MFV r354378,r354379,r354386: 10499 Multi-modifier protection (MMP)

[FreeBSD/FreeBSD.git] / sys / cddl / contrib / opensolaris / uts / common / fs / zfs / mmp.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) 2017 by Lawrence Livermore National Security, LLC.
 * Copyright 2019 Joyent, Inc.
 */

#include <sys/abd.h>
#include <sys/mmp.h>
#include <sys/spa.h>
#include <sys/spa_impl.h>
#include <sys/time.h>
#include <sys/vdev.h>
#include <sys/vdev_impl.h>
#include <sys/zfs_context.h>
#include <sys/callb.h>

/*
 * Multi-Modifier Protection (MMP) attempts to prevent a user from importing
 * or opening a pool on more than one host at a time.  In particular, it
 * prevents "zpool import -f" on a host from succeeding while the pool is
 * already imported on another host.  There are many other ways in which a
 * device could be used by two hosts for different purposes at the same time
 * resulting in pool damage.  This implementation does not attempt to detect
 * those cases.
 *
 * MMP operates by ensuring there are frequent visible changes on disk (a
 * "heartbeat") at all times.  And by altering the import process to check
 * for these changes and failing the import when they are detected.  This
 * functionality is enabled by setting the 'multihost' pool property to on.
 *
 * Uberblocks written by the txg_sync thread always go into the first
 * (N-MMP_BLOCKS_PER_LABEL) slots, the remaining slots are reserved for MMP.
 * They are used to hold uberblocks which are exactly the same as the last
 * synced uberblock except that the ub_timestamp and mmp_config are frequently
 * updated.  Like all other uberblocks, the slot is written with an embedded
 * checksum, and slots with invalid checksums are ignored.  This provides the
 * "heartbeat", with no risk of overwriting good uberblocks that must be
 * preserved, e.g. previous txgs and associated block pointers.
 *
 * Three optional fields are added to uberblock structure; ub_mmp_magic,
 * ub_mmp_config, and ub_mmp_delay.  The ub_mmp_magic value allows zfs to tell
 * whether the other ub_mmp_* fields are valid.  The ub_mmp_config field tells
 * the importing host the settings of zfs_multihost_interval and
 * zfs_multihost_fail_intervals on the host which last had (or currently has)
 * the pool imported.  These determine how long a host must wait to detect
 * activity in the pool, before concluding the pool is not in use.  The
 * mmp_delay field is a decaying average of the amount of time between
 * completion of successive MMP writes, in nanoseconds.  It indicates whether
 * MMP is enabled.
 *
 * During import an activity test may now be performed to determine if
 * the pool is in use.  The activity test is typically required if the
 * ZPOOL_CONFIG_HOSTID does not match the system hostid, the pool state is
 * POOL_STATE_ACTIVE, and the pool is not a root pool.
 *
 * The activity test finds the "best" uberblock (highest txg, timestamp, and, if
 * ub_mmp_magic is valid, sequence number from ub_mmp_config).  It then waits
 * some time, and finds the "best" uberblock again.  If any of the mentioned
 * fields have different values in the newly read uberblock, the pool is in use
 * by another host and the import fails.  In order to assure the accuracy of the
 * activity test, the default values result in an activity test duration of 20x
 * the mmp write interval.
 *
 * The duration of the "zpool import" activity test depends on the information
 * available in the "best" uberblock:
 *
 * 1) If uberblock was written by zfs-0.8 or newer and fail_intervals > 0:
 *    ub_mmp_config.fail_intervals * ub_mmp_config.multihost_interval * 2
 *
 *    In this case, a weak guarantee is provided.  Since the host which last had
 *    the pool imported will suspend the pool if no mmp writes land within
 *    fail_intervals * multihost_interval ms, the absence of writes during that
 *    time means either the pool is not imported, or it is imported but the pool
 *    is suspended and no further writes will occur.
 *
 *    Note that resuming the suspended pool on the remote host would invalidate
 *    this guarantee, and so it is not allowed.
 *
 *    The factor of 2 provides a conservative safety factor and derives from
 *    MMP_IMPORT_SAFETY_FACTOR;
 *
 * 2) If uberblock was written by zfs-0.8 or newer and fail_intervals == 0:
 *    (ub_mmp_config.multihost_interval + ub_mmp_delay) *
 *        zfs_multihost_import_intervals
 *
 *    In this case no guarantee can provided.  However, as long as some devices
 *    are healthy and connected, it is likely that at least one write will land
 *    within (multihost_interval + mmp_delay) because multihost_interval is
 *    enough time for a write to be attempted to each leaf vdev, and mmp_delay
 *    is enough for one to land, based on past delays.  Multiplying by
 *    zfs_multihost_import_intervals provides a conservative safety factor.
 *
 * 3) If uberblock was written by zfs-0.7:
 *    (zfs_multihost_interval + ub_mmp_delay) * zfs_multihost_import_intervals
 *
 *    The same logic as case #2 applies, but we do not know remote tunables.
 *
 *    We use the local value for zfs_multihost_interval because the original MMP
 *    did not record this value in the uberblock.
 *
 *    ub_mmp_delay >= (zfs_multihost_interval / leaves), so if the other host
 *    has a much larger zfs_multihost_interval set, ub_mmp_delay will reflect
 *    that.  We will have waited enough time for zfs_multihost_import_intervals
 *    writes to be issued and all but one to land.
 *
 *    single device pool example delays
 *
 *    import_delay = (1 + 1) * 20   =  40s #defaults, no I/O delay
 *    import_delay = (1 + 10) * 20  = 220s #defaults, 10s I/O delay
 *    import_delay = (10 + 10) * 20 = 400s #10s multihost_interval,
 *                                          no I/O delay
 *    100 device pool example delays
 *
 *    import_delay = (1 + .01) * 20 =  20s #defaults, no I/O delay
 *    import_delay = (1 + 10) * 20  = 220s #defaults, 10s I/O delay
 *    import_delay = (10 + .1) * 20 = 202s #10s multihost_interval,
 *                                          no I/O delay
 *
 * 4) Otherwise, this uberblock was written by a pre-MMP zfs:
 *    zfs_multihost_import_intervals * zfs_multihost_interval
 *
 *    In this case local tunables are used.  By default this product = 10s, long
 *    enough for a pool with any activity at all to write at least one
 *    uberblock.  No guarantee can be provided.
 *
 * Additionally, the duration is then extended by a random 25% to attempt to to
 * detect simultaneous imports.  For example, if both partner hosts are rebooted
 * at the same time and automatically attempt to import the pool.
 */

