Improve speculative prefetcher for block cloning

[zfs.git] / module / 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 https://opensource.org/licenses/CDDL-1.0.
 * 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, 2017 by Delphix. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/arc_impl.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>
#include <sys/wmsum.h>

/*
 * This tunable disables predictive prefetch.  Note that it leaves "prescient"
 * prefetch (e.g. prefetch for zfs send) intact.  Unlike predictive prefetch,
 * prescient prefetch never issues i/os that end up not being needed,
 * so it can't hurt performance.
 */

static int zfs_prefetch_disable = B_FALSE;

/* max # of streams per zfetch */
static unsigned int     zfetch_max_streams = 8;
/* min time before stream reclaim */
static unsigned int     zfetch_min_sec_reap = 1;
/* max time before stream delete */
static unsigned int     zfetch_max_sec_reap = 2;
#ifdef _ILP32
/* min bytes to prefetch per stream (default 2MB) */
static unsigned int     zfetch_min_distance = 2 * 1024 * 1024;
/* max bytes to prefetch per stream (default 8MB) */
unsigned int    zfetch_max_distance = 8 * 1024 * 1024;
#else
/* min bytes to prefetch per stream (default 4MB) */
static unsigned int     zfetch_min_distance = 4 * 1024 * 1024;
/* max bytes to prefetch per stream (default 64MB) */
unsigned int    zfetch_max_distance = 64 * 1024 * 1024;
#endif
/* max bytes to prefetch indirects for per stream (default 128MB) */
unsigned int    zfetch_max_idistance = 128 * 1024 * 1024;
/* max request reorder distance within a stream (default 16MB) */
unsigned int    zfetch_max_reorder = 16 * 1024 * 1024;
/* Max log2 fraction of holes in a stream */
unsigned int    zfetch_hole_shift = 2;

typedef struct zfetch_stats {
        kstat_named_t zfetchstat_hits;
        kstat_named_t zfetchstat_future;
        kstat_named_t zfetchstat_stride;
        kstat_named_t zfetchstat_past;
        kstat_named_t zfetchstat_misses;
        kstat_named_t zfetchstat_max_streams;
        kstat_named_t zfetchstat_io_issued;
        kstat_named_t zfetchstat_io_active;
} zfetch_stats_t;

static zfetch_stats_t zfetch_stats = {
        { "hits",                       KSTAT_DATA_UINT64 },
        { "future",                     KSTAT_DATA_UINT64 },
        { "stride",                     KSTAT_DATA_UINT64 },
        { "past",                       KSTAT_DATA_UINT64 },
        { "misses",                     KSTAT_DATA_UINT64 },
        { "max_streams",                KSTAT_DATA_UINT64 },
        { "io_issued",                  KSTAT_DATA_UINT64 },
        { "io_active",                  KSTAT_DATA_UINT64 },
};

struct {
        wmsum_t zfetchstat_hits;
        wmsum_t zfetchstat_future;
        wmsum_t zfetchstat_stride;
        wmsum_t zfetchstat_past;
        wmsum_t zfetchstat_misses;
        wmsum_t zfetchstat_max_streams;
        wmsum_t zfetchstat_io_issued;
        aggsum_t zfetchstat_io_active;
} zfetch_sums;

#define ZFETCHSTAT_BUMP(stat)                                   \
        wmsum_add(&zfetch_sums.stat, 1)
#define ZFETCHSTAT_ADD(stat, val)                               \
        wmsum_add(&zfetch_sums.stat, val)


static kstat_t          *zfetch_ksp;

static int
zfetch_kstats_update(kstat_t *ksp, int rw)
{
        zfetch_stats_t *zs = ksp->ks_data;

        if (rw == KSTAT_WRITE)
                return (EACCES);
        zs->zfetchstat_hits.value.ui64 =
            wmsum_value(&zfetch_sums.zfetchstat_hits);
        zs->zfetchstat_future.value.ui64 =
            wmsum_value(&zfetch_sums.zfetchstat_future);
        zs->zfetchstat_stride.value.ui64 =
            wmsum_value(&zfetch_sums.zfetchstat_stride);
        zs->zfetchstat_past.value.ui64 =
            wmsum_value(&zfetch_sums.zfetchstat_past);
        zs->zfetchstat_misses.value.ui64 =
            wmsum_value(&zfetch_sums.zfetchstat_misses);
        zs->zfetchstat_max_streams.value.ui64 =
            wmsum_value(&zfetch_sums.zfetchstat_max_streams);
        zs->zfetchstat_io_issued.value.ui64 =
            wmsum_value(&zfetch_sums.zfetchstat_io_issued);
        zs->zfetchstat_io_active.value.ui64 =
            aggsum_value(&zfetch_sums.zfetchstat_io_active);
        return (0);
}

