5960 zfs recv should prefetch indirect blocks

[unleashed.git] / usr / src / uts / common / fs / zfs / dmu_zfetch.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 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

/*
 * Copyright (c) 2013 by Delphix. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/dnode.h>
#include <sys/dmu_objset.h>
#include <sys/dmu_zfetch.h>
#include <sys/dmu.h>
#include <sys/dbuf.h>
#include <sys/kstat.h>

/*
 * I'm against tune-ables, but these should probably exist as tweakable globals
 * until we can get this working the way we want it to.
 */

int zfs_prefetch_disable = 0;

/* max # of streams per zfetch */
uint32_t        zfetch_max_streams = 8;
/* min time before stream reclaim */
uint32_t        zfetch_min_sec_reap = 2;
/* max number of blocks to fetch at a time */
uint32_t        zfetch_block_cap = 256;
/* number of bytes in a array_read at which we stop prefetching (1Mb) */
uint64_t        zfetch_array_rd_sz = 1024 * 1024;

/* forward decls for static routines */
static boolean_t        dmu_zfetch_colinear(zfetch_t *, zstream_t *);
static void             dmu_zfetch_dofetch(zfetch_t *, zstream_t *);
static uint64_t         dmu_zfetch_fetch(dnode_t *, uint64_t, uint64_t);
static uint64_t         dmu_zfetch_fetchsz(dnode_t *, uint64_t, uint64_t);
static boolean_t        dmu_zfetch_find(zfetch_t *, zstream_t *, int);
static int              dmu_zfetch_stream_insert(zfetch_t *, zstream_t *);
static zstream_t        *dmu_zfetch_stream_reclaim(zfetch_t *);
static void             dmu_zfetch_stream_remove(zfetch_t *, zstream_t *);
static int              dmu_zfetch_streams_equal(zstream_t *, zstream_t *);

typedef struct zfetch_stats {
        kstat_named_t zfetchstat_hits;
        kstat_named_t zfetchstat_misses;
        kstat_named_t zfetchstat_colinear_hits;
        kstat_named_t zfetchstat_colinear_misses;
        kstat_named_t zfetchstat_stride_hits;
        kstat_named_t zfetchstat_stride_misses;
        kstat_named_t zfetchstat_reclaim_successes;
        kstat_named_t zfetchstat_reclaim_failures;
        kstat_named_t zfetchstat_stream_resets;
        kstat_named_t zfetchstat_stream_noresets;
        kstat_named_t zfetchstat_bogus_streams;
} zfetch_stats_t;

static zfetch_stats_t zfetch_stats = {
        { "hits",                       KSTAT_DATA_UINT64 },
        { "misses",                     KSTAT_DATA_UINT64 },
        { "colinear_hits",              KSTAT_DATA_UINT64 },
        { "colinear_misses",            KSTAT_DATA_UINT64 },
        { "stride_hits",                KSTAT_DATA_UINT64 },
        { "stride_misses",              KSTAT_DATA_UINT64 },
        { "reclaim_successes",          KSTAT_DATA_UINT64 },
        { "reclaim_failures",           KSTAT_DATA_UINT64 },
        { "streams_resets",             KSTAT_DATA_UINT64 },
        { "streams_noresets",           KSTAT_DATA_UINT64 },
        { "bogus_streams",              KSTAT_DATA_UINT64 },
};

#define ZFETCHSTAT_INCR(stat, val) \
        atomic_add_64(&zfetch_stats.stat.value.ui64, (val));

#define ZFETCHSTAT_BUMP(stat)           ZFETCHSTAT_INCR(stat, 1);

kstat_t         *zfetch_ksp;