/*
 * Used to control the frequency of mmp writes which are performed when the
 * 'multihost' pool property is on.  This is one factor used to determine the
 * length of the activity check during import.
 *
 * On average an mmp write will be issued for each leaf vdev every
 * zfs_multihost_interval milliseconds.  In practice, the observed period can
 * vary with the I/O load and this observed value is the ub_mmp_delay which is
 * stored in the uberblock.  The minimum allowed value is 100 ms.
 */
ulong_t zfs_multihost_interval = MMP_DEFAULT_INTERVAL;
#ifdef __FreeBSD__
SYSCTL_DECL(_vfs_zfs);
SYSCTL_ULONG(_vfs_zfs, OID_AUTO, multihost_interval, CTLFLAG_RWTUN,
    &zfs_multihost_interval, 0, "Interval between MMP writes, milliseconds");
#endif

/*
 * Used to control the duration of the activity test on import.  Smaller values
 * of zfs_multihost_import_intervals will reduce the import time but increase
 * the risk of failing to detect an active pool.  The total activity check time
 * is never allowed to drop below one second.  A value of 0 is ignored and
 * treated as if it was set to 1.
 */
uint_t zfs_multihost_import_intervals = MMP_DEFAULT_IMPORT_INTERVALS;
#ifdef __FreeBSD__
SYSCTL_UINT(_vfs_zfs, OID_AUTO, multihost_import_intervals, CTLFLAG_RWTUN,
    &zfs_multihost_import_intervals, 0,
    "MMP activity check period for pool import, "
    "in units of multihost_interval");
#endif