void
zfetch_init(void)
{
        wmsum_init(&zfetch_sums.zfetchstat_hits, 0);
        wmsum_init(&zfetch_sums.zfetchstat_future, 0);
        wmsum_init(&zfetch_sums.zfetchstat_stride, 0);
        wmsum_init(&zfetch_sums.zfetchstat_past, 0);
        wmsum_init(&zfetch_sums.zfetchstat_misses, 0);
        wmsum_init(&zfetch_sums.zfetchstat_max_streams, 0);
        wmsum_init(&zfetch_sums.zfetchstat_io_issued, 0);
        aggsum_init(&zfetch_sums.zfetchstat_io_active, 0);

        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;
                zfetch_ksp->ks_update = zfetch_kstats_update;
                kstat_install(zfetch_ksp);
        }
}

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

        wmsum_fini(&zfetch_sums.zfetchstat_hits);
        wmsum_fini(&zfetch_sums.zfetchstat_future);
        wmsum_fini(&zfetch_sums.zfetchstat_stride);
        wmsum_fini(&zfetch_sums.zfetchstat_past);
        wmsum_fini(&zfetch_sums.zfetchstat_misses);
        wmsum_fini(&zfetch_sums.zfetchstat_max_streams);
        wmsum_fini(&zfetch_sums.zfetchstat_io_issued);
        ASSERT0(aggsum_value(&zfetch_sums.zfetchstat_io_active));
        aggsum_fini(&zfetch_sums.zfetchstat_io_active);
}

/*
 * 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_numstreams = 0;

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

        mutex_init(&zf->zf_lock, NULL, MUTEX_DEFAULT, NULL);
}

static void
dmu_zfetch_stream_fini(zstream_t *zs)
{
        ASSERT(!list_link_active(&zs->zs_node));
        zfs_refcount_destroy(&zs->zs_callers);
        zfs_refcount_destroy(&zs->zs_refs);
        kmem_free(zs, sizeof (*zs));
}

static void
dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
{
        ASSERT(MUTEX_HELD(&zf->zf_lock));
        list_remove(&zf->zf_stream, zs);
        zf->zf_numstreams--;
        membar_producer();
        if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0)
                dmu_zfetch_stream_fini(zs);
}

/*
 * Clean-up state associated with a zfetch structure (e.g. destroy the
 * streams).  This doesn't free the zfetch_t itself, that's left to the caller.
 */
void
dmu_zfetch_fini(zfetch_t *zf)
{
        zstream_t *zs;

        mutex_enter(&zf->zf_lock);
        while ((zs = list_head(&zf->zf_stream)) != NULL)
                dmu_zfetch_stream_remove(zf, zs);
        mutex_exit(&zf->zf_lock);
        list_destroy(&zf->zf_stream);
        mutex_destroy(&zf->zf_lock);

        zf->zf_dnode = NULL;
}

/*
 * If there aren't too many active streams already, create one more.
 * In process delete/reuse all streams without hits for zfetch_max_sec_reap.
 * If needed, reuse oldest stream without hits for zfetch_min_sec_reap or ever.
 * The "blkid" argument is the next block that we expect this stream to access.
 */
static void
dmu_zfetch_stream_create(zfetch_t *zf, uint64_t blkid)
{
        zstream_t *zs, *zs_next, *zs_old = NULL;
        uint_t now = gethrestime_sec(), t;

        ASSERT(MUTEX_HELD(&zf->zf_lock));

