[mirror_zfs-debian.git] / module / zfs / spa_stats.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
 */

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

/*
 * Keeps stats on last N reads per spa_t, disabled by default.
 */
int zfs_read_history = 0;

/*
 * Include cache hits in history, disabled by default.
 */
int zfs_read_history_hits = 0;

/*
 * Keeps stats on the last N txgs, disabled by default.
 */
int zfs_txg_history = 0;

/*
 * Keeps stats on the last N MMP updates, disabled by default.
 */
int zfs_multihost_history = 0;

/*
 * ==========================================================================
 * SPA Read History Routines
 * ==========================================================================
 */

/*
 * Read statistics - Information exported regarding each arc_read call
 */
typedef struct spa_read_history {
        uint64_t        uid;            /* unique identifier */
        hrtime_t        start;          /* time read completed */
        uint64_t        objset;         /* read from this objset */
        uint64_t        object;         /* read of this object number */
        uint64_t        level;          /* block's indirection level */
        uint64_t        blkid;          /* read of this block id */
        char            origin[24];     /* read originated from here */
        uint32_t        aflags;         /* ARC flags (cached, prefetch, etc.) */
        pid_t           pid;            /* PID of task doing read */
        char            comm[16];       /* process name of task doing read */
        list_node_t     srh_link;
} spa_read_history_t;

static int
spa_read_history_headers(char *buf, size_t size)
{
        (void) snprintf(buf, size, "%-8s %-16s %-8s %-8s %-8s %-8s %-8s "
            "%-24s %-8s %-16s\n", "UID", "start", "objset", "object",
            "level", "blkid", "aflags", "origin", "pid", "process");

        return (0);
}

static int
spa_read_history_data(char *buf, size_t size, void *data)
{
        spa_read_history_t *srh = (spa_read_history_t *)data;

        (void) snprintf(buf, size, "%-8llu %-16llu 0x%-6llx "
            "%-8lli %-8lli %-8lli 0x%-6x %-24s %-8i %-16s\n",
            (u_longlong_t)srh->uid, srh->start,
            (longlong_t)srh->objset, (longlong_t)srh->object,
            (longlong_t)srh->level, (longlong_t)srh->blkid,
            srh->aflags, srh->origin, srh->pid, srh->comm);

        return (0);
}

/*
 * Calculate the address for the next spa_stats_history_t entry.  The
 * ssh->lock will be held until ksp->ks_ndata entries are processed.
 */
static void *
spa_read_history_addr(kstat_t *ksp, loff_t n)
{
        spa_t *spa = ksp->ks_private;
        spa_stats_history_t *ssh = &spa->spa_stats.read_history;

        ASSERT(MUTEX_HELD(&ssh->lock));

        if (n == 0)
                ssh->private = list_tail(&ssh->list);
        else if (ssh->private)
                ssh->private = list_prev(&ssh->list, ssh->private);

        return (ssh->private);
}

/*
 * When the kstat is written discard all spa_read_history_t entries.  The
 * ssh->lock will be held until ksp->ks_ndata entries are processed.
 */
static int
spa_read_history_update(kstat_t *ksp, int rw)
{
        spa_t *spa = ksp->ks_private;
        spa_stats_history_t *ssh = &spa->spa_stats.read_history;

        if (rw == KSTAT_WRITE) {
                spa_read_history_t *srh;

                while ((srh = list_remove_head(&ssh->list))) {
                        ssh->size--;
                        kmem_free(srh, sizeof (spa_read_history_t));
                }

                ASSERT3U(ssh->size, ==, 0);
        }

        ksp->ks_ndata = ssh->size;
        ksp->ks_data_size = ssh->size * sizeof (spa_read_history_t);

        return (0);
}

static void
spa_read_history_init(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.read_history;
        char name[KSTAT_STRLEN];
        kstat_t *ksp;

        mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
        list_create(&ssh->list, sizeof (spa_read_history_t),
            offsetof(spa_read_history_t, srh_link));

        ssh->count = 0;
        ssh->size = 0;
        ssh->private = NULL;