/*
 * Given a zfetch structure and a zstream structure, determine whether the
 * blocks to be read are part of a co-linear pair of existing prefetch
 * streams.  If a set is found, coalesce the streams, removing one, and
 * configure the prefetch so it looks for a strided access pattern.
 *
 * In other words: if we find two sequential access streams that are
 * the same length and distance N appart, and this read is N from the
 * last stream, then we are probably in a strided access pattern.  So
 * combine the two sequential streams into a single strided stream.
 *
 * Returns whether co-linear streams were found.
 */
static boolean_t
dmu_zfetch_colinear(zfetch_t *zf, zstream_t *zh)
{
        zstream_t       *z_walk;
        zstream_t       *z_comp;

        if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
                return (0);

        if (zh == NULL) {
                rw_exit(&zf->zf_rwlock);
                return (0);
        }

        for (z_walk = list_head(&zf->zf_stream); z_walk;
            z_walk = list_next(&zf->zf_stream, z_walk)) {
                for (z_comp = list_next(&zf->zf_stream, z_walk); z_comp;
                    z_comp = list_next(&zf->zf_stream, z_comp)) {
                        int64_t         diff;

                        if (z_walk->zst_len != z_walk->zst_stride ||
                            z_comp->zst_len != z_comp->zst_stride) {
                                continue;
                        }

                        diff = z_comp->zst_offset - z_walk->zst_offset;
                        if (z_comp->zst_offset + diff == zh->zst_offset) {
                                z_walk->zst_offset = zh->zst_offset;
                                z_walk->zst_direction = diff < 0 ? -1 : 1;
                                z_walk->zst_stride =
                                    diff * z_walk->zst_direction;
                                z_walk->zst_ph_offset =
                                    zh->zst_offset + z_walk->zst_stride;
                                dmu_zfetch_stream_remove(zf, z_comp);
                                mutex_destroy(&z_comp->zst_lock);
                                kmem_free(z_comp, sizeof (zstream_t));

                                dmu_zfetch_dofetch(zf, z_walk);

                                rw_exit(&zf->zf_rwlock);
                                return (1);
                        }

                        diff = z_walk->zst_offset - z_comp->zst_offset;
                        if (z_walk->zst_offset + diff == zh->zst_offset) {
                                z_walk->zst_offset = zh->zst_offset;
                                z_walk->zst_direction = diff < 0 ? -1 : 1;
                                z_walk->zst_stride =
                                    diff * z_walk->zst_direction;
                                z_walk->zst_ph_offset =
                                    zh->zst_offset + z_walk->zst_stride;
                                dmu_zfetch_stream_remove(zf, z_comp);
                                mutex_destroy(&z_comp->zst_lock);
                                kmem_free(z_comp, sizeof (zstream_t));

                                dmu_zfetch_dofetch(zf, z_walk);

                                rw_exit(&zf->zf_rwlock);
                                return (1);
                        }
                }
        }

        rw_exit(&zf->zf_rwlock);
        return (0);
}

/*
 * Given a zstream_t, determine the bounds of the prefetch.  Then call the
 * routine that actually prefetches the individual blocks.
 */
static void
dmu_zfetch_dofetch(zfetch_t *zf, zstream_t *zs)
{
        uint64_t        prefetch_tail;
        uint64_t        prefetch_limit;
        uint64_t        prefetch_ofst;
        uint64_t        prefetch_len;
        uint64_t        blocks_fetched;

        zs->zst_stride = MAX((int64_t)zs->zst_stride, zs->zst_len);
        zs->zst_cap = MIN(zfetch_block_cap, 2 * zs->zst_cap);

        prefetch_tail = MAX((int64_t)zs->zst_ph_offset,
            (int64_t)(zs->zst_offset + zs->zst_stride));
        /*
         * XXX: use a faster division method?
         */
        prefetch_limit = zs->zst_offset + zs->zst_len +
            (zs->zst_cap * zs->zst_stride) / zs->zst_len;

        while (prefetch_tail < prefetch_limit) {
                prefetch_ofst = zs->zst_offset + zs->zst_direction *
                    (prefetch_tail - zs->zst_offset);

                prefetch_len = zs->zst_len;

                /*
                 * Don't prefetch beyond the end of the file, if working
                 * backwards.
                 */
                if ((zs->zst_direction == ZFETCH_BACKWARD) &&
                    (prefetch_ofst > prefetch_tail)) {
                        prefetch_len += prefetch_ofst;
                        prefetch_ofst = 0;
                }