        /*
         * Delete too old streams, reusing the first found one.
         */
        t = now - zfetch_max_sec_reap;
        for (zs = list_head(&zf->zf_stream); zs != NULL; zs = zs_next) {
                zs_next = list_next(&zf->zf_stream, zs);
                /*
                 * Skip if still active.  1 -- zf_stream reference.
                 */
                if ((int)(zs->zs_atime - t) >= 0)
                        continue;
                if (zfs_refcount_count(&zs->zs_refs) != 1)
                        continue;
                if (zs_old)
                        dmu_zfetch_stream_remove(zf, zs);
                else
                        zs_old = zs;
        }
        if (zs_old) {
                zs = zs_old;
                list_remove(&zf->zf_stream, zs);
                goto reuse;
        }

        /*
         * The maximum number of streams is normally zfetch_max_streams,
         * but for small files we lower it such that it's at least possible
         * for all the streams to be non-overlapping.
         */
        uint32_t max_streams = MAX(1, MIN(zfetch_max_streams,
            (zf->zf_dnode->dn_maxblkid << zf->zf_dnode->dn_datablkshift) /
            zfetch_max_distance));
        if (zf->zf_numstreams >= max_streams) {
                t = now - zfetch_min_sec_reap;
                for (zs = list_head(&zf->zf_stream); zs != NULL;
                    zs = list_next(&zf->zf_stream, zs)) {
                        if ((int)(zs->zs_atime - t) >= 0)
                                continue;
                        if (zfs_refcount_count(&zs->zs_refs) != 1)
                                continue;
                        if (zs_old == NULL ||
                            (int)(zs_old->zs_atime - zs->zs_atime) >= 0)
                                zs_old = zs;
                }
                if (zs_old) {
                        zs = zs_old;
                        list_remove(&zf->zf_stream, zs);
                        goto reuse;
                }
                ZFETCHSTAT_BUMP(zfetchstat_max_streams);
                return;
        }

        zs = kmem_zalloc(sizeof (*zs), KM_SLEEP);
        zfs_refcount_create(&zs->zs_callers);
        zfs_refcount_create(&zs->zs_refs);
        /* One reference for zf_stream. */
        zfs_refcount_add(&zs->zs_refs, NULL);
        zf->zf_numstreams++;

reuse:
        list_insert_head(&zf->zf_stream, zs);
        zs->zs_blkid = blkid;
        /* Allow immediate stream reuse until first hit. */
        zs->zs_atime = now - zfetch_min_sec_reap;
        memset(zs->zs_ranges, 0, sizeof (zs->zs_ranges));
        zs->zs_pf_dist = 0;
        zs->zs_ipf_dist = 0;
        zs->zs_pf_start = blkid;
        zs->zs_pf_end = blkid;
        zs->zs_ipf_start = blkid;
        zs->zs_ipf_end = blkid;
        zs->zs_missed = B_FALSE;
        zs->zs_more = B_FALSE;
}

static void
dmu_zfetch_done(void *arg, uint64_t level, uint64_t blkid, boolean_t io_issued)
{
        zstream_t *zs = arg;

        if (io_issued && level == 0 && blkid < zs->zs_blkid)
                zs->zs_more = B_TRUE;
        if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0)
                dmu_zfetch_stream_fini(zs);
        aggsum_add(&zfetch_sums.zfetchstat_io_active, -1);
}

/*
 * Process stream hit access for nblks blocks starting at zs_blkid.  Return
 * number of blocks to proceed for after aggregation with future ranges.
 */
static uint64_t
dmu_zfetch_hit(zstream_t *zs, uint64_t nblks)
{
        uint_t i, j;

        /* Optimize sequential accesses (no future ranges). */
        if (zs->zs_ranges[0].start == 0)
                goto done;

        /* Look for intersections with further ranges. */
        for (i = 0; i < ZFETCH_RANGES; i++) {
                zsrange_t *r = &zs->zs_ranges[i];
                if (r->start == 0 || r->start > nblks)
                        break;
                if (r->end >= nblks) {
                        nblks = r->end;
                        i++;
                        break;
                }
        }