        (void) snprintf(name, KSTAT_STRLEN, "zfs/%s", spa_name(spa));

        ksp = kstat_create(name, 0, "reads", "misc",
            KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
        ssh->kstat = ksp;

        if (ksp) {
                ksp->ks_lock = &ssh->lock;
                ksp->ks_data = NULL;
                ksp->ks_private = spa;
                ksp->ks_update = spa_read_history_update;
                kstat_set_raw_ops(ksp, spa_read_history_headers,
                    spa_read_history_data, spa_read_history_addr);
                kstat_install(ksp);
        }
}

static void
spa_read_history_destroy(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.read_history;
        spa_read_history_t *srh;
        kstat_t *ksp;

        ksp = ssh->kstat;
        if (ksp)
                kstat_delete(ksp);

        mutex_enter(&ssh->lock);
        while ((srh = list_remove_head(&ssh->list))) {
                ssh->size--;
                kmem_free(srh, sizeof (spa_read_history_t));
        }

        ASSERT3U(ssh->size, ==, 0);
        list_destroy(&ssh->list);
        mutex_exit(&ssh->lock);

        mutex_destroy(&ssh->lock);
}

void
spa_read_history_add(spa_t *spa, const zbookmark_phys_t *zb, uint32_t aflags)
{
        spa_stats_history_t *ssh = &spa->spa_stats.read_history;
        spa_read_history_t *srh, *rm;

        ASSERT3P(spa, !=, NULL);
        ASSERT3P(zb,  !=, NULL);

        if (zfs_read_history == 0 && ssh->size == 0)
                return;

        if (zfs_read_history_hits == 0 && (aflags & ARC_FLAG_CACHED))
                return;

        srh = kmem_zalloc(sizeof (spa_read_history_t), KM_SLEEP);
        strlcpy(srh->comm, getcomm(), sizeof (srh->comm));
        srh->start  = gethrtime();
        srh->objset = zb->zb_objset;
        srh->object = zb->zb_object;
        srh->level  = zb->zb_level;
        srh->blkid  = zb->zb_blkid;
        srh->aflags = aflags;
        srh->pid    = getpid();

        mutex_enter(&ssh->lock);

        srh->uid = ssh->count++;
        list_insert_head(&ssh->list, srh);
        ssh->size++;

        while (ssh->size > zfs_read_history) {
                ssh->size--;
                rm = list_remove_tail(&ssh->list);
                kmem_free(rm, sizeof (spa_read_history_t));
        }

        mutex_exit(&ssh->lock);
}

/*
 * ==========================================================================
 * SPA TXG History Routines
 * ==========================================================================
 */

/*
 * Txg statistics - Information exported regarding each txg sync
 */

typedef struct spa_txg_history {
        uint64_t        txg;            /* txg id */
        txg_state_t     state;          /* active txg state */
        uint64_t        nread;          /* number of bytes read */
        uint64_t        nwritten;       /* number of bytes written */
        uint64_t        reads;          /* number of read operations */
        uint64_t        writes;         /* number of write operations */
        uint64_t        ndirty;         /* number of dirty bytes */
        hrtime_t        times[TXG_STATE_COMMITTED]; /* completion times */
        list_node_t     sth_link;
} spa_txg_history_t;

static int
spa_txg_history_headers(char *buf, size_t size)
{
        (void) snprintf(buf, size, "%-8s %-16s %-5s %-12s %-12s %-12s "
            "%-8s %-8s %-12s %-12s %-12s %-12s\n", "txg", "birth", "state",
            "ndirty", "nread", "nwritten", "reads", "writes",
            "otime", "qtime", "wtime", "stime");

        return (0);
}

static int
spa_txg_history_data(char *buf, size_t size, void *data)
{
        spa_txg_history_t *sth = (spa_txg_history_t *)data;
        uint64_t open = 0, quiesce = 0, wait = 0, sync = 0;
        char state;