/*
 * Controls the behavior of the pool when mmp write failures or delays are
 * detected.
 *
 * When zfs_multihost_fail_intervals = 0, mmp write failures or delays are
 * ignored.  The failures will still be reported to the ZED which depending on
 * its configuration may take action such as suspending the pool or taking a
 * device offline.
 *
 * When zfs_multihost_fail_intervals > 0, the pool will be suspended if
 * zfs_multihost_fail_intervals * zfs_multihost_interval milliseconds pass
 * without a successful mmp write.  This guarantees the activity test will see
 * mmp writes if the pool is imported.  A value of 1 is ignored and treated as
 * if it was set to 2, because a single leaf vdev pool will issue a write once
 * per multihost_interval and thus any variation in latency would cause the
 * pool to be suspended.
 */
uint_t zfs_multihost_fail_intervals = MMP_DEFAULT_FAIL_INTERVALS;
#ifdef __FreeBSD__
SYSCTL_UINT(_vfs_zfs, OID_AUTO, multihost_fail_intervals, CTLFLAG_RWTUN,
    &zfs_multihost_fail_intervals, 0,
    "How long to tolerate MMP write failures before suspending a pool, "
    "in units of multihost_interval");
#endif

char *mmp_tag = "mmp_write_uberblock";
static void mmp_thread(void *arg);

void
mmp_init(spa_t *spa)
{
        mmp_thread_t *mmp = &spa->spa_mmp;

        mutex_init(&mmp->mmp_thread_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&mmp->mmp_thread_cv, NULL, CV_DEFAULT, NULL);
        mutex_init(&mmp->mmp_io_lock, NULL, MUTEX_DEFAULT, NULL);
        mmp->mmp_kstat_id = 1;

        /*
         * mmp_write_done() calculates mmp_delay based on prior mmp_delay and
         * the elapsed time since the last write.  For the first mmp write,
         * there is no "last write", so we start with fake non-zero values.
         */
        mmp->mmp_last_write = gethrtime();
        mmp->mmp_delay = MSEC2NSEC(MMP_INTERVAL_OK(zfs_multihost_interval));
}

void
mmp_fini(spa_t *spa)
{
        mmp_thread_t *mmp = &spa->spa_mmp;

        mutex_destroy(&mmp->mmp_thread_lock);
        cv_destroy(&mmp->mmp_thread_cv);
        mutex_destroy(&mmp->mmp_io_lock);
}

static void
mmp_thread_enter(mmp_thread_t *mmp, callb_cpr_t *cpr)
{
        CALLB_CPR_INIT(cpr, &mmp->mmp_thread_lock, callb_generic_cpr, FTAG);
        mutex_enter(&mmp->mmp_thread_lock);
}

static void
mmp_thread_exit(mmp_thread_t *mmp, kthread_t **mpp, callb_cpr_t *cpr)
{
        ASSERT(*mpp != NULL);
        *mpp = NULL;
        cv_broadcast(&mmp->mmp_thread_cv);
        CALLB_CPR_EXIT(cpr);            /* drops &mmp->mmp_thread_lock */
        thread_exit();
}

void
mmp_thread_start(spa_t *spa)
{
        mmp_thread_t *mmp = &spa->spa_mmp;

        if (spa_writeable(spa)) {
                mutex_enter(&mmp->mmp_thread_lock);
                if (!mmp->mmp_thread) {
                        mmp->mmp_thread = thread_create(NULL, 0, mmp_thread,
                            spa, 0, &p0, TS_RUN, minclsyspri);
                        zfs_dbgmsg("MMP thread started pool '%s' "
                            "gethrtime %llu", spa_name(spa), gethrtime());
                }
                mutex_exit(&mmp->mmp_thread_lock);
        }
}

