynamically
 * switch to using best-fit allocations.
 */
int metaslab_df_free_pct = 4;

/*
 * A metaslab is considered "free" if it contains a contiguous
 * segment which is greater than metaslab_min_alloc_size.
 */
uint64_t metaslab_min_alloc_size = DMU_MAX_ACCESS;

/*
 * Max number of space_maps to prefetch.
 */
int metaslab_prefetch_limit = SPA_DVAS_PER_BP;

/*
 * Percentage bonus multiplier for metaslabs that are in the bonus area.
 */
int metaslab_smo_bonus_pct = 150;

/*
 * Should we be willing to write data to degraded vdevs?
 */
boolean_t zfs_write_to_degraded = B_FALSE;
SYSCTL_INT(_vfs_zfs, OID_AUTO, write_to_degraded, CTLFLAG_RWTUN,
    &zfs_write_to_degraded, 0, "Allow writing data to degraded vdevs");
TUNABLE_INT("vfs.zfs.write_to_degraded", &zfs_write_to_degraded);

/*
 * ==========================================================================
 * Metaslab classes
 * ==========================================================================
 */
metaslab_class_t *
metaslab_class_create(spa_t *spa, space_map_ops_t *ops)
{
        metaslab_class_t *mc;

        mc = kmem_zalloc(sizeof (metaslab_class_t), KM_SLEEP);

        mc->mc_spa = spa;
        mc->mc_rotor = NULL;
        mc->mc_ops = ops;

        return (mc);
}

void
metaslab_class_destroy(metaslab_class_t *mc)
{
        ASSERT(mc->mc_rotor == NULL);
        ASSERT(mc->mc_alloc == 0);
        ASSERT(mc->mc_deferred == 0);
        ASSERT(mc->mc_space == 0);
        ASSERT(mc->mc_dspace == 0);

        kmem_free(mc, sizeof (metaslab_class_t));
}

int
metaslab_class_validate(metaslab_class_t *mc)
{
        metaslab_group_t *mg;
        vdev_t *vd;

        /*
         * Must hold one of the spa_config locks.
         */
        ASSERT(spa_config_held(mc->mc_spa, SCL_ALL, RW_READER) ||
            spa_config_held(mc->mc_spa, SCL_ALL, RW_WRITER));

        if ((mg = mc->mc_rotor) == NULL)
                return (0);

        do {
                vd = mg->mg_vd;
                ASSERT(vd->vdev_mg != NULL);
                ASSERT3P(vd->vdev_top, ==, vd);
                ASSERT3P(mg->mg_class, ==, mc);
                ASSERT3P(vd->vdev_ops, !=, &vdev_hole_ops);
        } while ((mg = mg->mg_next) != mc->mc_rotor);

        return (0);
}

void
metaslab_class_space_update(metaslab_class_t *mc, int64_t alloc_delta,
    int64_t defer_delta, int64_t space_delta, int64_t dspace_delta)
{
        atomic_add_64(&mc->mc_alloc, alloc_delta);
        atomic_add_64(&mc->mc_deferred, defer_delta);
        atomic_add_64(&mc->mc_space, space_delta);
        atomic_add_64(&mc->mc_dspace, dspace_delta);
}

void
metaslab_class_minblocksize_update(metaslab_class_t *mc)
{
        metaslab_group_t *mg;
        vdev_t *vd;
        uint64_t minashift = UINT64_MAX;

        if ((mg = mc->mc_rotor) == NULL) {
                mc->mc_minblocksize = SPA_MINBLOCKSIZE;
                return;
        }

        do {
                vd = mg->mg_vd;
                if (vd->vdev_ashift < minashift)
                        minashift = vd->vdev_ashift;
        } while ((mg = mg->mg_next) != mc->mc_rotor);

        mc->mc_minblocksize = 1ULL << minashift;
}

uint64_t
metaslab_class_get_alloc(metaslab_class_t *mc)
{
        return (mc->mc_alloc);
}

uint64_t
metaslab_class_get_deferred(metaslab_class_t *mc)
{
        return (mc->mc_deferred);
}

uint64_t
metaslab_class_get_space(metaslab_class_t *mc)
{
        return (mc->mc_space);
}

uint64_t
metaslab_class_get_dspace(metaslab_class_t *mc)
{
        return (spa_deflate(mc->mc_spa) ? mc->mc_dspace : mc->mc_space);
}

uint64_t
metaslab_class_get_minblocksize(metaslab_class_t *mc)
{
        return (mc->mc_minblocksize);
}

/*
 * ==========================================================================
 * Metaslab groups
 * ==========================================================================
 */
static int
metaslab_compare(const void *x1, const void *x2)
{
        const metaslab_t *m1 = x1;
        const metaslab_t *m2 = x2;

        if (m1->ms_weight < m2->ms_weight)
                return (1);
        if (m1->ms_weight > m2->ms_weight)
                return (-1);

        /*
         * If the weights are identical, use the offset to force uniqueness.
         */
        if (m1->ms_map->sm_start < m2->ms_map->sm_start)
                return (-1);
        if (m1->ms_map->sm_start > m2->ms_map->sm_start)
                return (1);

        ASSERT3P(m1, ==, m2);

        return (0);
}

metaslab_group_t *
metaslab_group_create(metaslab_class_t *mc, vdev_t *vd)
{
        metaslab_group_t *mg;

        mg = kmem_zalloc(sizeof (metaslab_group_t), KM_SLEEP);
        mutex_init(&mg->mg_lock, NULL, MUTEX_DEFAULT, NULL);
        avl_create(&mg->mg_metaslab_tree, metaslab_compare,
            sizeof (metaslab_t), offsetof(struct metaslab, ms_group_node));
        mg->mg_vd = vd;
        mg->mg_class = mc;
        mg->mg_activation_count = 0;

        return (mg);
}

void
metaslab_group_destroy(metaslab_group_t *mg)
{
        ASSERT(mg->mg_prev == NULL);
        ASSERT(mg->mg_next == NULL);
        /*
         * We may have gone below zero with the activation count
         * either because we never activated in the first place or
         * because we're done, and possibly removing the vdev.
         */
        ASSERT(mg->mg_activation_count <= 0);

        avl_destroy(&mg->mg_metaslab_tree);
        mutex_destroy(&mg->mg_lock);
        kmem_free(mg, sizeof (metaslab_group_t));
}

void
metaslab_group_activate(metaslab_group_t *mg)
{
        metaslab_class_t *mc = mg->mg_class;
        metaslab_group_t *mgprev, *mgnext;

        ASSERT(spa_config_held(mc->mc_spa, SCL_ALLOC, RW_WRITER));

        ASSERT(mc->mc_rotor != mg);
        ASSERT(mg->mg_prev == NULL);
        ASSERT(mg->mg_next == NULL);
        ASSERT(mg->mg_activation_count <= 0);

        if (++mg->mg_activation_count <= 0)
                return;

        mg->mg_aliquot = metaslab_aliquot * MAX(1, mg->mg_vd->vdev_children);

        if ((mgprev = mc->mc_rotor) == NULL) {
                mg->mg_prev = mg;
                mg->mg_next = mg;
        } else {
                mgnext = mgprev->mg_next;
                mg->mg_prev = mgprev;
                mg->mg_next = mgnext;
                mgprev->mg_next = mg;
                mgnext->mg_prev = mg;
        }
        mc->mc_rotor = mg;
        metaslab_class_minblocksize_update(mc);
}

void
metaslab_group_passivate(metaslab_group_t *mg)
{
        metaslab_class_t *mc = mg->mg_class;
        metaslab_group_t *mgprev, *mgnext;