        switch (sth->state) {
                case TXG_STATE_BIRTH:           state = 'B';    break;
                case TXG_STATE_OPEN:            state = 'O';    break;
                case TXG_STATE_QUIESCED:        state = 'Q';    break;
                case TXG_STATE_WAIT_FOR_SYNC:   state = 'W';    break;
                case TXG_STATE_SYNCED:          state = 'S';    break;
                case TXG_STATE_COMMITTED:       state = 'C';    break;
                default:                        state = '?';    break;
        }

        if (sth->times[TXG_STATE_OPEN])
                open = sth->times[TXG_STATE_OPEN] -
                    sth->times[TXG_STATE_BIRTH];

        if (sth->times[TXG_STATE_QUIESCED])
                quiesce = sth->times[TXG_STATE_QUIESCED] -
                    sth->times[TXG_STATE_OPEN];

        if (sth->times[TXG_STATE_WAIT_FOR_SYNC])
                wait = sth->times[TXG_STATE_WAIT_FOR_SYNC] -
                    sth->times[TXG_STATE_QUIESCED];

        if (sth->times[TXG_STATE_SYNCED])
                sync = sth->times[TXG_STATE_SYNCED] -
                    sth->times[TXG_STATE_WAIT_FOR_SYNC];

        (void) snprintf(buf, size, "%-8llu %-16llu %-5c %-12llu "
            "%-12llu %-12llu %-8llu %-8llu %-12llu %-12llu %-12llu %-12llu\n",
            (longlong_t)sth->txg, sth->times[TXG_STATE_BIRTH], state,
            (u_longlong_t)sth->ndirty,
            (u_longlong_t)sth->nread, (u_longlong_t)sth->nwritten,
            (u_longlong_t)sth->reads, (u_longlong_t)sth->writes,
            (u_longlong_t)open, (u_longlong_t)quiesce, (u_longlong_t)wait,
            (u_longlong_t)sync);

        return (0);
}

/*
 * Calculate the address for the next spa_stats_history_t entry.  The
 * ssh->lock will be held until ksp->ks_ndata entries are processed.
 */
static void *
spa_txg_history_addr(kstat_t *ksp, loff_t n)
{
        spa_t *spa = ksp->ks_private;
        spa_stats_history_t *ssh = &spa->spa_stats.txg_history;

        ASSERT(MUTEX_HELD(&ssh->lock));

        if (n == 0)
                ssh->private = list_tail(&ssh->list);
        else if (ssh->private)
                ssh->private = list_prev(&ssh->list, ssh->private);

        return (ssh->private);
}

/*
 * When the kstat is written discard all spa_txg_history_t entries.  The
 * ssh->lock will be held until ksp->ks_ndata entries are processed.
 */
static int
spa_txg_history_update(kstat_t *ksp, int rw)
{
        spa_t *spa = ksp->ks_private;
        spa_stats_history_t *ssh = &spa->spa_stats.txg_history;

        ASSERT(MUTEX_HELD(&ssh->lock));

        if (rw == KSTAT_WRITE) {
                spa_txg_history_t *sth;

                while ((sth = list_remove_head(&ssh->list))) {
                        ssh->size--;
                        kmem_free(sth, sizeof (spa_txg_history_t));
                }

                ASSERT3U(ssh->size, ==, 0);
        }

        ksp->ks_ndata = ssh->size;
        ksp->ks_data_size = ssh->size * sizeof (spa_txg_history_t);

        return (0);
}

static void
spa_txg_history_init(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
        char name[KSTAT_STRLEN];
        kstat_t *ksp;

        mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
        list_create(&ssh->list, sizeof (spa_txg_history_t),
            offsetof(spa_txg_history_t, sth_link));

        ssh->count = 0;
        ssh->size = 0;
        ssh->private = NULL;

        (void) snprintf(name, KSTAT_STRLEN, "zfs/%s", spa_name(spa));

        ksp = kstat_create(name, 0, "txgs", "misc",
            KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
        ssh->kstat = ksp;

        if (ksp) {
                ksp->ks_lock = &ssh->lock;
                ksp->ks_data = NULL;
                ksp->ks_private = spa;
                ksp->ks_update = spa_txg_history_update;
                kstat_set_raw_ops(ksp, spa_txg_history_headers,
                    spa_txg_history_data, spa_txg_history_addr);
                kstat_install(ksp);
        }
}

static void
spa_txg_history_destroy(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
        spa_txg_history_t *sth;
        kstat_t *ksp;

        ksp = ssh->kstat;
        if (ksp)
                kstat_delete(ksp);

        mutex_enter(&ssh->lock);
        while ((sth = list_remove_head(&ssh->list))) {
                ssh->size--;
                kmem_free(sth, sizeof (spa_txg_history_t));
        }