        /* Delete all found intersecting ranges, updates remaining. */
        for (j = 0; i < ZFETCH_RANGES; i++, j++) {
                if (zs->zs_ranges[i].start == 0)
                        break;
                ASSERT3U(zs->zs_ranges[i].start, >, nblks);
                ASSERT3U(zs->zs_ranges[i].end, >, nblks);
                zs->zs_ranges[j].start = zs->zs_ranges[i].start - nblks;
                zs->zs_ranges[j].end = zs->zs_ranges[i].end - nblks;
        }
        if (j < ZFETCH_RANGES) {
                zs->zs_ranges[j].start = 0;
                zs->zs_ranges[j].end = 0;
        }

done:
        zs->zs_blkid += nblks;
        return (nblks);
}

/*
 * Process future stream access for nblks blocks starting at blkid.  Return
 * number of blocks to proceed for if future ranges reach fill threshold.
 */
static uint64_t
dmu_zfetch_future(zstream_t *zs, uint64_t blkid, uint64_t nblks)
{
        ASSERT3U(blkid, >, zs->zs_blkid);
        blkid -= zs->zs_blkid;
        ASSERT3U(blkid + nblks, <=, UINT16_MAX);

        /* Search for first and last intersection or insert point. */
        uint_t f = ZFETCH_RANGES, l = 0, i;
        for (i = 0; i < ZFETCH_RANGES; i++) {
                zsrange_t *r = &zs->zs_ranges[i];
                if (r->start == 0 || r->start > blkid + nblks)
                        break;
                if (r->end < blkid)
                        continue;
                if (f > i)
                        f = i;
                if (l < i)
                        l = i;
        }
        if (f <= l) {
                /* Got some intersecting range, expand it if needed. */
                if (zs->zs_ranges[f].start > blkid)
                        zs->zs_ranges[f].start = blkid;
                zs->zs_ranges[f].end = MAX(zs->zs_ranges[l].end, blkid + nblks);
                if (f < l) {
                        /* Got more than one intersection, remove others. */
                        for (f++, l++; l < ZFETCH_RANGES; f++, l++) {
                                zs->zs_ranges[f].start = zs->zs_ranges[l].start;
                                zs->zs_ranges[f].end = zs->zs_ranges[l].end;
                        }
                        zs->zs_ranges[f].start = 0;
                        zs->zs_ranges[f].end = 0;
                }
        } else if (i < ZFETCH_RANGES) {
                /* Got no intersecting ranges, insert new one. */
                for (l = ZFETCH_RANGES - 1; l > i; l--) {
                        zs->zs_ranges[l].start = zs->zs_ranges[l - 1].start;
                        zs->zs_ranges[l].end = zs->zs_ranges[l - 1].end;
                }
                zs->zs_ranges[i].start = blkid;
                zs->zs_ranges[i].end = blkid + nblks;
        } else {
                /* No space left to insert.  Drop the range. */
                return (0);
        }

        /* Check if with the new access addition we reached fill threshold. */
        if (zfetch_hole_shift >= 16)
                return (0);
        uint_t hole = 0;
        for (i = f = l = 0; i < ZFETCH_RANGES; i++) {
                zsrange_t *r = &zs->zs_ranges[i];
                if (r->start == 0)
                        break;
                hole += r->start - f;
                f = r->end;
                if (hole <= r->end >> zfetch_hole_shift)
                        l = r->end;
        }
        if (l > 0)
                return (dmu_zfetch_hit(zs, l));

        return (0);
}

/*
 * This is the predictive prefetch entry point.  dmu_zfetch_prepare()
 * associates dnode access specified with blkid and nblks arguments with
 * prefetch stream, predicts further accesses based on that stats and returns
 * the stream pointer on success.  That pointer must later be passed to
 * dmu_zfetch_run() to initiate the speculative prefetch for the stream and
 * release it.  dmu_zfetch() is a wrapper for simple cases when window between
 * prediction and prefetch initiation is not needed.
 * fetch_data argument specifies whether actual data blocks should be fetched:
 *   FALSE -- prefetch only indirect blocks for predicted data blocks;
 *   TRUE -- prefetch predicted data blocks plus following indirect blocks.
 */
zstream_t *
dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks,
    boolean_t fetch_data, boolean_t have_lock)
{
        zstream_t *zs;
        spa_t *spa = zf->zf_dnode->dn_objset->os_spa;
        zfs_prefetch_type_t os_prefetch = zf->zf_dnode->dn_objset->os_prefetch;
        int64_t ipf_start, ipf_end;

        if (zfs_prefetch_disable || os_prefetch == ZFS_PREFETCH_NONE)
                return (NULL);

        if (os_prefetch == ZFS_PREFETCH_METADATA)
                fetch_data = B_FALSE;