        ASSERT(spa_config_held(mc->mc_spa, SCL_ALLOC, RW_WRITER));

        if (--mg->mg_activation_count != 0) {
                ASSERT(mc->mc_rotor != mg);
                ASSERT(mg->mg_prev == NULL);
                ASSERT(mg->mg_next == NULL);
                ASSERT(mg->mg_activation_count < 0);
                return;
        }

        mgprev = mg->mg_prev;
        mgnext = mg->mg_next;

        if (mg == mgnext) {
                mc->mc_rotor = NULL;
        } else {
                mc->mc_rotor = mgnext;
                mgprev->mg_next = mgnext;
                mgnext->mg_prev = mgprev;
        }

        mg->mg_prev = NULL;
        mg->mg_next = NULL;
        metaslab_class_minblocksize_update(mc);
}

static void
metaslab_group_add(metaslab_group_t *mg, metaslab_t *msp)
{
        mutex_enter(&mg->mg_lock);
        ASSERT(msp->ms_group == NULL);
        msp->ms_group = mg;
        msp->ms_weight = 0;
        avl_add(&mg->mg_metaslab_tree, msp);
        mutex_exit(&mg->mg_lock);
}

static void
metaslab_group_remove(metaslab_group_t *mg, metaslab_t *msp)
{
        mutex_enter(&mg->mg_lock);
        ASSERT(msp->ms_group == mg);
        avl_remove(&mg->mg_metaslab_tree, msp);
        msp->ms_group = NULL;
        mutex_exit(&mg->mg_lock);
}

static void
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].
         */
        ASSERT(weight >= SPA_MINBLOCKSIZE-1 || weight == 0);
        ASSERT(MUTEX_HELD(&msp->ms_lock));

        mutex_enter(&mg->mg_lock);
        ASSERT(msp->ms_group == mg);
        avl_remove(&mg->mg_metaslab_tree, msp);
        msp->ms_weight = weight;
        avl_add(&mg->mg_metaslab_tree, msp);
        mutex_exit(&mg->mg_lock);
}

/*
 * ==========================================================================
 * Common allocator routines
 * ==========================================================================
 */
static int
metaslab_segsize_compare(const void *x1, const void *x2)
{
        const space_seg_t *s1 = x1;
        const space_seg_t *s2 = x2;
        uint64_t ss_size1 = s1->ss_end - s1->ss_start;
        uint64_t ss_size2 = s2->ss_end - s2->ss_start;

        if (ss_size1 < ss_size2)
                return (-1);
        if (ss_size1 > ss_size2)
                return (1);

        if (s1->ss_start < s2->ss_start)
                return (-1);
        if (s1->ss_start > s2->ss_start)
                return (1);

        return (0);
}

/*
 * This is a helper function that can be used by the allocator to find
 * a suitable block to allocate. This will search the specified AVL
 * tree looking for a block that matches the specified criteria.
 */
static uint64_t
metaslab_block_picker(avl_tree_t *t, uint64_t *cursor, uint64_t size,
    uint64_t align)
{
        space_seg_t *ss, ssearch;
        avl_index_t where;

        ssearch.ss_start = *cursor;
        ssearch.ss_end = *cursor + size;

        ss = avl_find(t, &ssearch, &where);
        if (ss == NULL)
                ss = avl_nearest(t, where, AVL_AFTER);

        while (ss != NULL) {
                uint64_t offset = P2ROUNDUP(ss->ss_start, align);

                if (offset + size <= ss->ss_end) {
                        *cursor = offset + size;
                        return (offset);
                }
                ss = AVL_NEXT(t, ss);
        }

        /*
         * If we know we've searched the whole map (*cursor == 0), give up.
         * Otherwise, reset the cursor to the beginning and try again.
         */
        if (*cursor == 0)
                return (-1ULL);

        *cursor = 0;
        return (metaslab_block_picker(t, cursor, size, align));
}

static void
metaslab_pp_load(space_map_t *sm)
{
        space_seg_t *ss;

        ASSERT(sm->sm_ppd == NULL);
        sm->sm_ppd = kmem_zalloc(64 * sizeof (uint64_t), KM_SLEEP);

        sm->sm_pp_root = kmem_alloc(sizeof (avl_tree_t), KM_SLEEP);
        avl_create(sm->sm_pp_root, metaslab_segsize_compare,
            sizeof (space_seg_t), offsetof(struct space_seg, ss_pp_node));

        for (ss = avl_first(&sm->sm_root); ss; ss = AVL_NEXT(&sm->sm_root, ss))
                avl_add(sm->sm_pp_root, ss);
}

static void
metaslab_pp_unload(space_map_t *sm)
{
        void *cookie = NULL;

        kmem_free(sm->sm_ppd, 64 * sizeof (uint64_t));
        sm->sm_ppd = NULL;

        while (avl_destroy_nodes(sm->sm_pp_root, &cookie) != NULL) {
                /* tear down the tree */
        }

        avl_destroy(sm->sm_pp_root);
        kmem_free(sm->sm_pp_root, sizeof (avl_tree_t));
        sm->sm_pp_root = NULL;
}

/* ARGSUSED */
static void
metaslab_pp_claim(space_map_t *sm, uint64_t start, uint64_t size)
{
        /* No need to update cursor */
}

/* ARGSUSED */
static void
metaslab_pp_free(space_map_t *sm, uint64_t start, uint64_t size)
{
        /* No need to update cursor */
}

/*
 * Return the maximum contiguous segment within the metaslab.
 */
uint64_t
metaslab_pp_maxsize(space_map_t *sm)
{
        avl_tree_t *t = sm->sm_pp_root;
        space_seg_t *ss;

        if (t == NULL || (ss = avl_last(t)) == NULL)
                return (0ULL);

        return (ss->ss_end - ss->ss_start);
}

/*
 * ==========================================================================
 * The first-fit block allocator
 * ==========================================================================
 */
static uint64_t
metaslab_ff_alloc(space_map_t *sm, uint64_t size)
{
        avl_tree_t *t = &sm->sm_root;
        uint64_t align = size & -size;
        uint64_t *cursor = (uint64_t *)sm->sm_ppd + highbit(align) - 1;

        return (metaslab_block_picker(t, cursor, size, align));
}

/* ARGSUSED */
boolean_t
metaslab_ff_fragmented(space_map_t *sm)
{
        return (B_TRUE);
}

static space_map_ops_t metaslab_ff_ops = {
        metaslab_pp_load,
        metaslab_pp_unload,
        metaslab_ff_alloc,
        metaslab_pp_claim,
        metaslab_pp_free,
        metaslab_pp_maxsize,
        metaslab_ff_fragmented
};

/*
 * ==========================================================================
 * Dynamic block allocator -
 * Uses the first fit allocation scheme until space get low and then
 * adjusts to a best fit allocation method. Uses metaslab_df_alloc_threshold
 * and metaslab_df_free_pct to determine when to switch the allocation scheme.
 * ==========================================================================
 */
static uint64_t
metaslab_df_alloc(space_map_t *sm, uint64_t size)
{
        avl_tree_t *t = &sm->sm_root;
        uint64_t align = size & -size;
        uint64_t *cursor = (uint64_t *)sm->sm_ppd + highbit(align) - 1;
        uint64_t max_size = metaslab_pp_maxsize(sm);
        int free_pct = sm->sm_space * 100 / sm->sm_size;

        ASSERT(MUTEX_HELD(sm->sm_lock));
        ASSERT3U(avl_numnodes(&sm->sm_root), ==, avl_numnodes(sm->sm_pp_root));

        if (max_size < size)
                return (-1ULL);

        /*
         * If we're running low on space switch to using the size
         * sorted AVL tree (best-fit).
         */
        if (max_size < metaslab_df_alloc_threshold ||
            free_pct < metaslab_df_free_pct) {
                t = sm->sm_pp_root;
                *cursor = 0;
        }

        return (metaslab_block_picker(t, cursor, size, 1ULL));
}

static boolean_t
metaslab_df_fragmented(space_map_t *sm)
{
        uint64_t max_size = metaslab_pp_maxsize(sm);
        int free_pct = sm->sm_space * 100 / sm->sm_size;

        if (max_size >= metaslab_df_alloc_threshold &&
            free_pct >= metaslab_df_free_pct)
                return (B_FALSE);

        return (B_TRUE);
}

static space_map_ops_t metaslab_df_ops = {
        metaslab_pp_load,
        metaslab_pp_unload,
        metaslab_df_alloc,
        metaslab_pp_claim,
        metaslab_pp_free,
        metaslab_pp_maxsize,
        metaslab_df_fragmented
};