void
mmp_thread_stop(spa_t *spa)
{
        mmp_thread_t *mmp = &spa->spa_mmp;

        mutex_enter(&mmp->mmp_thread_lock);
        mmp->mmp_thread_exiting = 1;
        cv_broadcast(&mmp->mmp_thread_cv);

        while (mmp->mmp_thread) {
                cv_wait(&mmp->mmp_thread_cv, &mmp->mmp_thread_lock);
        }
        mutex_exit(&mmp->mmp_thread_lock);
        zfs_dbgmsg("MMP thread stopped pool '%s' gethrtime %llu",
            spa_name(spa), gethrtime());

        ASSERT(mmp->mmp_thread == NULL);
        mmp->mmp_thread_exiting = 0;
}

typedef enum mmp_vdev_state_flag {
        MMP_FAIL_NOT_WRITABLE   = (1 << 0),
        MMP_FAIL_WRITE_PENDING  = (1 << 1),
} mmp_vdev_state_flag_t;

/*
 * Find a leaf vdev to write an MMP block to.  It must not have an outstanding
 * mmp write (if so a new write will also likely block).  If there is no usable
 * leaf, a nonzero error value is returned. The error value returned is a bit
 * field.
 *
 * MMP_FAIL_WRITE_PENDING   One or more leaf vdevs are writeable, but have an
 *                          outstanding MMP write.
 * MMP_FAIL_NOT_WRITABLE    One or more leaf vdevs are not writeable.
 */

static int
mmp_next_leaf(spa_t *spa)
{
        vdev_t *leaf;
        vdev_t *starting_leaf;
        int fail_mask = 0;

        ASSERT(MUTEX_HELD(&spa->spa_mmp.mmp_io_lock));
        ASSERT(spa_config_held(spa, SCL_STATE, RW_READER));
        ASSERT(list_link_active(&spa->spa_leaf_list.list_head) == B_TRUE);
        ASSERT(!list_is_empty(&spa->spa_leaf_list));

        if (spa->spa_mmp.mmp_leaf_last_gen != spa->spa_leaf_list_gen) {
                spa->spa_mmp.mmp_last_leaf = list_head(&spa->spa_leaf_list);
                spa->spa_mmp.mmp_leaf_last_gen = spa->spa_leaf_list_gen;
        }

        leaf = spa->spa_mmp.mmp_last_leaf;
        if (leaf == NULL)
                leaf = list_head(&spa->spa_leaf_list);
        starting_leaf = leaf;

        do {
                leaf = list_next(&spa->spa_leaf_list, leaf);
                if (leaf == NULL)
                        leaf = list_head(&spa->spa_leaf_list);

                if (!vdev_writeable(leaf)) {
                        fail_mask |= MMP_FAIL_NOT_WRITABLE;
                } else if (leaf->vdev_mmp_pending != 0) {
                        fail_mask |= MMP_FAIL_WRITE_PENDING;
                } else {
                        spa->spa_mmp.mmp_last_leaf = leaf;
                        return (0);
                }
        } while (leaf != starting_leaf);

        ASSERT(fail_mask);

        return (fail_mask);
}

/*
 * MMP writes are issued on a fixed schedule, but may complete at variable,
 * much longer, intervals.  The mmp_delay captures long periods between
 * successful writes for any reason, including disk latency, scheduling delays,
 * etc.
 *
 * The mmp_delay is usually calculated as a decaying average, but if the latest
 * delay is higher we do not average it, so that we do not hide sudden spikes
 * which the importing host must wait for.
 *
 * If writes are occurring frequently, such as due to a high rate of txg syncs,
 * the mmp_delay could become very small.  Since those short delays depend on
 * activity we cannot count on, we never allow mmp_delay to get lower than rate
 * expected if only mmp_thread writes occur.
 *
 * If an mmp write was skipped or fails, and we have already waited longer than
 * mmp_delay, we need to update it so the next write reflects the longer delay.
 *
 * Do not set mmp_delay if the multihost property is not on, so as not to
 * trigger an activity check on import.
 */
static void
mmp_delay_update(spa_t *spa, boolean_t write_completed)
{
        mmp_thread_t *mts = &spa->spa_mmp;
        hrtime_t delay = gethrtime() - mts->mmp_last_write;