        ASSERT3U(ssh->size, ==, 0);
        list_destroy(&ssh->list);
        mutex_exit(&ssh->lock);

        mutex_destroy(&ssh->lock);
}

/*
 * Add a new txg to historical record.
 */
void
spa_txg_history_add(spa_t *spa, uint64_t txg, hrtime_t birth_time)
{
        spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
        spa_txg_history_t *sth, *rm;

        if (zfs_txg_history == 0 && ssh->size == 0)
                return;

        sth = kmem_zalloc(sizeof (spa_txg_history_t), KM_SLEEP);
        sth->txg = txg;
        sth->state = TXG_STATE_OPEN;
        sth->times[TXG_STATE_BIRTH] = birth_time;

        mutex_enter(&ssh->lock);

        list_insert_head(&ssh->list, sth);
        ssh->size++;

        while (ssh->size > zfs_txg_history) {
                ssh->size--;
                rm = list_remove_tail(&ssh->list);
                kmem_free(rm, sizeof (spa_txg_history_t));
        }

        mutex_exit(&ssh->lock);
}

/*
 * Set txg state completion time and increment current state.
 */
int
spa_txg_history_set(spa_t *spa, uint64_t txg, txg_state_t completed_state,
    hrtime_t completed_time)
{
        spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
        spa_txg_history_t *sth;
        int error = ENOENT;

        if (zfs_txg_history == 0)
                return (0);

        mutex_enter(&ssh->lock);
        for (sth = list_head(&ssh->list); sth != NULL;
            sth = list_next(&ssh->list, sth)) {
                if (sth->txg == txg) {
                        sth->times[completed_state] = completed_time;
                        sth->state++;
                        error = 0;
                        break;
                }
        }
        mutex_exit(&ssh->lock);

        return (error);
}

/*
 * Set txg IO stats.
 */
static int
spa_txg_history_set_io(spa_t *spa, uint64_t txg, uint64_t nread,
    uint64_t nwritten, uint64_t reads, uint64_t writes, uint64_t ndirty)
{
        spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
        spa_txg_history_t *sth;
        int error = ENOENT;

        if (zfs_txg_history == 0)
                return (0);

        mutex_enter(&ssh->lock);
        for (sth = list_head(&ssh->list); sth != NULL;
            sth = list_next(&ssh->list, sth)) {
                if (sth->txg == txg) {
                        sth->nread = nread;
                        sth->nwritten = nwritten;
                        sth->reads = reads;
                        sth->writes = writes;
                        sth->ndirty = ndirty;
                        error = 0;
                        break;
                }
        }
        mutex_exit(&ssh->lock);

        return (error);
}

txg_stat_t *
spa_txg_history_init_io(spa_t *spa, uint64_t txg, dsl_pool_t *dp)
{
        txg_stat_t *ts;

        if (zfs_txg_history == 0)
                return (NULL);

        ts = kmem_alloc(sizeof (txg_stat_t), KM_SLEEP);

        spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
        vdev_get_stats(spa->spa_root_vdev, &ts->vs1);
        spa_config_exit(spa, SCL_ALL, FTAG);

        ts->txg = txg;
        ts->ndirty = dp->dp_dirty_pertxg[txg & TXG_MASK];

        spa_txg_history_set(spa, txg, TXG_STATE_WAIT_FOR_SYNC, gethrtime());

        return (ts);
}

void
spa_txg_history_fini_io(spa_t *spa, txg_stat_t *ts)
{
        if (ts == NULL)
                return;

        if (zfs_txg_history == 0) {
                kmem_free(ts, sizeof (txg_stat_t));
                return;
        }

        spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
        vdev_get_stats(spa->spa_root_vdev, &ts->vs2);
        spa_config_exit(spa, SCL_ALL, FTAG);

        spa_txg_history_set(spa, ts->txg, TXG_STATE_SYNCED, gethrtime());
        spa_txg_history_set_io(spa, ts->txg,
            ts->vs2.vs_bytes[ZIO_TYPE_READ] - ts->vs1.vs_bytes[ZIO_TYPE_READ],
            ts->vs2.vs_bytes[ZIO_TYPE_WRITE] - ts->vs1.vs_bytes[ZIO_TYPE_WRITE],
            ts->vs2.vs_ops[ZIO_TYPE_READ] - ts->vs1.vs_ops[ZIO_TYPE_READ],
            ts->vs2.vs_ops[ZIO_TYPE_WRITE] - ts->vs1.vs_ops[ZIO_TYPE_WRITE],
            ts->ndirty);

        kmem_free(ts, sizeof (txg_stat_t));
}

/*
 * ==========================================================================
 * SPA TX Assign Histogram Routines
 * ==========================================================================
 */

/*
 * Tx statistics - Information exported regarding dmu_tx_assign time.
 */

/*
 * When the kstat is written zero all buckets.  When the kstat is read
 * count the number of trailing buckets set to zero and update ks_ndata
 * such that they are not output.
 */
static int
spa_tx_assign_update(kstat_t *ksp, int rw)
{
        spa_t *spa = ksp->ks_private;
        spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
        int i;

        if (rw == KSTAT_WRITE) {
                for (i = 0; i < ssh->count; i++)
                        ((kstat_named_t *)ssh->private)[i].value.ui64 = 0;
        }

        for (i = ssh->count; i > 0; i--)
                if (((kstat_named_t *)ssh->private)[i-1].value.ui64 != 0)
                        break;

        ksp->ks_ndata = i;
        ksp->ks_data_size = i * sizeof (kstat_named_t);

        return (0);
}

static void
spa_tx_assign_init(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
        char name[KSTAT_STRLEN];
        kstat_named_t *ks;
        kstat_t *ksp;
        int i;

        mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);

        ssh->count = 42; /* power of two buckets for 1ns to 2,199s */
        ssh->size = ssh->count * sizeof (kstat_named_t);
        ssh->private = kmem_alloc(ssh->size, KM_SLEEP);

        (void) snprintf(name, KSTAT_STRLEN, "zfs/%s", spa_name(spa));

        for (i = 0; i < ssh->count; i++) {
                ks = &((kstat_named_t *)ssh->private)[i];
                ks->data_type = KSTAT_DATA_UINT64;
                ks->value.ui64 = 0;
                (void) snprintf(ks->name, KSTAT_STRLEN, "%llu ns",
                    (u_longlong_t)1 << i);
        }

        ksp = kstat_create(name, 0, "dmu_tx_assign", "misc",
            KSTAT_TYPE_NAMED, 0, KSTAT_FLAG_VIRTUAL);
        ssh->kstat = ksp;

        if (ksp) {
                ksp->ks_lock = &ssh->lock;
                ksp->ks_data = ssh->private;
                ksp->ks_ndata = ssh->count;
                ksp->ks_data_size = ssh->size;
                ksp->ks_private = spa;
                ksp->ks_update = spa_tx_assign_update;
                kstat_install(ksp);
        }
}

static void
spa_tx_assign_destroy(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
        kstat_t *ksp;

        ksp = ssh->kstat;
        if (ksp)
                kstat_delete(ksp);

        kmem_free(ssh->private, ssh->size);
        mutex_destroy(&ssh->lock);
}

void
spa_tx_assign_add_nsecs(spa_t *spa, uint64_t nsecs)
{
        spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
        uint64_t idx = 0;

        while (((1ULL << idx) < nsecs) && (idx < ssh->size - 1))
                idx++;

        atomic_inc_64(&((kstat_named_t *)ssh->private)[idx].value.ui64);
}

/*
 * ==========================================================================
 * SPA IO History Routines
 * ==========================================================================
 */
static int
spa_io_history_update(kstat_t *ksp, int rw)
{
        if (rw == KSTAT_WRITE)
                memset(ksp->ks_data, 0, ksp->ks_data_size);

        return (0);
}

static void
spa_io_history_init(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.io_history;
        char name[KSTAT_STRLEN];
        kstat_t *ksp;

        mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);