/*
 * ==========================================================================
 * Other experimental allocators
 * ==========================================================================
 */
static uint64_t
metaslab_cdf_alloc(space_map_t *sm, uint64_t size)
{
        avl_tree_t *t = &sm->sm_root;
        uint64_t *cursor = (uint64_t *)sm->sm_ppd;
        uint64_t *extent_end = (uint64_t *)sm->sm_ppd + 1;
        uint64_t max_size = metaslab_pp_maxsize(sm);
        uint64_t rsize = size;
        uint64_t offset = 0;

        ASSERT(MUTEX_HELD(sm->sm_lock));
        ASSERT3U(avl_numnodes(&sm->sm_root), ==, avl_numnodes(sm->sm_pp_root));

        if (max_size < size)
                return (-1ULL);

        ASSERT3U(*extent_end, >=, *cursor);

        /*
         * If we're running low on space switch to using the size
         * sorted AVL tree (best-fit).
         */
        if ((*cursor + size) > *extent_end) {

                t = sm->sm_pp_root;
                *cursor = *extent_end = 0;

                if (max_size > 2 * SPA_MAXBLOCKSIZE)
                        rsize = MIN(metaslab_min_alloc_size, max_size);
                offset = metaslab_block_picker(t, extent_end, rsize, 1ULL);
                if (offset != -1)
                        *cursor = offset + size;
        } else {
                offset = metaslab_block_picker(t, cursor, rsize, 1ULL);
        }
        ASSERT3U(*cursor, <=, *extent_end);
        return (offset);
}

static boolean_t
metaslab_cdf_fragmented(space_map_t *sm)
{
        uint64_t max_size = metaslab_pp_maxsize(sm);

        if (max_size > (metaslab_min_alloc_size * 10))
                return (B_FALSE);
        return (B_TRUE);
}

static space_map_ops_t metaslab_cdf_ops = {
        metaslab_pp_load,
        metaslab_pp_unload,
        metaslab_cdf_alloc,
        metaslab_pp_claim,
        metaslab_pp_free,
        metaslab_pp_maxsize,
        metaslab_cdf_fragmented
};

uint64_t metaslab_ndf_clump_shift = 4;

static uint64_t
metaslab_ndf_alloc(space_map_t *sm, uint64_t size)
{
        avl_tree_t *t = &sm->sm_root;
        avl_index_t where;
        space_seg_t *ss, ssearch;
        uint64_t hbit = highbit(size);
        uint64_t *cursor = (uint64_t *)sm->sm_ppd + hbit - 1;
        uint64_t max_size = metaslab_pp_maxsize(sm);

        ASSERT(MUTEX_HELD(sm->sm_lock));
        ASSERT3U(avl_numnodes(&sm->sm_root), ==, avl_numnodes(sm->sm_pp_root));

        if (max_size < size)
                return (-1ULL);

        ssearch.ss_start = *cursor;
        ssearch.ss_end = *cursor + size;

        ss = avl_find(t, &ssearch, &where);
        if (ss == NULL || (ss->ss_start + size > ss->ss_end)) {
                t = sm->sm_pp_root;

                ssearch.ss_start = 0;
                ssearch.ss_end = MIN(max_size,
                    1ULL << (hbit + metaslab_ndf_clump_shift));
                ss = avl_find(t, &ssearch, &where);
                if (ss == NULL)
                        ss = avl_nearest(t, where, AVL_AFTER);
                ASSERT(ss != NULL);
        }

        if (ss != NULL) {
                if (ss->ss_start + size <= ss->ss_end) {
                        *cursor = ss->ss_start + size;
                        return (ss->ss_start);
                }
        }
        return (-1ULL);
}

static boolean_t
metaslab_ndf_fragmented(space_map_t *sm)
{
        uint64_t max_size = metaslab_pp_maxsize(sm);

        if (max_size > (metaslab_min_alloc_size << metaslab_ndf_clump_shift))
                return (B_FALSE);
        return (B_TRUE);
}


static space_map_ops_t metaslab_ndf_ops = {
        metaslab_pp_load,
        metaslab_pp_unload,
        metaslab_ndf_alloc,
        metaslab_pp_claim,
        metaslab_pp_free,
        metaslab_pp_maxsize,
        metaslab_ndf_fragmented
};

space_map_ops_t *zfs_metaslab_ops = &metaslab_df_ops;

/*
 * ==========================================================================
 * Metaslabs
 * ==========================================================================
 */
metaslab_t *
metaslab_init(metaslab_group_t *mg, space_map_obj_t *smo,
        uint64_t start, uint64_t size, uint64_t txg)
{
        vdev_t *vd = mg->mg_vd;
        metaslab_t *msp;

        msp = kmem_zalloc(sizeof (metaslab_t), KM_SLEEP);
        mutex_init(&msp->ms_lock, NULL, MUTEX_DEFAULT, NULL);

        msp->ms_smo_syncing = *smo;

        /*
         * We create the main space map here, but we don't create the
         * allocmaps and freemaps until metaslab_sync_done().  This serves
         * two purposes: it allows metaslab_sync_done() to detect the
         * addition of new space; and for debugging, it ensures that we'd
         * data fault on any attempt to use this metaslab before it's ready.
         */
        msp->ms_map = kmem_zalloc(sizeof (space_map_t), KM_SLEEP);
        space_map_create(msp->ms_map, start, size,
            vd->vdev_ashift, &msp->ms_lock);

        metaslab_group_add(mg, msp);

        if (metaslab_debug && smo->smo_object != 0) {
                mutex_enter(&msp->ms_lock);
                VERIFY(space_map_load(msp->ms_map, mg->mg_class->mc_ops,
                    SM_FREE, smo, spa_meta_objset(vd->vdev_spa)) == 0);
                mutex_exit(&msp->ms_lock);
        }

        /*
         * If we're opening an existing pool (txg == 0) or creating
         * a new one (txg == TXG_INITIAL), all space is available now.
         * If we're adding space to an existing pool, the new space
         * does not become available until after this txg has synced.
         */
        if (txg <= TXG_INITIAL)
                metaslab_sync_done(msp, 0);

        if (txg != 0) {
                vdev_dirty(vd, 0, NULL, txg);
                vdev_dirty(vd, VDD_METASLAB, msp, txg);
        }

        return (msp);
}

void
metaslab_fini(metaslab_t *msp)
{
        metaslab_group_t *mg = msp->ms_group;

        vdev_space_update(mg->mg_vd,
            -msp->ms_smo.smo_alloc, 0, -msp->ms_map->sm_size);

        metaslab_group_remove(mg, msp);

        mutex_enter(&msp->ms_lock);

        space_map_unload(msp->ms_map);
        space_map_destroy(msp->ms_map);
        kmem_free(msp->ms_map, sizeof (*msp->ms_map));

        for (int t = 0; t < TXG_SIZE; t++) {
                space_map_destroy(msp->ms_allocmap[t]);
                space_map_destroy(msp->ms_freemap[t]);
                kmem_free(msp->ms_allocmap[t], sizeof (*msp->ms_allocmap[t]));
                kmem_free(msp->ms_freemap[t], sizeof (*msp->ms_freemap[t]));
        }

        for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                space_map_destroy(msp->ms_defermap[t]);
                kmem_free(msp->ms_defermap[t], sizeof (*msp->ms_defermap[t]));
        }