        ASSERT(MUTEX_HELD(&mts->mmp_io_lock));

        if (spa_multihost(spa) == B_FALSE) {
                mts->mmp_delay = 0;
                return;
        }

        if (delay > mts->mmp_delay)
                mts->mmp_delay = delay;

        if (write_completed == B_FALSE)
                return;

        mts->mmp_last_write = gethrtime();

        /*
         * strictly less than, in case delay was changed above.
         */
        if (delay < mts->mmp_delay) {
                hrtime_t min_delay =
                    MSEC2NSEC(MMP_INTERVAL_OK(zfs_multihost_interval)) /
                    MAX(1, vdev_count_leaves(spa));
                mts->mmp_delay = MAX(((delay + mts->mmp_delay * 127) / 128),
                    min_delay);
        }
}

static void
mmp_write_done(zio_t *zio)
{
        spa_t *spa = zio->io_spa;
        vdev_t *vd = zio->io_vd;
        mmp_thread_t *mts = zio->io_private;

        mutex_enter(&mts->mmp_io_lock);
        uint64_t mmp_kstat_id = vd->vdev_mmp_kstat_id;
        hrtime_t mmp_write_duration = gethrtime() - vd->vdev_mmp_pending;

        mmp_delay_update(spa, (zio->io_error == 0));

        vd->vdev_mmp_pending = 0;
        vd->vdev_mmp_kstat_id = 0;

        mutex_exit(&mts->mmp_io_lock);
        spa_config_exit(spa, SCL_STATE, mmp_tag);

        abd_free(zio->io_abd);
}

/*
 * When the uberblock on-disk is updated by a spa_sync,
 * creating a new "best" uberblock, update the one stored
 * in the mmp thread state, used for mmp writes.
 */
void
mmp_update_uberblock(spa_t *spa, uberblock_t *ub)
{
        mmp_thread_t *mmp = &spa->spa_mmp;

        mutex_enter(&mmp->mmp_io_lock);
        mmp->mmp_ub = *ub;
        mmp->mmp_seq = 1;
        mmp->mmp_ub.ub_timestamp = gethrestime_sec();
        mmp_delay_update(spa, B_TRUE);
        mutex_exit(&mmp->mmp_io_lock);
}

/*
 * Choose a random vdev, label, and MMP block, and write over it
 * with a copy of the last-synced uberblock, whose timestamp
 * has been updated to reflect that the pool is in use.
 */
static void
mmp_write_uberblock(spa_t *spa)
{
        int flags = ZIO_FLAG_CONFIG_WRITER | ZIO_FLAG_CANFAIL;
        mmp_thread_t *mmp = &spa->spa_mmp;
        uberblock_t *ub;
        vdev_t *vd = NULL;
        int label, error;
        uint64_t offset;

        hrtime_t lock_acquire_time = gethrtime();
        spa_config_enter(spa, SCL_STATE, mmp_tag, RW_READER);
        lock_acquire_time = gethrtime() - lock_acquire_time;
        if (lock_acquire_time > (MSEC2NSEC(MMP_MIN_INTERVAL) / 10))
                zfs_dbgmsg("MMP SCL_STATE acquisition pool '%s' took %llu ns "
                    "gethrtime %llu", spa_name(spa), lock_acquire_time,
                    gethrtime());

        mutex_enter(&mmp->mmp_io_lock);

        error = mmp_next_leaf(spa);