        (void) snprintf(name, KSTAT_STRLEN, "zfs/%s", spa_name(spa));

        ksp = kstat_create(name, 0, "io", "disk", KSTAT_TYPE_IO, 1, 0);
        ssh->kstat = ksp;

        if (ksp) {
                ksp->ks_lock = &ssh->lock;
                ksp->ks_private = spa;
                ksp->ks_update = spa_io_history_update;
                kstat_install(ksp);
        }
}

static void
spa_io_history_destroy(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.io_history;

        if (ssh->kstat)
                kstat_delete(ssh->kstat);

        mutex_destroy(&ssh->lock);
}

/*
 * ==========================================================================
 * SPA MMP History Routines
 * ==========================================================================
 */

/*
 * MMP statistics - Information exported regarding each MMP update
 */

typedef struct spa_mmp_history {
        uint64_t        txg;            /* txg of last sync */
        uint64_t        timestamp;      /* UTC time of of last sync */
        uint64_t        mmp_delay;      /* nanosec since last MMP write */
        uint64_t        vdev_guid;      /* unique ID of leaf vdev */
        char            *vdev_path;
        uint64_t        vdev_label;     /* vdev label */
        list_node_t     smh_link;
} spa_mmp_history_t;

static int
spa_mmp_history_headers(char *buf, size_t size)
{
        (void) snprintf(buf, size, "%-10s %-10s %-12s %-24s %-10s %s\n",
            "txg", "timestamp", "mmp_delay", "vdev_guid", "vdev_label",
            "vdev_path");
        return (0);
}

static int
spa_mmp_history_data(char *buf, size_t size, void *data)
{
        spa_mmp_history_t *smh = (spa_mmp_history_t *)data;

        (void) snprintf(buf, size, "%-10llu %-10llu %-12llu %-24llu %-10llu "
            "%s\n",
            (u_longlong_t)smh->txg, (u_longlong_t)smh->timestamp,
            (u_longlong_t)smh->mmp_delay, (u_longlong_t)smh->vdev_guid,
            (u_longlong_t)smh->vdev_label,
            (smh->vdev_path ? smh->vdev_path : "-"));

        return (0);
}

/*
 * Calculate the address for the next spa_stats_history_t entry.  The
 * ssh->lock will be held until ksp->ks_ndata entries are processed.
 */
static void *
spa_mmp_history_addr(kstat_t *ksp, loff_t n)
{
        spa_t *spa = ksp->ks_private;
        spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;

        ASSERT(MUTEX_HELD(&ssh->lock));

        if (n == 0)
                ssh->private = list_tail(&ssh->list);
        else if (ssh->private)
                ssh->private = list_prev(&ssh->list, ssh->private);

        return (ssh->private);
}

/*
 * When the kstat is written discard all spa_mmp_history_t entries.  The
 * ssh->lock will be held until ksp->ks_ndata entries are processed.
 */
static int
spa_mmp_history_update(kstat_t *ksp, int rw)
{
        spa_t *spa = ksp->ks_private;
        spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;

        ASSERT(MUTEX_HELD(&ssh->lock));

        if (rw == KSTAT_WRITE) {
                spa_mmp_history_t *smh;

                while ((smh = list_remove_head(&ssh->list))) {
                        ssh->size--;
                        if (smh->vdev_path)
                                strfree(smh->vdev_path);
                        kmem_free(smh, sizeof (spa_mmp_history_t));
                }