        ASSERT0(msp->ms_deferspace);

        mutex_exit(&msp->ms_lock);
        mutex_destroy(&msp->ms_lock);

        kmem_free(msp, sizeof (metaslab_t));
}

#define METASLAB_WEIGHT_PRIMARY         (1ULL << 63)
#define METASLAB_WEIGHT_SECONDARY       (1ULL << 62)
#define METASLAB_ACTIVE_MASK            \
        (METASLAB_WEIGHT_PRIMARY | METASLAB_WEIGHT_SECONDARY)

static uint64_t
metaslab_weight(metaslab_t *msp)
{
        metaslab_group_t *mg = msp->ms_group;
        space_map_t *sm = msp->ms_map;
        space_map_obj_t *smo = &msp->ms_smo;
        vdev_t *vd = mg->mg_vd;
        uint64_t weight, space;

        ASSERT(MUTEX_HELD(&msp->ms_lock));

        /*
         * This vdev is in the process of being removed so there is nothing
         * for us to do here.
         */
        if (vd->vdev_removing) {
                ASSERT0(smo->smo_alloc);
                ASSERT0(vd->vdev_ms_shift);
                return (0);
        }

        /*
         * The baseline weight is the metaslab's free space.
         */
        space = sm->sm_size - smo->smo_alloc;
        weight = space;

        /*
         * Modern disks have uniform bit density and constant angular velocity.
         * Therefore, the outer recording zones are faster (higher bandwidth)
         * than the inner zones by the ratio of outer to inner track diameter,
         * which is typically around 2:1.  We account for this by assigning
         * higher weight to lower metaslabs (multiplier ranging from 2x to 1x).
         * 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 -
            ((sm->sm_start >> vd->vdev_ms_shift) * weight) / vd->vdev_ms_count;
        ASSERT(weight >= space && weight <= 2 * space);

        /*
         * For locality, assign higher weight to metaslabs which have
         * a lower offset than what we've already activated.
         */
        if (sm->sm_start <= mg->mg_bonus_area)
                weight *= (metaslab_smo_bonus_pct / 100);
        ASSERT(weight >= space &&
            weight <= 2 * (metaslab_smo_bonus_pct / 100) * space);

        if (sm->sm_loaded && !sm->sm_ops->smop_fragmented(sm)) {
                /*
                 * 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.
                 */
                weight |= (msp->ms_weight & METASLAB_ACTIVE_MASK);
        }
        return (weight);
}

static void
metaslab_prefetch(metaslab_group_t *mg)
{
        spa_t *spa = mg->mg_vd->vdev_spa;
        metaslab_t *msp;
        avl_tree_t *t = &mg->mg_metaslab_tree;
        int m;

        mutex_enter(&mg->mg_lock);

        /*
         * Prefetch the next potential metaslabs
         */
        for (msp = avl_first(t), m = 0; msp; msp = AVL_NEXT(t, msp), m++) {
                space_map_t *sm = msp->ms_map;
                space_map_obj_t *smo = &msp->ms_smo;

                /* If we have reached our prefetch limit then we're done */
                if (m >= metaslab_prefetch_limit)
                        break;

                if (!sm->sm_loaded && smo->smo_object != 0) {
                        mutex_exit(&mg->mg_lock);
                        dmu_prefetch(spa_meta_objset(spa), smo->smo_object,
                            0ULL, smo->smo_objsize);
                        mutex_enter(&mg->mg_lock);
                }
        }
        mutex_exit(&mg->mg_lock);
}

static int
metaslab_activate(metaslab_t *msp, uint64_t activation_weight)
{
        metaslab_group_t *mg = msp->ms_group;
        space_map_t *sm = msp->ms_map;
        space_map_ops_t *sm_ops = msp->ms_group->mg_class->mc_ops;

        ASSERT(MUTEX_HELD(&msp->ms_lock));

        if ((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0) {
                space_map_load_wait(sm);
                if (!sm->sm_loaded) {
                        space_map_obj_t *smo = &msp->ms_smo;

                        int error = space_map_load(sm, sm_ops, SM_FREE, smo,
                            spa_meta_objset(msp->ms_group->mg_vd->vdev_spa));
                        if (error)  {
                                metaslab_group_sort(msp->ms_group, msp, 0);
                                return (error);
                        }
                        for (int t = 0; t < TXG_DEFER_SIZE; t++)
                                space_map_walk(msp->ms_defermap[t],
                                    space_map_claim, sm);

                }

                /*
                 * Track the bonus area as we activate new metaslabs.
                 */
                if (sm->sm_start > mg->mg_bonus_area) {
                        mutex_enter(&mg->mg_lock);
                        mg->mg_bonus_area = sm->sm_start;
                        mutex_exit(&mg->mg_lock);
                }

                metaslab_group_sort(msp->ms_group, msp,
                    msp->ms_weight | activation_weight);
        }
        ASSERT(sm->sm_loaded);
        ASSERT(msp->ms_weight & METASLAB_ACTIVE_MASK);

        return (0);
}

static void
metaslab_passivate(metaslab_t *msp, uint64_t size)
{
        /*
         * If size < SPA_MINBLOCKSIZE, then we will not allocate from
         * this metaslab again.  In that case, it had better be empty,
         * or we would be leaving space on the table.
         */
        ASSERT(size >= SPA_MINBLOCKSIZE || msp->ms_map->sm_space == 0);
        metaslab_group_sort(msp->ms_group, msp, MIN(msp->ms_weight, size));
        ASSERT((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0);
}

/*
 * Determine if the in-core space map representation can be condensed on-disk.
 * We would like to use the following criteria to make our decision:
 *
 * 1. The size of the space map object should not dramatically increase as a
 * result of writing out our in-core free map.
 *
 * 2. The minimal on-disk space map representation is zfs_condense_pct/100
 * times the size than the in-core representation (i.e. zfs_condense_pct = 110
 * and in-core = 1MB, minimal = 1.1.MB).
 *
 * Checking the first condition is tricky since we don't want to walk
 * the entire AVL tree calculating the estimated on-disk size. Instead we
 * use the size-ordered AVL tree in the space map and calculate the
 * size required for the largest segment in our in-core free map. If the
 * size required to represent that segment on disk is larger than the space
 * map object then we avoid condensing this map.
 *
 * To determine the second criterion we use a best-case estimate and assume
 * each segment can be represented on-disk as a single 64-bit entry. We refer
 * to this best-case estimate as the space map's minimal form.
 */
static boolean_t
metaslab_should_condense(metaslab_t *msp)
{
        space_map_t *sm = msp->ms_map;
        space_map_obj_t *smo = &msp->ms_smo_syncing;
        space_seg_t *ss;
        uint64_t size, entries, segsz;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT(sm->sm_loaded);

        /*
         * Use the sm_pp_root AVL tree, which is ordered by size, to obtain
         * the largest segment in the in-core free map. If the tree is
         * empty then we should condense the map.
         */
        ss = avl_last(sm->sm_pp_root);
        if (ss == NULL)
                return (B_TRUE);

        /*
         * Calculate the number of 64-bit entries this segment would
         * require when written to disk. If this single segment would be
         * larger on-disk than the entire current on-disk structure, then
         * clearly condensing will increase the on-disk structure size.
         */
        size = (ss->ss_end - ss->ss_start) >> sm->sm_shift;
        entries = size / (MIN(size, SM_RUN_MAX));
        segsz = entries * sizeof (uint64_t);

        return (segsz <= smo->smo_objsize &&
            smo->smo_objsize >= (zfs_condense_pct *
            sizeof (uint64_t) * avl_numnodes(&sm->sm_root)) / 100);
}

/*
 * Condense the on-disk space map representation to its minimized form.
 * The minimized form consists of a small number of allocations followed by
 * the in-core free map.
 */
static void
metaslab_condense(metaslab_t *msp, uint64_t txg, dmu_tx_t *tx)
{
        spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
        space_map_t *freemap = msp->ms_freemap[txg & TXG_MASK];
        space_map_t condense_map;
        space_map_t *sm = msp->ms_map;
        objset_t *mos = spa_meta_objset(spa);
        space_map_obj_t *smo = &msp->ms_smo_syncing;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT3U(spa_sync_pass(spa), ==, 1);
        ASSERT(sm->sm_loaded);

        spa_dbgmsg(spa, "condensing: txg %llu, msp[%llu] %p, "
            "smo size %llu, segments %lu", txg,
            (msp->ms_map->sm_start / msp->ms_map->sm_size), msp,
            smo->smo_objsize, avl_numnodes(&sm->sm_root));