                /* don't prefetch more than we're supposed to */
                if (prefetch_len > zs->zst_len)
                        break;

                blocks_fetched = dmu_zfetch_fetch(zf->zf_dnode,
                    prefetch_ofst, zs->zst_len);

                prefetch_tail += zs->zst_stride;
                /* stop if we've run out of stuff to prefetch */
                if (blocks_fetched < zs->zst_len)
                        break;
        }
        zs->zst_ph_offset = prefetch_tail;
        zs->zst_last = ddi_get_lbolt();
}

void
zfetch_init(void)
{

        zfetch_ksp = kstat_create("zfs", 0, "zfetchstats", "misc",
            KSTAT_TYPE_NAMED, sizeof (zfetch_stats) / sizeof (kstat_named_t),
            KSTAT_FLAG_VIRTUAL);

        if (zfetch_ksp != NULL) {
                zfetch_ksp->ks_data = &zfetch_stats;
                kstat_install(zfetch_ksp);
        }
}

void
zfetch_fini(void)
{
        if (zfetch_ksp != NULL) {
                kstat_delete(zfetch_ksp);
                zfetch_ksp = NULL;
        }
}

/*
 * This takes a pointer to a zfetch structure and a dnode.  It performs the
 * necessary setup for the zfetch structure, grokking data from the
 * associated dnode.
 */
void
dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
{
        if (zf == NULL) {
                return;
        }

        zf->zf_dnode = dno;
        zf->zf_stream_cnt = 0;
        zf->zf_alloc_fail = 0;

        list_create(&zf->zf_stream, sizeof (zstream_t),
            offsetof(zstream_t, zst_node));

        rw_init(&zf->zf_rwlock, NULL, RW_DEFAULT, NULL);
}

/*
 * This function computes the actual size, in blocks, that can be prefetched,
 * and fetches it.
 */
static uint64_t
dmu_zfetch_fetch(dnode_t *dn, uint64_t blkid, uint64_t nblks)
{
        uint64_t        fetchsz;
        uint64_t        i;

        fetchsz = dmu_zfetch_fetchsz(dn, blkid, nblks);

        for (i = 0; i < fetchsz; i++) {
                dbuf_prefetch(dn, 0, blkid + i, ZIO_PRIORITY_ASYNC_READ,
                    ARC_FLAG_PREFETCH);
        }

        return (fetchsz);
}

/*
 * this function returns the number of blocks that would be prefetched, based
 * upon the supplied dnode, blockid, and nblks.  This is used so that we can
 * update streams in place, and then prefetch with their old value after the
 * fact.  This way, we can delay the prefetch, but subsequent accesses to the
 * stream won't result in the same data being prefetched multiple times.
 */
static uint64_t
dmu_zfetch_fetchsz(dnode_t *dn, uint64_t blkid, uint64_t nblks)
{
        uint64_t        fetchsz;

        if (blkid > dn->dn_maxblkid) {
                return (0);
        }

        /* compute fetch size */
        if (blkid + nblks + 1 > dn->dn_maxblkid) {
                fetchsz = (dn->dn_maxblkid - blkid) + 1;
                ASSERT(blkid + fetchsz - 1 <= dn->dn_maxblkid);
        } else {
                fetchsz = nblks;
        }


        return (fetchsz);
}

/*
 * given a zfetch and a zstream structure, see if there is an associated zstream
 * for this block read.  If so, it starts a prefetch for the stream it
 * located and returns true, otherwise it returns false
 */
static boolean_t
dmu_zfetch_find(zfetch_t *zf, zstream_t *zh, int prefetched)
{
        zstream_t       *zs;
        int64_t         diff;
        int             reset = !prefetched;
        int             rc = 0;

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

        /*
         * XXX: This locking strategy is a bit coarse; however, it's impact has
         * yet to be tested.  If this turns out to be an issue, it can be
         * modified in a number of different ways.
         */

        rw_enter(&zf->zf_rwlock, RW_READER);
top:

        for (zs = list_head(&zf->zf_stream); zs;
            zs = list_next(&zf->zf_stream, zs)) {