                ASSERT3U(ssh->size, ==, 0);
        }

        ksp->ks_ndata = ssh->size;
        ksp->ks_data_size = ssh->size * sizeof (spa_mmp_history_t);

        return (0);
}

static void
spa_mmp_history_init(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
        char name[KSTAT_STRLEN];
        kstat_t *ksp;

        mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
        list_create(&ssh->list, sizeof (spa_mmp_history_t),
            offsetof(spa_mmp_history_t, smh_link));

        ssh->count = 0;
        ssh->size = 0;
        ssh->private = NULL;

        (void) snprintf(name, KSTAT_STRLEN, "zfs/%s", spa_name(spa));

        ksp = kstat_create(name, 0, "multihost", "misc",
            KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
        ssh->kstat = ksp;

        if (ksp) {
                ksp->ks_lock = &ssh->lock;
                ksp->ks_data = NULL;
                ksp->ks_private = spa;
                ksp->ks_update = spa_mmp_history_update;
                kstat_set_raw_ops(ksp, spa_mmp_history_headers,
                    spa_mmp_history_data, spa_mmp_history_addr);
                kstat_install(ksp);
        }
}

static void
spa_mmp_history_destroy(spa_t *spa)
{
        spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
        spa_mmp_history_t *smh;
        kstat_t *ksp;

        ksp = ssh->kstat;
        if (ksp)
                kstat_delete(ksp);

        mutex_enter(&ssh->lock);
        while ((smh = list_remove_head(&ssh->list))) {
                ssh->size--;
                if (smh->vdev_path)
                        strfree(smh->vdev_path);
                kmem_free(smh, sizeof (spa_mmp_history_t));
        }

        ASSERT3U(ssh->size, ==, 0);
        list_destroy(&ssh->list);
        mutex_exit(&ssh->lock);

        mutex_destroy(&ssh->lock);
}

/*
 * Add a new MMP update to historical record.
 */
void
spa_mmp_history_add(uint64_t txg, uint64_t timestamp, uint64_t mmp_delay,
    vdev_t *vd, int label)
{
        spa_t *spa = vd->vdev_spa;
        spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
        spa_mmp_history_t *smh, *rm;

        if (zfs_multihost_history == 0 && ssh->size == 0)
                return;

        smh = kmem_zalloc(sizeof (spa_mmp_history_t), KM_SLEEP);
        smh->txg = txg;
        smh->timestamp = timestamp;
        smh->mmp_delay = mmp_delay;
        smh->vdev_guid = vd->vdev_guid;
        if (vd->vdev_path)
                smh->vdev_path = strdup(vd->vdev_path);
        smh->vdev_label = label;

        mutex_enter(&ssh->lock);

        list_insert_head(&ssh->list, smh);
        ssh->size++;

        while (ssh->size > zfs_multihost_history) {
                ssh->size--;
                rm = list_remove_tail(&ssh->list);
                if (rm->vdev_path)
                        strfree(rm->vdev_path);
                kmem_free(rm, sizeof (spa_mmp_history_t));
        }

        mutex_exit(&ssh->lock);
}

void
spa_stats_init(spa_t *spa)
{
        spa_read_history_init(spa);
        spa_txg_history_init(spa);
        spa_tx_assign_init(spa);
        spa_io_history_init(spa);
        spa_mmp_history_init(spa);
}

void
spa_stats_destroy(spa_t *spa)
{
        spa_tx_assign_destroy(spa);
        spa_txg_history_destroy(spa);
        spa_read_history_destroy(spa);
        spa_io_history_destroy(spa);
        spa_mmp_history_destroy(spa);
}

#if defined(_KERNEL) && defined(HAVE_SPL)
/* CSTYLED */
module_param(zfs_read_history, int, 0644);
MODULE_PARM_DESC(zfs_read_history,
        "Historical statistics for the last N reads");

module_param(zfs_read_history_hits, int, 0644);
MODULE_PARM_DESC(zfs_read_history_hits,
        "Include cache hits in read history");

module_param(zfs_txg_history, int, 0644);
MODULE_PARM_DESC(zfs_txg_history,
        "Historical statistics for the last N txgs");

module_param(zfs_multihost_history, int, 0644);
MODULE_PARM_DESC(zfs_multihost_history,
        "Historical statistics for last N multihost writes");
/* END CSTYLED */
#endif