        /*
         * Create an map that is a 100% allocated map. We remove segments
         * that have been freed in this txg, any deferred frees that exist,
         * and any allocation in the future. Removing segments should be
         * a relatively inexpensive operation since we expect these maps to
         * a small number of nodes.
         */
        space_map_create(&condense_map, sm->sm_start, sm->sm_size,
            sm->sm_shift, sm->sm_lock);
        space_map_add(&condense_map, condense_map.sm_start,
            condense_map.sm_size);

        /*
         * Remove what's been freed in this txg from the condense_map.
         * Since we're in sync_pass 1, we know that all the frees from
         * this txg are in the freemap.
         */
        space_map_walk(freemap, space_map_remove, &condense_map);

        for (int t = 0; t < TXG_DEFER_SIZE; t++)
                space_map_walk(msp->ms_defermap[t],
                    space_map_remove, &condense_map);

        for (int t = 1; t < TXG_CONCURRENT_STATES; t++)
                space_map_walk(msp->ms_allocmap[(txg + t) & TXG_MASK],
                    space_map_remove, &condense_map);

        /*
         * We're about to drop the metaslab's lock thus allowing
         * other consumers to change it's content. Set the
         * space_map's sm_condensing flag to ensure that
         * allocations on this metaslab do not occur while we're
         * in the middle of committing it to disk. This is only critical
         * for the ms_map as all other space_maps use per txg
         * views of their content.
         */
        sm->sm_condensing = B_TRUE;

        mutex_exit(&msp->ms_lock);
        space_map_truncate(smo, mos, tx);
        mutex_enter(&msp->ms_lock);

        /*
         * While we would ideally like to create a space_map representation
         * that consists only of allocation records, doing so can be
         * prohibitively expensive because the in-core free map can be
         * large, and therefore computationally expensive to subtract
         * from the condense_map. Instead we sync out two maps, a cheap
         * allocation only map followed by the in-core free map. While not
         * optimal, this is typically close to optimal, and much cheaper to
         * compute.
         */
        space_map_sync(&condense_map, SM_ALLOC, smo, mos, tx);
        space_map_vacate(&condense_map, NULL, NULL);
        space_map_destroy(&condense_map);

        space_map_sync(sm, SM_FREE, smo, mos, tx);
        sm->sm_condensing = B_FALSE;

        spa_dbgmsg(spa, "condensed: txg %llu, msp[%llu] %p, "
            "smo size %llu", txg,
            (msp->ms_map->sm_start / msp->ms_map->sm_size), msp,
            smo->smo_objsize);
}

/*
 * Write a metaslab to disk in the context of the specified transaction group.
 */
void
metaslab_sync(metaslab_t *msp, uint64_t txg)
{
        vdev_t *vd = msp->ms_group->mg_vd;
        spa_t *spa = vd->vdev_spa;
        objset_t *mos = spa_meta_objset(spa);
        space_map_t *allocmap = msp->ms_allocmap[txg & TXG_MASK];
        space_map_t **freemap = &msp->ms_freemap[txg & TXG_MASK];
        space_map_t **freed_map = &msp->ms_freemap[TXG_CLEAN(txg) & TXG_MASK];
        space_map_t *sm = msp->ms_map;
        space_map_obj_t *smo = &msp->ms_smo_syncing;
        dmu_buf_t *db;
        dmu_tx_t *tx;

        ASSERT(!vd->vdev_ishole);

        /*
         * This metaslab has just been added so there's no work to do now.
         */
        if (*freemap == NULL) {
                ASSERT3P(allocmap, ==, NULL);
                return;
        }

        ASSERT3P(allocmap, !=, NULL);
        ASSERT3P(*freemap, !=, NULL);
        ASSERT3P(*freed_map, !=, NULL);

        if (allocmap->sm_space == 0 && (*freemap)->sm_space == 0)
                return;

        /*
         * The only state that can actually be changing concurrently with
         * metaslab_sync() is the metaslab's ms_map.  No other thread can
         * be modifying this txg's allocmap, freemap, freed_map, or smo.
         * Therefore, we only hold ms_lock to satify space_map ASSERTs.
         * We drop it whenever we call into the DMU, because the DMU
         * can call down to us (e.g. via zio_free()) at any time.
         */

        tx = dmu_tx_create_assigned(spa_get_dsl(spa), txg);

        if (smo->smo_object == 0) {
                ASSERT(smo->smo_objsize == 0);
                ASSERT(smo->smo_alloc == 0);
                smo->smo_object = dmu_object_alloc(mos,
                    DMU_OT_SPACE_MAP, 1 << SPACE_MAP_BLOCKSHIFT,
                    DMU_OT_SPACE_MAP_HEADER, sizeof (*smo), tx);
                ASSERT(smo->smo_object != 0);
                dmu_write(mos, vd->vdev_ms_array, sizeof (uint64_t) *
                    (sm->sm_start >> vd->vdev_ms_shift),
                    sizeof (uint64_t), &smo->smo_object, tx);
        }

        mutex_enter(&msp->ms_lock);

        if (sm->sm_loaded && spa_sync_pass(spa) == 1 &&
            metaslab_should_condense(msp)) {
                metaslab_condense(msp, txg, tx);
        } else {
                space_map_sync(allocmap, SM_ALLOC, smo, mos, tx);
                space_map_sync(*freemap, SM_FREE, smo, mos, tx);
        }

        space_map_vacate(allocmap, NULL, NULL);

        /*
         * For sync pass 1, we avoid walking the entire space map and
         * instead will just swap the pointers for freemap and
         * freed_map. We can safely do this since the freed_map is
         * guaranteed to be empty on the initial pass.
         */
        if (spa_sync_pass(spa) == 1) {
                ASSERT0((*freed_map)->sm_space);
                ASSERT0(avl_numnodes(&(*freed_map)->sm_root));
                space_map_swap(freemap, freed_map);
        } else {
                space_map_vacate(*freemap, space_map_add, *freed_map);
        }

        ASSERT0(msp->ms_allocmap[txg & TXG_MASK]->sm_space);
        ASSERT0(msp->ms_freemap[txg & TXG_MASK]->sm_space);

        mutex_exit(&msp->ms_lock);

        VERIFY0(dmu_bonus_hold(mos, smo->smo_object, FTAG, &db));
        dmu_buf_will_dirty(db, tx);
        ASSERT3U(db->db_size, >=, sizeof (*smo));
        bcopy(smo, db->db_data, sizeof (*smo));
        dmu_buf_rele(db, FTAG);

        dmu_tx_commit(tx);
}

/*
 * Called after a transaction group has completely synced to mark
 * all of the metaslab's free space as usable.
 */
void
metaslab_sync_done(metaslab_t *msp, uint64_t txg)
{
        space_map_obj_t *smo = &msp->ms_smo;
        space_map_obj_t *smosync = &msp->ms_smo_syncing;
        space_map_t *sm = msp->ms_map;
        space_map_t **freed_map = &msp->ms_freemap[TXG_CLEAN(txg) & TXG_MASK];
        space_map_t **defer_map = &msp->ms_defermap[txg % TXG_DEFER_SIZE];
        metaslab_group_t *mg = msp->ms_group;
        vdev_t *vd = mg->mg_vd;
        int64_t alloc_delta, defer_delta;

        ASSERT(!vd->vdev_ishole);

        mutex_enter(&msp->ms_lock);