                /*
                 * XXX - should this be an assert?
                 */
                if (zs->zst_len == 0) {
                        /* bogus stream */
                        ZFETCHSTAT_BUMP(zfetchstat_bogus_streams);
                        continue;
                }

                /*
                 * We hit this case when we are in a strided prefetch stream:
                 * we will read "len" blocks before "striding".
                 */
                if (zh->zst_offset >= zs->zst_offset &&
                    zh->zst_offset < zs->zst_offset + zs->zst_len) {
                        if (prefetched) {
                                /* already fetched */
                                ZFETCHSTAT_BUMP(zfetchstat_stride_hits);
                                rc = 1;
                                goto out;
                        } else {
                                ZFETCHSTAT_BUMP(zfetchstat_stride_misses);
                        }
                }

                /*
                 * This is the forward sequential read case: we increment
                 * len by one each time we hit here, so we will enter this
                 * case on every read.
                 */
                if (zh->zst_offset == zs->zst_offset + zs->zst_len) {

                        reset = !prefetched && zs->zst_len > 1;

                        mutex_enter(&zs->zst_lock);

                        if (zh->zst_offset != zs->zst_offset + zs->zst_len) {
                                mutex_exit(&zs->zst_lock);
                                goto top;
                        }
                        zs->zst_len += zh->zst_len;
                        diff = zs->zst_len - zfetch_block_cap;
                        if (diff > 0) {
                                zs->zst_offset += diff;
                                zs->zst_len = zs->zst_len > diff ?
                                    zs->zst_len - diff : 0;
                        }
                        zs->zst_direction = ZFETCH_FORWARD;

                        break;

                /*
                 * Same as above, but reading backwards through the file.
                 */
                } else if (zh->zst_offset == zs->zst_offset - zh->zst_len) {
                        /* backwards sequential access */

                        reset = !prefetched && zs->zst_len > 1;

                        mutex_enter(&zs->zst_lock);

                        if (zh->zst_offset != zs->zst_offset - zh->zst_len) {
                                mutex_exit(&zs->zst_lock);
                                goto top;
                        }

                        zs->zst_offset = zs->zst_offset > zh->zst_len ?
                            zs->zst_offset - zh->zst_len : 0;
                        zs->zst_ph_offset = zs->zst_ph_offset > zh->zst_len ?
                            zs->zst_ph_offset - zh->zst_len : 0;
                        zs->zst_len += zh->zst_len;

                        diff = zs->zst_len - zfetch_block_cap;
                        if (diff > 0) {
                                zs->zst_ph_offset = zs->zst_ph_offset > diff ?
                                    zs->zst_ph_offset - diff : 0;
                                zs->zst_len = zs->zst_len > diff ?
                                    zs->zst_len - diff : zs->zst_len;
                        }
                        zs->zst_direction = ZFETCH_BACKWARD;

                        break;

                } else if ((zh->zst_offset - zs->zst_offset - zs->zst_stride <
                    zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
                        /* strided forward access */

                        mutex_enter(&zs->zst_lock);

                        if ((zh->zst_offset - zs->zst_offset - zs->zst_stride >=
                            zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
                                mutex_exit(&zs->zst_lock);
                                goto top;
                        }

                        zs->zst_offset += zs->zst_stride;
                        zs->zst_direction = ZFETCH_FORWARD;

                        break;

                } else if ((zh->zst_offset - zs->zst_offset + zs->zst_stride <
                    zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
                        /* strided reverse access */

                        mutex_enter(&zs->zst_lock);

                        if ((zh->zst_offset - zs->zst_offset + zs->zst_stride >=
                            zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
                                mutex_exit(&zs->zst_lock);
                                goto top;
                        }

                        zs->zst_offset = zs->zst_offset > zs->zst_stride ?
                            zs->zst_offset - zs->zst_stride : 0;
                        zs->zst_ph_offset = (zs->zst_ph_offset >
                            (2 * zs->zst_stride)) ?
                            (zs->zst_ph_offset - (2 * zs->zst_stride)) : 0;
                        zs->zst_direction = ZFETCH_BACKWARD;