        /*
         * spa_mmp_history has two types of entries:
         * Issued MMP write: records time issued, error status, etc.
         * Skipped MMP write: an MMP write could not be issued because no
         * suitable leaf vdev was available.  See comment above struct
         * spa_mmp_history for details.
         */

        if (error) {
                mmp_delay_update(spa, B_FALSE);
                if (mmp->mmp_skip_error == error) {
                        /*
                         * ZoL porting note: the following is TBD
                         * spa_mmp_history_set_skip(spa, mmp->mmp_kstat_id - 1);
                         */
                } else {
                        mmp->mmp_skip_error = error;
                        /*
                         * ZoL porting note: the following is TBD
                         * spa_mmp_history_add(spa, mmp->mmp_ub.ub_txg,
                         * gethrestime_sec(), mmp->mmp_delay, NULL, 0,
                         * mmp->mmp_kstat_id++, error);
                         */
                        zfs_dbgmsg("MMP error choosing leaf pool '%s' "
                            "gethrtime %llu fail_mask %#x", spa_name(spa),
                            gethrtime(), error);
                }
                mutex_exit(&mmp->mmp_io_lock);
                spa_config_exit(spa, SCL_STATE, mmp_tag);
                return;
        }

        vd = spa->spa_mmp.mmp_last_leaf;
        if (mmp->mmp_skip_error != 0) {
                mmp->mmp_skip_error = 0;
                zfs_dbgmsg("MMP write after skipping due to unavailable "
                    "leaves, pool '%s' gethrtime %llu leaf %#llu",
                    spa_name(spa), gethrtime(), vd->vdev_guid);
        }

        if (mmp->mmp_zio_root == NULL)
                mmp->mmp_zio_root = zio_root(spa, NULL, NULL,
                    flags | ZIO_FLAG_GODFATHER);

        if (mmp->mmp_ub.ub_timestamp != gethrestime_sec()) {
                /*
                 * Want to reset mmp_seq when timestamp advances because after
                 * an mmp_seq wrap new values will not be chosen by
                 * uberblock_compare() as the "best".
                 */
                mmp->mmp_ub.ub_timestamp = gethrestime_sec();
                mmp->mmp_seq = 1;
        }

        ub = &mmp->mmp_ub;
        ub->ub_mmp_magic = MMP_MAGIC;
        ub->ub_mmp_delay = mmp->mmp_delay;
        ub->ub_mmp_config = MMP_SEQ_SET(mmp->mmp_seq) |
            MMP_INTERVAL_SET(MMP_INTERVAL_OK(zfs_multihost_interval)) |
            MMP_FAIL_INT_SET(MMP_FAIL_INTVS_OK(
            zfs_multihost_fail_intervals));
        vd->vdev_mmp_pending = gethrtime();
        vd->vdev_mmp_kstat_id = mmp->mmp_kstat_id;

        zio_t *zio  = zio_null(mmp->mmp_zio_root, spa, NULL, NULL, NULL, flags);
        abd_t *ub_abd = abd_alloc_for_io(VDEV_UBERBLOCK_SIZE(vd), B_TRUE);
        abd_zero(ub_abd, VDEV_UBERBLOCK_SIZE(vd));
        abd_copy_from_buf(ub_abd, ub, sizeof (uberblock_t));

        mmp->mmp_seq++;
        mmp->mmp_kstat_id++;
        mutex_exit(&mmp->mmp_io_lock);

        offset = VDEV_UBERBLOCK_OFFSET(vd, VDEV_UBERBLOCK_COUNT(vd) -
            MMP_BLOCKS_PER_LABEL + spa_get_random(MMP_BLOCKS_PER_LABEL));

        label = spa_get_random(VDEV_LABELS);
        vdev_label_write(zio, vd, label, ub_abd, offset,
            VDEV_UBERBLOCK_SIZE(vd), mmp_write_done, mmp,
            flags | ZIO_FLAG_DONT_PROPAGATE);