        /*
         * If this metaslab is just becoming available, initialize its
         * allocmaps, freemaps, and defermap and add its capacity to the vdev.
         */
        if (*freed_map == NULL) {
                ASSERT(*defer_map == NULL);
                for (int t = 0; t < TXG_SIZE; t++) {
                        msp->ms_allocmap[t] = kmem_zalloc(sizeof (space_map_t),
                            KM_SLEEP);
                        space_map_create(msp->ms_allocmap[t], sm->sm_start,
                            sm->sm_size, sm->sm_shift, sm->sm_lock);
                        msp->ms_freemap[t] = kmem_zalloc(sizeof (space_map_t),
                            KM_SLEEP);
                        space_map_create(msp->ms_freemap[t], sm->sm_start,
                            sm->sm_size, sm->sm_shift, sm->sm_lock);
                }

                for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                        msp->ms_defermap[t] = kmem_zalloc(sizeof (space_map_t),
                            KM_SLEEP);
                        space_map_create(msp->ms_defermap[t], sm->sm_start,
                            sm->sm_size, sm->sm_shift, sm->sm_lock);
                }

                freed_map = &msp->ms_freemap[TXG_CLEAN(txg) & TXG_MASK];
                defer_map = &msp->ms_defermap[txg % TXG_DEFER_SIZE];

                vdev_space_update(vd, 0, 0, sm->sm_size);
        }

        alloc_delta = smosync->smo_alloc - smo->smo_alloc;
        defer_delta = (*freed_map)->sm_space - (*defer_map)->sm_space;

        vdev_space_update(vd, alloc_delta + defer_delta, defer_delta, 0);

        ASSERT(msp->ms_allocmap[txg & TXG_MASK]->sm_space == 0);
        ASSERT(msp->ms_freemap[txg & TXG_MASK]->sm_space == 0);

        /*
         * If there's a space_map_load() in progress, wait for it to complete
         * so that we have a consistent view of the in-core space map.
         */
        space_map_load_wait(sm);

        /*
         * Move the frees from the defer_map to this map (if it's loaded).
         * Swap the freed_map and the defer_map -- this is safe to do
         * because we've just emptied out the defer_map.
         */
        space_map_vacate(*defer_map, sm->sm_loaded ? space_map_free : NULL, sm);
        ASSERT0((*defer_map)->sm_space);
        ASSERT0(avl_numnodes(&(*defer_map)->sm_root));
        space_map_swap(freed_map, defer_map);

        *smo = *smosync;

        msp->ms_deferspace += defer_delta;
        ASSERT3S(msp->ms_deferspace, >=, 0);
        ASSERT3S(msp->ms_deferspace, <=, sm->sm_size);
        if (msp->ms_deferspace != 0) {
                /*
                 * Keep syncing this metaslab until all deferred frees
                 * are back in circulation.
                 */
                vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
        }

        /*
         * If the map is loaded but no longer active, evict it as soon as all
         * future allocations have synced.  (If we unloaded it now and then
         * loaded a moment later, the map wouldn't reflect those allocations.)
         */
        if (sm->sm_loaded && (msp->ms_weight & METASLAB_ACTIVE_MASK) == 0) {
                int evictable = 1;

                for (int t = 1; t < TXG_CONCURRENT_STATES; t++)
                        if (msp->ms_allocmap[(txg + t) & TXG_MASK]->sm_space)
                                evictable = 0;

                if (evictable && !metaslab_debug)
                        space_map_unload(sm);
        }

        metaslab_group_sort(mg, msp, metaslab_weight(msp));

        mutex_exit(&msp->ms_lock);
}

void
metaslab_sync_reassess(metaslab_group_t *mg)
{
        vdev_t *vd = mg->mg_vd;
        int64_t failures = mg->mg_alloc_failures;

        /*
         * Re-evaluate all metaslabs which have lower offsets than the
         * bonus area.
         */
        for (int m = 0; m < vd->vdev_ms_count; m++) {
                metaslab_t *msp = vd->vdev_ms[m];

                if (msp->ms_map->sm_start > mg->mg_bonus_area)
                        break;

                mutex_enter(&msp->ms_lock);
                metaslab_group_sort(mg, msp, metaslab_weight(msp));
                mutex_exit(&msp->ms_lock);
        }

        atomic_add_64(&mg->mg_alloc_failures, -failures);

        /*
         * Prefetch the next potential metaslabs
         */
        metaslab_prefetch(mg);
}

static uint64_t
metaslab_distance(metaslab_t *msp, dva_t *dva)
{
        uint64_t ms_shift = msp->ms_group->mg_vd->vdev_ms_shift;
        uint64_t offset = DVA_GET_OFFSET(dva) >> ms_shift;
        uint64_t start = msp->ms_map->sm_start >> ms_shift;

        if (msp->ms_group->mg_vd->vdev_id != DVA_GET_VDEV(dva))
                return (1ULL << 63);

        if (offset < start)
                return ((start - offset) << ms_shift);
        if (offset > start)
                return ((offset - start) << ms_shift);
        return (0);
}

static uint64_t
metaslab_group_alloc(metaslab_group_t *mg, uint64_t psize, uint64_t asize,
    uint64_t txg, uint64_t min_distance, dva_t *dva, int d, int flags)
{
        spa_t *spa = mg->mg_vd->vdev_spa;
        metaslab_t *msp = NULL;
        uint64_t offset = -1ULL;
        avl_tree_t *t = &mg->mg_metaslab_tree;
        uint64_t activation_weight;
        uint64_t target_distance;
        int i;

        activation_weight = METASLAB_WEIGHT_PRIMARY;
        for (i = 0; i < d; i++) {
                if (DVA_GET_VDEV(&dva[i]) == mg->mg_vd->vdev_id) {
                        activation_weight = METASLAB_WEIGHT_SECONDARY;
                        break;
                }
        }

        for (;;) {
                boolean_t was_active;

                mutex_enter(&mg->mg_lock);
                for (msp = avl_first(t); msp; msp = AVL_NEXT(t, msp)) {
                        if (msp->ms_weight < asize) {
                                spa_dbgmsg(spa, "%s: failed to meet weight "
                                    "requirement: vdev %llu, txg %llu, mg %p, "
                                    "msp %p, psize %llu, asize %llu, "
                                    "failures %llu, weight %llu",
                                    spa_name(spa), mg->mg_vd->vdev_id, txg,
                                    mg, msp, psize, asize,
                                    mg->mg_alloc_failures, msp->ms_weight);
                                mutex_exit(&mg->mg_lock);
                                return (-1ULL);
                        }

                        /*
                         * If the selected metaslab is condensing, skip it.
                         */
                        if (msp->ms_map->sm_condensing)
                                continue;

                        was_active = msp->ms_weight & METASLAB_ACTIVE_MASK;
                        if (activation_weight == METASLAB_WEIGHT_PRIMARY)
                                break;

                        target_distance = min_distance +
                            (msp->ms_smo.smo_alloc ? 0 : min_distance >> 1);

                        for (i = 0; i < d; i++)
                                if (metaslab_distance(msp, &dva[i]) <
                                    target_distance)
                                        break;
                        if (i == d)
                                break;
                }
                mutex_exit(&mg->mg_lock);
                if (msp == NULL)
                        return (-1ULL);

                /*
                 * If we've already reached the allowable number of failed
                 * allocation attempts on this metaslab group then we
                 * consider skipping it. We skip it only if we're allowed
                 * to "fast" gang, the physical size is larger than
                 * a gang block, and we're attempting to allocate from
                 * the primary metaslab.
                 */
                if (mg->mg_alloc_failures > zfs_mg_alloc_failures &&
                    CAN_FASTGANG(flags) && psize > SPA_GANGBLOCKSIZE &&
                    activation_weight == METASLAB_WEIGHT_PRIMARY) {
                        spa_dbgmsg(spa, "%s: skipping metaslab group: "
                            "vdev %llu, txg %llu, mg %p, psize %llu, "
                            "asize %llu, failures %llu", spa_name(spa),
                            mg->mg_vd->vdev_id, txg, mg, psize, asize,
                            mg->mg_alloc_failures);
                        return (-1ULL);
                }

                mutex_enter(&msp->ms_lock);