                        break;
                }
        }

        if (zs) {
                if (reset) {
                        zstream_t *remove = zs;

                        ZFETCHSTAT_BUMP(zfetchstat_stream_resets);
                        rc = 0;
                        mutex_exit(&zs->zst_lock);
                        rw_exit(&zf->zf_rwlock);
                        rw_enter(&zf->zf_rwlock, RW_WRITER);
                        /*
                         * Relocate the stream, in case someone removes
                         * it while we were acquiring the WRITER lock.
                         */
                        for (zs = list_head(&zf->zf_stream); zs;
                            zs = list_next(&zf->zf_stream, zs)) {
                                if (zs == remove) {
                                        dmu_zfetch_stream_remove(zf, zs);
                                        mutex_destroy(&zs->zst_lock);
                                        kmem_free(zs, sizeof (zstream_t));
                                        break;
                                }
                        }
                } else {
                        ZFETCHSTAT_BUMP(zfetchstat_stream_noresets);
                        rc = 1;
                        dmu_zfetch_dofetch(zf, zs);
                        mutex_exit(&zs->zst_lock);
                }
        }
out:
        rw_exit(&zf->zf_rwlock);
        return (rc);
}

/*
 * Clean-up state associated with a zfetch structure.  This frees allocated
 * structure members, empties the zf_stream tree, and generally makes things
 * nice.  This doesn't free the zfetch_t itself, that's left to the caller.
 */
void
dmu_zfetch_rele(zfetch_t *zf)
{
        zstream_t       *zs;
        zstream_t       *zs_next;

        ASSERT(!RW_LOCK_HELD(&zf->zf_rwlock));

        for (zs = list_head(&zf->zf_stream); zs; zs = zs_next) {
                zs_next = list_next(&zf->zf_stream, zs);

                list_remove(&zf->zf_stream, zs);
                mutex_destroy(&zs->zst_lock);
                kmem_free(zs, sizeof (zstream_t));
        }
        list_destroy(&zf->zf_stream);
        rw_destroy(&zf->zf_rwlock);

        zf->zf_dnode = NULL;
}

/*
 * Given a zfetch and zstream structure, insert the zstream structure into the
 * AVL tree contained within the zfetch structure.  Peform the appropriate
 * book-keeping.  It is possible that another thread has inserted a stream which
 * matches one that we are about to insert, so we must be sure to check for this
 * case.  If one is found, return failure, and let the caller cleanup the
 * duplicates.
 */
static int
dmu_zfetch_stream_insert(zfetch_t *zf, zstream_t *zs)
{
        zstream_t       *zs_walk;
        zstream_t       *zs_next;

        ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));

        for (zs_walk = list_head(&zf->zf_stream); zs_walk; zs_walk = zs_next) {
                zs_next = list_next(&zf->zf_stream, zs_walk);

                if (dmu_zfetch_streams_equal(zs_walk, zs)) {
                        return (0);
                }
        }

        list_insert_head(&zf->zf_stream, zs);
        zf->zf_stream_cnt++;
        return (1);
}


/*
 * Walk the list of zstreams in the given zfetch, find an old one (by time), and
 * reclaim it for use by the caller.
 */
static zstream_t *
dmu_zfetch_stream_reclaim(zfetch_t *zf)
{
        zstream_t       *zs;

        if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
                return (0);

        for (zs = list_head(&zf->zf_stream); zs;
            zs = list_next(&zf->zf_stream, zs)) {

                if (((ddi_get_lbolt() - zs->zst_last)/hz) > zfetch_min_sec_reap)
                        break;
        }

        if (zs) {
                dmu_zfetch_stream_remove(zf, zs);
                mutex_destroy(&zs->zst_lock);
                bzero(zs, sizeof (zstream_t));
        } else {
                zf->zf_alloc_fail++;
        }
        rw_exit(&zf->zf_rwlock);

        return (zs);
}

/*
 * Given a zfetch and zstream structure, remove the zstream structure from its
 * container in the zfetch structure.  Perform the appropriate book-keeping.
 */
static void
dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
{
        ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));