        /*
         * ZoL porting note: the following is TBD
         * (void) spa_mmp_history_add(spa, ub->ub_txg, ub->ub_timestamp,
         * ub->ub_mmp_delay, vd, label, vd->vdev_mmp_kstat_id, 0);
         */

        zio_nowait(zio);
}

static void
mmp_thread(void *arg)
{
        spa_t *spa = (spa_t *)arg;
        mmp_thread_t *mmp = &spa->spa_mmp;
        boolean_t suspended = spa_suspended(spa);
        boolean_t multihost = spa_multihost(spa);
        uint64_t mmp_interval = MSEC2NSEC(MMP_INTERVAL_OK(
            zfs_multihost_interval));
        uint32_t mmp_fail_intervals = MMP_FAIL_INTVS_OK(
            zfs_multihost_fail_intervals);
        hrtime_t mmp_fail_ns = mmp_fail_intervals * mmp_interval;
        boolean_t last_spa_suspended = suspended;
        boolean_t last_spa_multihost = multihost;
        uint64_t last_mmp_interval = mmp_interval;
        uint32_t last_mmp_fail_intervals = mmp_fail_intervals;
        hrtime_t last_mmp_fail_ns = mmp_fail_ns;
        callb_cpr_t cpr;
        int skip_wait = 0;

        mmp_thread_enter(mmp, &cpr);

        while (!mmp->mmp_thread_exiting) {
                hrtime_t next_time = gethrtime() +
                    MSEC2NSEC(MMP_DEFAULT_INTERVAL);
                int leaves = MAX(vdev_count_leaves(spa), 1);

                /* Detect changes in tunables or state */

                last_spa_suspended = suspended;
                last_spa_multihost = multihost;
                suspended = spa_suspended(spa);
                multihost = spa_multihost(spa);

                last_mmp_interval = mmp_interval;
                last_mmp_fail_intervals = mmp_fail_intervals;
                last_mmp_fail_ns = mmp_fail_ns;
                mmp_interval = MSEC2NSEC(MMP_INTERVAL_OK(
                    zfs_multihost_interval));
                mmp_fail_intervals = MMP_FAIL_INTVS_OK(
                    zfs_multihost_fail_intervals);

                /* Smooth so pool is not suspended when reducing tunables */
                if (mmp_fail_intervals * mmp_interval < mmp_fail_ns) {
                        mmp_fail_ns = (mmp_fail_ns * 31 +
                            mmp_fail_intervals * mmp_interval) / 32;
                } else {
                        mmp_fail_ns = mmp_fail_intervals *
                            mmp_interval;
                }

                if (mmp_interval != last_mmp_interval ||
                    mmp_fail_intervals != last_mmp_fail_intervals) {
                        /*
                         * We want other hosts to see new tunables as quickly as
                         * possible.  Write out at higher frequency than usual.
                         */
                        skip_wait += leaves;
                }

                if (multihost)
                        next_time = gethrtime() + mmp_interval / leaves;

                if (mmp_fail_ns != last_mmp_fail_ns) {
                        zfs_dbgmsg("MMP interval change pool '%s' "
                            "gethrtime %llu last_mmp_interval %llu "
                            "mmp_interval %llu last_mmp_fail_intervals %u "
                            "mmp_fail_intervals %u mmp_fail_ns %llu "
                            "skip_wait %d leaves %d next_time %llu",
                            spa_name(spa), gethrtime(), last_mmp_interval,
                            mmp_interval, last_mmp_fail_intervals,
                            mmp_fail_intervals, mmp_fail_ns, skip_wait, leaves,
                            next_time);
                }

                /*
                 * MMP off => on, or suspended => !suspended:
                 * No writes occurred recently.  Update mmp_last_write to give
                 * us some time to try.
                 */
                if ((!last_spa_multihost && multihost) ||
                    (last_spa_suspended && !suspended)) {
                        zfs_dbgmsg("MMP state change pool '%s': gethrtime %llu "
                            "last_spa_multihost %u multihost %u "
                            "last_spa_suspended %u suspended %u",
                            spa_name(spa), last_spa_multihost, multihost,
                            last_spa_suspended, suspended);
                        mutex_enter(&mmp->mmp_io_lock);
                        mmp->mmp_last_write = gethrtime();
                        mmp->mmp_delay = mmp_interval;
                        mutex_exit(&mmp->mmp_io_lock);
                }