                /*
                 * Ensure that the metaslab we have selected is still
                 * capable of handling our request. It's possible that
                 * another thread may have changed the weight while we
                 * were blocked on the metaslab lock.
                 */
                if (msp->ms_weight < asize || (was_active &&
                    !(msp->ms_weight & METASLAB_ACTIVE_MASK) &&
                    activation_weight == METASLAB_WEIGHT_PRIMARY)) {
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                if ((msp->ms_weight & METASLAB_WEIGHT_SECONDARY) &&
                    activation_weight == METASLAB_WEIGHT_PRIMARY) {
                        metaslab_passivate(msp,
                            msp->ms_weight & ~METASLAB_ACTIVE_MASK);
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                if (metaslab_activate(msp, activation_weight) != 0) {
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                /*
                 * If this metaslab is currently condensing then pick again as
                 * we can't manipulate this metaslab until it's committed
                 * to disk.
                 */
                if (msp->ms_map->sm_condensing) {
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                if ((offset = space_map_alloc(msp->ms_map, asize)) != -1ULL)
                        break;

                atomic_inc_64(&mg->mg_alloc_failures);

                metaslab_passivate(msp, space_map_maxsize(msp->ms_map));

                mutex_exit(&msp->ms_lock);
        }

        if (msp->ms_allocmap[txg & TXG_MASK]->sm_space == 0)
                vdev_dirty(mg->mg_vd, VDD_METASLAB, msp, txg);

        space_map_add(msp->ms_allocmap[txg & TXG_MASK], offset, asize);

        mutex_exit(&msp->ms_lock);

        return (offset);
}

/*
 * Allocate a block for the specified i/o.
 */
static int
metaslab_alloc_dva(spa_t *spa, metaslab_class_t *mc, uint64_t psize,
    dva_t *dva, int d, dva_t *hintdva, uint64_t txg, int flags)
{
        metaslab_group_t *mg, *rotor;
        vdev_t *vd;
        int dshift = 3;
        int all_zero;
        int zio_lock = B_FALSE;
        boolean_t allocatable;
        uint64_t offset = -1ULL;
        uint64_t asize;
        uint64_t distance;

        ASSERT(!DVA_IS_VALID(&dva[d]));

        /*
         * For testing, make some blocks above a certain size be gang blocks.
         */
        if (psize >= metaslab_gang_bang && (ddi_get_lbolt() & 3) == 0)
                return (SET_ERROR(ENOSPC));

        /*
         * Start at the rotor and loop through all mgs until we find something.
         * Note that there's no locking on mc_rotor or mc_aliquot because
         * nothing actually breaks if we miss a few updates -- we just won't
         * allocate quite as evenly.  It all balances out over time.
         *
         * If we are doing ditto or log blocks, try to spread them across
         * consecutive vdevs.  If we're forced to reuse a vdev before we've
         * allocated all of our ditto blocks, then try and spread them out on
         * that vdev as much as possible.  If it turns out to not be possible,
         * gradually lower our standards until anything becomes acceptable.
         * Also, allocating on consecutive vdevs (as opposed to random vdevs)
         * gives us hope of containing our fault domains to something we're
         * able to reason about.  Otherwise, any two top-level vdev failures
         * will guarantee the loss of data.  With consecutive allocation,
         * only two adjacent top-level vdev failures will result in data loss.
         *
         * If we are doing gang blocks (hintdva is non-NULL), try to keep
         * ourselves on the same vdev as our gang block header.  That
         * way, we can hope for locality in vdev_cache, plus it makes our
         * fault domains something tractable.
         */
        if (hintdva) {
                vd = vdev_lookup_top(spa, DVA_GET_VDEV(&hintdva[d]));

                /*
                 * It's possible the vdev we're using as the hint no
                 * longer exists (i.e. removed). Consult the rotor when
                 * all else fails.
                 */
                if (vd != NULL) {
                        mg = vd->vdev_mg;

                        if (flags & METASLAB_HINTBP_AVOID &&
                            mg->mg_next != NULL)
                                mg = mg->mg_next;
                } else {
                        mg = mc->mc_rotor;
                }
        } else if (d != 0) {
                vd = vdev_lookup_top(spa, DVA_GET_VDEV(&dva[d - 1]));
                mg = vd->vdev_mg->mg_next;
        } else {
                mg = mc->mc_rotor;
        }

        /*
         * If the hint put us into the wrong metaslab class, or into a
         * metaslab group that has been passivated, just follow the rotor.
         */
        if (mg->mg_class != mc || mg->mg_activation_count <= 0)
                mg = mc->mc_rotor;

        rotor = mg;
top:
        all_zero = B_TRUE;
        do {
                ASSERT(mg->mg_activation_count == 1);

                vd = mg->mg_vd;

                /*
                 * Don't allocate from faulted devices.
                 */
                if (zio_lock) {
                        spa_config_enter(spa, SCL_ZIO, FTAG, RW_READER);
                        allocatable = vdev_allocatable(vd);
                        spa_config_exit(spa, SCL_ZIO, FTAG);
                } else {
                        allocatable = vdev_allocatable(vd);
                }
                if (!allocatable)
                        goto next;

                /*
                 * Avoid writing single-copy data to a failing vdev
                 * unless the user instructs us that it is okay.
                 */
                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)) {
                        all_zero = B_FALSE;
                        goto next;
                }

                ASSERT(mg->mg_class == mc);

                distance = vd->vdev_asize >> dshift;
                if (distance <= (1ULL << vd->vdev_ms_shift))
                        distance = 0;
                else
                        all_zero = B_FALSE;

                asize = vdev_psize_to_asize(vd, psize);
                ASSERT(P2PHASE(asize, 1ULL << vd->vdev_ashift) == 0);

                offset = metaslab_group_alloc(mg, psize, asize, txg, distance,
                    dva, d, flags);
                if (offset != -1ULL) {
                        /*
                         * If we've just selected this metaslab group,
                         * figure out whether the corresponding vdev is
                         * over- or under-used relative to the pool,
                         * and set an allocation bias to even it out.
                         */
                        if (mc->mc_aliquot == 0) {
                                vdev_stat_t *vs = &vd->vdev_stat;
                                int64_t vu, cu;

                                vu = (vs->vs_alloc * 100) / (vs->vs_space + 1);
                                cu = (mc->mc_alloc * 100) / (mc->mc_space + 1);

                                /*
                                 * Calculate how much more or less we should
                                 * try to allocate from this device during
                                 * this iteration around the rotor.
                                 * For example, if a device is 80% full
                                 * and the pool is 20% full then we should
                                 * reduce allocations by 60% on this device.
                                 *
                                 * mg_bias = (20 - 80) * 512K / 100 = -307K
                                 *
                                 * This reduces allocations by 307K for this
                                 * iteration.
                                 */
                                mg->mg_bias = ((cu - vu) *
                                    (int64_t)mg->mg_aliquot) / 100;
                        }

                        if (atomic_add_64_nv(&mc->mc_aliquot, asize) >=
                            mg->mg_aliquot + mg->mg_bias) {
                                mc->mc_rotor = mg->mg_next;
                                mc->mc_aliquot = 0;
                        }

                        DVA_SET_VDEV(&dva[d], vd->vdev_id);
                        DVA_SET_OFFSET(&dva[d], offset);
                        DVA_SET_GANG(&dva[d], !!(flags & METASLAB_GANG_HEADER));
                        DVA_SET_ASIZE(&dva[d], asize);

                        return (0);
                }
next:
                mc->mc_rotor = mg->mg_next;
                mc->mc_aliquot = 0;
        } while ((mg = mg->mg_next) != rotor);

        if (!all_zero) {
                dshift++;
                ASSERT(dshift < 64);
                goto top;
        }

        if (!allocatable && !zio_lock) {
                dshift = 3;
                zio_lock = B_TRUE;
                goto top;
        }

        bzero(&dva[d], sizeof (dva_t));

        return (SET_ERROR(ENOSPC));
}

/*
 * Free the block represented by DVA in the context of the specified
 * transaction group.
 */
static void
metaslab_free_dva(spa_t *spa, const dva_t *dva, uint64_t txg, boolean_t now)
{
        uint64_t vdev = DVA_GET_VDEV(dva);
        uint64_t offset = DVA_GET_OFFSET(dva);
        uint64_t size = DVA_GET_ASIZE(dva);
        vdev_t *vd;
        metaslab_t *msp;

