fs/zfs/metaslab.c

	metaslab.c revision 0a4e9518a44f226be6d39383330b5b1792d2f184
/*
 * 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 2007 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma ident   "%Z%%M% %I% %E% SMI"

#include <sys/zfs_context.h>
#include <sys/spa_impl.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>

uint64_t metaslab_aliquot = 512ULL << 10;

/*
 * ==========================================================================
 * Metaslab classes
 * ==========================================================================
 */
metaslab_class_t *
metaslab_class_create(void)
{
    metaslab_class_t *mc;

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

    mc->mc_rotor = NULL;

    return (mc);
}

void
metaslab_class_destroy(metaslab_class_t *mc)
{
    metaslab_group_t *mg;

    while ((mg = mc->mc_rotor) != NULL) {
        metaslab_class_remove(mc, mg);
        metaslab_group_destroy(mg);
    }

    kmem_free(mc, sizeof (metaslab_class_t));
}

void
metaslab_class_add(metaslab_class_t *mc, metaslab_group_t *mg)
{
    metaslab_group_t *mgprev, *mgnext;

    ASSERT(mg->mg_class == NULL);

    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;
    mg->mg_class = mc;
}

void
metaslab_class_remove(metaslab_class_t *mc, metaslab_group_t *mg)
{
    metaslab_group_t *mgprev, *mgnext;

    ASSERT(mg->mg_class == mc);

    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;
    mg->mg_class = NULL;
}

/*
 * ==========================================================================
 * 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_aliquot = metaslab_aliquot * MAX(1, vd->vdev_children);
    mg->mg_vd = vd;
    metaslab_class_add(mc, mg);

    return (mg);
}

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

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);
}

/*
 * ==========================================================================
 * The first-fit block allocator
 * ==========================================================================
 */
static void
metaslab_ff_load(space_map_t *sm)
{
    ASSERT(sm->sm_ppd == NULL);
    sm->sm_ppd = kmem_zalloc(64 * sizeof (uint64_t), KM_SLEEP);
}

static void
metaslab_ff_unload(space_map_t *sm)
{
    kmem_free(sm->sm_ppd, 64 * sizeof (uint64_t));
    sm->sm_ppd = NULL;
}

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;
    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_ff_alloc(sm, size));
}

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

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

static space_map_ops_t metaslab_ff_ops = {
    metaslab_ff_load,
    metaslab_ff_unload,
    metaslab_ff_alloc,
    metaslab_ff_claim,
    metaslab_ff_free
};

/*
 * ==========================================================================
 * 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.
     */
    space_map_create(&msp->ms_map, start, size,
        vd->vdev_ashift, &msp->ms_lock);

    metaslab_group_add(mg, msp);

    /*
     * 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) {
        /*
         * The vdev is dirty, but the metaslab isn't -- it just needs
         * to have metaslab_sync_done() invoked from vdev_sync_done().
         * [We could just dirty the metaslab, but that would cause us
         * to allocate a space map object for it, which is wasteful
         * and would mess up the locality logic in metaslab_weight().]
         */
        ASSERT(TXG_CLEAN(txg) == spa_last_synced_txg(vd->vdev_spa));
        vdev_dirty(vd, 0, NULL, txg);
        vdev_dirty(vd, VDD_METASLAB, msp, TXG_CLEAN(txg));
    }

    return (msp);
}

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

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

    metaslab_group_remove(mg, msp);

    mutex_enter(&msp->ms_lock);

    space_map_unload(&msp->ms_map);
    space_map_destroy(&msp->ms_map);

    for (t = 0; t < TXG_SIZE; t++) {
        space_map_destroy(&msp->ms_allocmap[t]);
        space_map_destroy(&msp->ms_freemap[t]);
    }

    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)
#define METASLAB_SMO_BONUS_MULTIPLIER   2

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));

    /*
     * 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 we've used before.
     */
    if (smo->smo_object != 0)
        weight *= METASLAB_SMO_BONUS_MULTIPLIER;
    ASSERT(weight >= space &&
        weight <= 2 * METASLAB_SMO_BONUS_MULTIPLIER * space);

    /*
     * 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 int
metaslab_activate(metaslab_t *msp, uint64_t activation_weight)
{
    space_map_t *sm = &msp->ms_map;

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

    if ((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0) {
        int error = space_map_load(sm, &metaslab_ff_ops,
            SM_FREE, &msp->ms_smo,
            msp->ms_group->mg_vd->vdev_spa->spa_meta_objset);
        if (error) {
            metaslab_group_sort(msp->ms_group, msp, 0);
            return (error);
        }
        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);
}

/*
 * 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->spa_meta_objset;
    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;
    int t;

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

    /*
     * 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.
     */
    mutex_enter(&msp->ms_lock);

    if (smo->smo_object == 0) {
        ASSERT(smo->smo_objsize == 0);
        ASSERT(smo->smo_alloc == 0);
        mutex_exit(&msp->ms_lock);
        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);
    }

    space_map_walk(freemap, space_map_add, freed_map);

    if (sm->sm_loaded && spa_sync_pass(spa) == 1 && smo->smo_objsize >=
        2 * sizeof (uint64_t) * avl_numnodes(&sm->sm_root)) {
        /*
         * The in-core space map representation is twice as compact
         * as the on-disk one, so it's time to condense the latter
         * by generating a pure allocmap from first principles.
         *
         * This metaslab is 100% allocated,
         * minus the content of the in-core map (sm),
         * minus what's been freed this txg (freed_map),
         * minus allocations from txgs in the future
         * (because they haven't been committed yet).
         */
        space_map_vacate(allocmap, NULL, NULL);
        space_map_vacate(freemap, NULL, NULL);

        space_map_add(allocmap, allocmap->sm_start, allocmap->sm_size);

        space_map_walk(sm, space_map_remove, allocmap);
        space_map_walk(freed_map, space_map_remove, allocmap);