        list_remove(&zf->zf_stream, zs);
        zf->zf_stream_cnt--;
}

static int
dmu_zfetch_streams_equal(zstream_t *zs1, zstream_t *zs2)
{
        if (zs1->zst_offset != zs2->zst_offset)
                return (0);

        if (zs1->zst_len != zs2->zst_len)
                return (0);

        if (zs1->zst_stride != zs2->zst_stride)
                return (0);

        if (zs1->zst_ph_offset != zs2->zst_ph_offset)
                return (0);

        if (zs1->zst_cap != zs2->zst_cap)
                return (0);

        if (zs1->zst_direction != zs2->zst_direction)
                return (0);

        return (1);
}

/*
 * This is the prefetch entry point.  It calls all of the other dmu_zfetch
 * routines to create, delete, find, or operate upon prefetch streams.
 */
void
dmu_zfetch(zfetch_t *zf, uint64_t offset, uint64_t size, int prefetched)
{
        zstream_t       zst;
        zstream_t       *newstream;
        boolean_t       fetched;
        int             inserted;
        unsigned int    blkshft;
        uint64_t        blksz;

        if (zfs_prefetch_disable)
                return;

        /* files that aren't ln2 blocksz are only one block -- nothing to do */
        if (!zf->zf_dnode->dn_datablkshift)
                return;

        /* convert offset and size, into blockid and nblocks */
        blkshft = zf->zf_dnode->dn_datablkshift;
        blksz = (1 << blkshft);

        bzero(&zst, sizeof (zstream_t));
        zst.zst_offset = offset >> blkshft;
        zst.zst_len = (P2ROUNDUP(offset + size, blksz) -
            P2ALIGN(offset, blksz)) >> blkshft;

        fetched = dmu_zfetch_find(zf, &zst, prefetched);
        if (fetched) {
                ZFETCHSTAT_BUMP(zfetchstat_hits);
        } else {
                ZFETCHSTAT_BUMP(zfetchstat_misses);
                fetched = dmu_zfetch_colinear(zf, &zst);
                if (fetched) {
                        ZFETCHSTAT_BUMP(zfetchstat_colinear_hits);
                } else {
                        ZFETCHSTAT_BUMP(zfetchstat_colinear_misses);
                }
        }

        if (!fetched) {
                newstream = dmu_zfetch_stream_reclaim(zf);

                /*
                 * we still couldn't find a stream, drop the lock, and allocate
                 * one if possible.  Otherwise, give up and go home.
                 */
                if (newstream) {
                        ZFETCHSTAT_BUMP(zfetchstat_reclaim_successes);
                } else {
                        uint64_t        maxblocks;
                        uint32_t        max_streams;
                        uint32_t        cur_streams;

                        ZFETCHSTAT_BUMP(zfetchstat_reclaim_failures);
                        cur_streams = zf->zf_stream_cnt;
                        maxblocks = zf->zf_dnode->dn_maxblkid;

                        max_streams = MIN(zfetch_max_streams,
                            (maxblocks / zfetch_block_cap));
                        if (max_streams == 0) {
                                max_streams++;
                        }

                        if (cur_streams >= max_streams) {
                                return;
                        }
                        newstream = kmem_zalloc(sizeof (zstream_t), KM_SLEEP);
                }

                newstream->zst_offset = zst.zst_offset;
                newstream->zst_len = zst.zst_len;
                newstream->zst_stride = zst.zst_len;
                newstream->zst_ph_offset = zst.zst_len + zst.zst_offset;
                newstream->zst_cap = zst.zst_len;
                newstream->zst_direction = ZFETCH_FORWARD;
                newstream->zst_last = ddi_get_lbolt();

                mutex_init(&newstream->zst_lock, NULL, MUTEX_DEFAULT, NULL);

                rw_enter(&zf->zf_rwlock, RW_WRITER);
                inserted = dmu_zfetch_stream_insert(zf, newstream);
                rw_exit(&zf->zf_rwlock);

                if (!inserted) {
                        mutex_destroy(&newstream->zst_lock);
                        kmem_free(newstream, sizeof (zstream_t));
                }
        }
}