        ASSERT(DVA_IS_VALID(dva));

        if (txg > spa_freeze_txg(spa))
                return;

        if ((vd = vdev_lookup_top(spa, vdev)) == NULL ||
            (offset >> vd->vdev_ms_shift) >= vd->vdev_ms_count) {
                cmn_err(CE_WARN, "metaslab_free_dva(): bad DVA %llu:%llu",
                    (u_longlong_t)vdev, (u_longlong_t)offset);
                ASSERT(0);
                return;
        }

        msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];

        if (DVA_GET_GANG(dva))
                size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);

        mutex_enter(&msp->ms_lock);

        if (now) {
                space_map_remove(msp->ms_allocmap[txg & TXG_MASK],
                    offset, size);
                space_map_free(msp->ms_map, offset, size);
        } else {
                if (msp->ms_freemap[txg & TXG_MASK]->sm_space == 0)
                        vdev_dirty(vd, VDD_METASLAB, msp, txg);
                space_map_add(msp->ms_freemap[txg & TXG_MASK], offset, size);
        }

        mutex_exit(&msp->ms_lock);
}

/*
 * Intent log support: upon opening the pool after a crash, notify the SPA
 * of blocks that the intent log has allocated for immediate write, but
 * which are still considered free by the SPA because the last transaction
 * group didn't commit yet.
 */
static int
metaslab_claim_dva(spa_t *spa, const dva_t *dva, uint64_t txg)
{
        uint64_t vdev = DVA_GET_VDEV(dva);
        uint64_t offset = DVA_GET_OFFSET(dva);
        uint64_t size = DVA_GET_ASIZE(dva);
        vdev_t *vd;
        metaslab_t *msp;
        int error = 0;

        ASSERT(DVA_IS_VALID(dva));

        if ((vd = vdev_lookup_top(spa, vdev)) == NULL ||
            (offset >> vd->vdev_ms_shift) >= vd->vdev_ms_count)
                return (SET_ERROR(ENXIO));

        msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];

        if (DVA_GET_GANG(dva))
                size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);

        mutex_enter(&msp->ms_lock);

        if ((txg != 0 && spa_writeable(spa)) || !msp->ms_map->sm_loaded)
                error = metaslab_activate(msp, METASLAB_WEIGHT_SECONDARY);

        if (error == 0 && !space_map_contains(msp->ms_map, offset, size))
                error = SET_ERROR(ENOENT);

        if (error || txg == 0) {        /* txg == 0 indicates dry run */
                mutex_exit(&msp->ms_lock);
                return (error);
        }

        space_map_claim(msp->ms_map, offset, size);

        if (spa_writeable(spa)) {       /* don't dirty if we're zdb(1M) */
                if (msp->ms_allocmap[txg & TXG_MASK]->sm_space == 0)
                        vdev_dirty(vd, VDD_METASLAB, msp, txg);
                space_map_add(msp->ms_allocmap[txg & TXG_MASK], offset, size);
        }

        mutex_exit(&msp->ms_lock);

        return (0);
}

int
metaslab_alloc(spa_t *spa, metaslab_class_t *mc, uint64_t psize, blkptr_t *bp,
    int ndvas, uint64_t txg, blkptr_t *hintbp, int flags)
{
        dva_t *dva = bp->blk_dva;
        dva_t *hintdva = hintbp->blk_dva;
        int error = 0;

        ASSERT(bp->blk_birth == 0);
        ASSERT(BP_PHYSICAL_BIRTH(bp) == 0);

        spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);

        if (mc->mc_rotor == NULL) {     /* no vdevs in this class */
                spa_config_exit(spa, SCL_ALLOC, FTAG);
                return (SET_ERROR(ENOSPC));
        }

        ASSERT(ndvas > 0 && ndvas <= spa_max_replication(spa));
        ASSERT(BP_GET_NDVAS(bp) == 0);
        ASSERT(hintbp == NULL || ndvas <= BP_GET_NDVAS(hintbp));

        for (int d = 0; d < ndvas; d++) {
                error = metaslab_alloc_dva(spa, mc, psize, dva, d, hintdva,
                    txg, flags);
                if (error) {
                        for (d--; d >= 0; d--) {
                                metaslab_free_dva(spa, &dva[d], txg, B_TRUE);
                                bzero(&dva[d], sizeof (dva_t));
                        }
                        spa_config_exit(spa, SCL_ALLOC, FTAG);
                        return (error);
                }
        }
        ASSERT(error == 0);
        ASSERT(BP_GET_NDVAS(bp) == ndvas);

        spa_config_exit(spa, SCL_ALLOC, FTAG);

        BP_SET_BIRTH(bp, txg, txg);

        return (0);
}

void
metaslab_free(spa_t *spa, const blkptr_t *bp, uint64_t txg, boolean_t now)
{
        const dva_t *dva = bp->blk_dva;
        int ndvas = BP_GET_NDVAS(bp);

        ASSERT(!BP_IS_HOLE(bp));
        ASSERT(!now || bp->blk_birth >= spa_syncing_txg(spa));

        spa_config_enter(spa, SCL_FREE, FTAG, RW_READER);

        for (int d = 0; d < ndvas; d++)
                metaslab_free_dva(spa, &dva[d], txg, now);

        spa_config_exit(spa, SCL_FREE, FTAG);
}

int
metaslab_claim(spa_t *spa, const blkptr_t *bp, uint64_t txg)
{
        const dva_t *dva = bp->blk_dva;
        int ndvas = BP_GET_NDVAS(bp);
        int error = 0;

        ASSERT(!BP_IS_HOLE(bp));

        if (txg != 0) {
                /*
                 * First do a dry run to make sure all DVAs are claimable,
                 * so we don't have to unwind from partial failures below.
                 */
                if ((error = metaslab_claim(spa, bp, 0)) != 0)
                        return (error);
        }

        spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);

        for (int d = 0; d < ndvas; d++)
                if ((error = metaslab_claim_dva(spa, &dva[d], txg)) != 0)
                        break;

        spa_config_exit(spa, SCL_ALLOC, FTAG);

        ASSERT(error == 0 || txg == 0);

        return (error);
}

static void
checkmap(space_map_t *sm, uint64_t off, uint64_t size)
{
        space_seg_t *ss;
        avl_index_t where;

        mutex_enter(sm->sm_lock);
        ss = space_map_find(sm, off, size, &where);
        if (ss != NULL)
                panic("freeing free block; ss=%p", (void *)ss);
        mutex_exit(sm->sm_lock);
}

void
metaslab_check_free(spa_t *spa, const blkptr_t *bp)
{
        if ((zfs_flags & ZFS_DEBUG_ZIO_FREE) == 0)
                return;

        spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
        for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
                uint64_t vdid = DVA_GET_VDEV(&bp->blk_dva[i]);
                vdev_t *vd = vdev_lookup_top(spa, vdid);
                uint64_t off = DVA_GET_OFFSET(&bp->blk_dva[i]);
                uint64_t size = DVA_GET_ASIZE(&bp->blk_dva[i]);
                metaslab_t *ms = vd->vdev_ms[off >> vd->vdev_ms_shift];

                if (ms->ms_map->sm_loaded)
                        checkmap(ms->ms_map, off, size);

                for (int j = 0; j < TXG_SIZE; j++)
                        checkmap(ms->ms_freemap[j], off, size);
                for (int j = 0; j < TXG_DEFER_SIZE; j++)
                        checkmap(ms->ms_defermap[j], off, size);
        }
        spa_config_exit(spa, SCL_VDEV, FTAG);
}