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

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

    space_map_sync(allocmap, SM_ALLOC, smo, mos, tx);
    space_map_sync(freemap, SM_FREE, smo, mos, tx);

    mutex_exit(&msp->ms_lock);

    VERIFY(0 == 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];
    metaslab_group_t *mg = msp->ms_group;
    vdev_t *vd = mg->mg_vd;
    int t;

    mutex_enter(&msp->ms_lock);

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

    vdev_space_update(vd, 0, smosync->smo_alloc - smo->smo_alloc);

    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.
     * Then, add everything we freed in this txg to the map.
     */
    space_map_load_wait(sm);
    space_map_vacate(freed_map, sm->sm_loaded ? space_map_free : NULL, sm);

    *smo = *smosync;

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

        if (evictable)
            space_map_unload(sm);
    }

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

    mutex_exit(&msp->ms_lock);
}

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 size, uint64_t txg,
    uint64_t min_distance, dva_t *dva, int d)
{
    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;

    for (;;) {
        mutex_enter(&mg->mg_lock);
        for (msp = avl_first(t); msp; msp = AVL_NEXT(t, msp)) {
            if (msp->ms_weight < size) {
                mutex_exit(&mg->mg_lock);
                return (-1ULL);
            }

            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);

        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 < size) {
            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 ((offset = space_map_alloc(&msp->ms_map, size)) != -1ULL)
            break;

        metaslab_passivate(msp, size - 1);

        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, size);

    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, boolean_t hintdva_avoid)
{
    metaslab_group_t *mg, *rotor;
    vdev_t *vd;
    int dshift = 3;
    int all_zero;
    uint64_t offset = -1ULL;
    uint64_t asize;
    uint64_t distance;

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

    /*
     * Start at the rotor and loop through all mgs until we find something.
     * Note that there's no locking on mc_rotor or mc_allocated 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]));
        if (hintdva_avoid)
            mg = vd->vdev_mg->mg_next;
        else
            mg = vd->vdev_mg;
    } 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 class, just follow the rotor.
     */
    if (mg->mg_class != mc)
        mg = mc->mc_rotor;

    rotor = mg;
top:
    all_zero = B_TRUE;
    do {
        vd = mg->mg_vd;
        /*
         * Dont allocate from faulted devices
         */
        if (!vdev_writeable(vd))
            goto next;
        /*
         * Avoid writing single-copy data to a failing vdev
         */
        if ((vd->vdev_stat.vs_write_errors > 0 ||
            vd->vdev_state < VDEV_STATE_HEALTHY) &&
            d == 0 && dshift == 3) {
            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, asize, txg, distance, dva, d);
        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_allocated == 0) {
                vdev_stat_t *vs = &vd->vdev_stat;
                uint64_t alloc, space;
                int64_t vu, su;

                alloc = spa_get_alloc(spa);
                space = spa_get_space(spa);

                /*
                 * Determine percent used in units of 0..1024.
                 * (This is just to avoid floating point.)
                 */
                vu = (vs->vs_alloc << 10) / (vs->vs_space + 1);
                su = (alloc << 10) / (space + 1);

                /*
                 * Bias by at most +/- 25% of the aliquot.
                 */
                mg->mg_bias = ((su - vu) *
                    (int64_t)mg->mg_aliquot) / (1024 * 4);
            }

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

            DVA_SET_VDEV(&dva[d], vd->vdev_id);
            DVA_SET_OFFSET(&dva[d], offset);
            DVA_SET_GANG(&dva[d], 0);
            DVA_SET_ASIZE(&dva[d], asize);

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

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

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

    return (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);

        /*
         * verify that this region is actually allocated in
         * either a ms_allocmap or the ms_map
         */
        if (msp->ms_map.sm_loaded) {
            boolean_t allocd = B_FALSE;
            int i;

            if (!space_map_contains(&msp->ms_map, offset, size)) {
                allocd = B_TRUE;
            } else {
                for (i = 0; i < TXG_CONCURRENT_STATES; i++) {
                    space_map_t *sm = &msp->ms_allocmap
                        [(txg - i) & TXG_MASK];
                    if (space_map_contains(sm,
                        offset, size)) {
                        allocd = B_TRUE;
                        break;
                    }
                }
            }

            if (!allocd) {
                zfs_panic_recover("freeing free segment "
                    "(vdev=%llu offset=%llx size=%llx)",
                    (longlong_t)vdev, (longlong_t)offset,
                    (longlong_t)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;

    ASSERT(DVA_IS_VALID(dva));

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

    error = metaslab_activate(msp, METASLAB_WEIGHT_SECONDARY);
    if (error) {
        mutex_exit(&msp->ms_lock);
        return (error);
    }

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

    space_map_claim(&msp->ms_map, offset, size);
    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, boolean_t hintbp_avoid)
{
    dva_t *dva = bp->blk_dva;
    dva_t *hintdva = hintbp->blk_dva;
    int d;
    int error = 0;

    if (mc->mc_rotor == NULL)   /* no vdevs in this class */
        return (ENOSPC);

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

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

    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);
    int d;

    ASSERT(!BP_IS_HOLE(bp));

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

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 d, error;
    int last_error = 0;

    ASSERT(!BP_IS_HOLE(bp));

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

    return (last_error);
}