                /*
                 * MMP on => off:
                 * mmp_delay == 0 tells importing node to skip activity check.
                 */
                if (last_spa_multihost && !multihost) {
                        mutex_enter(&mmp->mmp_io_lock);
                        mmp->mmp_delay = 0;
                        mutex_exit(&mmp->mmp_io_lock);
                }

                /*
                 * Suspend the pool if no MMP write has succeeded in over
                 * mmp_interval * mmp_fail_intervals nanoseconds.
                 */
                if (multihost && !suspended && mmp_fail_intervals &&
                    (gethrtime() - mmp->mmp_last_write) > mmp_fail_ns) {
                        zfs_dbgmsg("MMP suspending pool '%s': gethrtime %llu "
                            "mmp_last_write %llu mmp_interval %llu "
                            "mmp_fail_intervals %llu mmp_fail_ns %llu",
                            spa_name(spa), (u_longlong_t)gethrtime(),
                            (u_longlong_t)mmp->mmp_last_write,
                            (u_longlong_t)mmp_interval,
                            (u_longlong_t)mmp_fail_intervals,
                            (u_longlong_t)mmp_fail_ns);
                        cmn_err(CE_WARN, "MMP writes to pool '%s' have not "
                            "succeeded in over %llu ms; suspending pool. "
                            "Hrtime %llu",
                            spa_name(spa),
                            NSEC2MSEC(gethrtime() - mmp->mmp_last_write),
                            gethrtime());
                        zio_suspend(spa, NULL, ZIO_SUSPEND_MMP);
                }

                if (multihost && !suspended)
                        mmp_write_uberblock(spa);

                if (skip_wait > 0) {
                        next_time = gethrtime() + MSEC2NSEC(MMP_MIN_INTERVAL) /
                            leaves;
                        skip_wait--;
                }

                CALLB_CPR_SAFE_BEGIN(&cpr);
#if defined(illumos)
                (void) cv_timedwait_sig_hrtime(&mmp->mmp_thread_cv,
                    &mmp->mmp_thread_lock, next_time);
#elif defined(_KERNEL)
                (void) cv_timedwait_sig_sbt(&mmp->mmp_thread_cv,
                    &mmp->mmp_thread_lock, nstosbt(next_time),
                    100 * SBT_1US, C_ABSOLUTE);
#else
                (void) cv_timedwait_sig_hires(&mmp->mmp_thread_cv,
                    &mmp->mmp_thread_lock, next_time, USEC2NSEC(100),
                    CALLOUT_FLAG_ABSOLUTE);
#endif
                CALLB_CPR_SAFE_END(&cpr, &mmp->mmp_thread_lock);
        }

        /* Outstanding writes are allowed to complete. */
        if (mmp->mmp_zio_root)
                zio_wait(mmp->mmp_zio_root);

        mmp->mmp_zio_root = NULL;
        mmp_thread_exit(mmp, &mmp->mmp_thread, &cpr);
}

/*
 * Signal the MMP thread to wake it, when it is sleeping on
 * its cv.  Used when some module parameter has changed and
 * we want the thread to know about it.
 * Only signal if the pool is active and mmp thread is
 * running, otherwise there is no thread to wake.
 */
static void
mmp_signal_thread(spa_t *spa)
{
        mmp_thread_t *mmp = &spa->spa_mmp;

        mutex_enter(&mmp->mmp_thread_lock);
        if (mmp->mmp_thread)
                cv_broadcast(&mmp->mmp_thread_cv);
        mutex_exit(&mmp->mmp_thread_lock);
}

void
mmp_signal_all_threads(void)
{
        spa_t *spa = NULL;

        mutex_enter(&spa_namespace_lock);
        while ((spa = spa_next(spa))) {
                if (spa->spa_state == POOL_STATE_ACTIVE)
                        mmp_signal_thread(spa);
        }
        mutex_exit(&spa_namespace_lock);
}