        /*
         * If we haven't yet loaded the indirect vdevs' mappings, we
         * can only read from blocks that we carefully ensure are on
         * concrete vdevs (or previously-loaded indirect vdevs).  So we
         * can't allow the predictive prefetcher to attempt reads of other
         * blocks (e.g. of the MOS's dnode object).
         */
        if (!spa_indirect_vdevs_loaded(spa))
                return (NULL);

        /*
         * As a fast path for small (single-block) files, ignore access
         * to the first block.
         */
        if (!have_lock && blkid == 0)
                return (NULL);

        if (!have_lock)
                rw_enter(&zf->zf_dnode->dn_struct_rwlock, RW_READER);

        /*
         * A fast path for small files for which no prefetch will
         * happen.
         */
        uint64_t maxblkid = zf->zf_dnode->dn_maxblkid;
        if (maxblkid < 2) {
                if (!have_lock)
                        rw_exit(&zf->zf_dnode->dn_struct_rwlock);
                return (NULL);
        }
        mutex_enter(&zf->zf_lock);

        /*
         * Find perfect prefetch stream.  Depending on whether the accesses
         * are block-aligned, first block of the new access may either follow
         * the last block of the previous access, or be equal to it.
         */
        unsigned int dbs = zf->zf_dnode->dn_datablkshift;
        uint64_t end_blkid = blkid + nblks;
        for (zs = list_head(&zf->zf_stream); zs != NULL;
            zs = list_next(&zf->zf_stream, zs)) {
                if (blkid == zs->zs_blkid) {
                        goto hit;
                } else if (blkid + 1 == zs->zs_blkid) {
                        blkid++;
                        nblks--;
                        goto hit;
                }
        }

        /*
         * Find close enough prefetch stream.  Access crossing stream position
         * is a hit in its new part.  Access ahead of stream position considered
         * a hit for metadata prefetch, since we do not care about fill percent,
         * or stored for future otherwise.  Access behind stream position is
         * silently ignored, since we already skipped it reaching fill percent.
         */
        uint_t max_reorder = MIN((zfetch_max_reorder >> dbs) + 1, UINT16_MAX);
        uint_t t = gethrestime_sec() - zfetch_max_sec_reap;
        for (zs = list_head(&zf->zf_stream); zs != NULL;
            zs = list_next(&zf->zf_stream, zs)) {
                if (blkid > zs->zs_blkid) {
                        if (end_blkid <= zs->zs_blkid + max_reorder) {
                                if (!fetch_data) {
                                        nblks = dmu_zfetch_hit(zs,
                                            end_blkid - zs->zs_blkid);
                                        ZFETCHSTAT_BUMP(zfetchstat_stride);
                                        goto future;
                                }
                                nblks = dmu_zfetch_future(zs, blkid, nblks);
                                if (nblks > 0)
                                        ZFETCHSTAT_BUMP(zfetchstat_stride);
                                else
                                        ZFETCHSTAT_BUMP(zfetchstat_future);
                                goto future;
                        }
                } else if (end_blkid >= zs->zs_blkid) {
                        nblks -= zs->zs_blkid - blkid;
                        blkid += zs->zs_blkid - blkid;
                        goto hit;
                } else if (end_blkid + max_reorder > zs->zs_blkid &&
                    (int)(zs->zs_atime - t) >= 0) {
                        ZFETCHSTAT_BUMP(zfetchstat_past);
                        zs->zs_atime = gethrestime_sec();
                        goto out;
                }
        }

        /*
         * This access is not part of any existing stream.  Create a new
         * stream for it unless we are at the end of file.
         */
        ASSERT0P(zs);
        if (end_blkid < maxblkid)
                dmu_zfetch_stream_create(zf, end_blkid);
        mutex_exit(&zf->zf_lock);
        ZFETCHSTAT_BUMP(zfetchstat_misses);
        ipf_start = 0;
        goto prescient;

hit:
        nblks = dmu_zfetch_hit(zs, nblks);
        ZFETCHSTAT_BUMP(zfetchstat_hits);

future:
        zs->zs_atime = gethrestime_sec();

        /* Exit if we already prefetched for this position before. */
        if (nblks == 0)
                goto out;

        /* If the file is ending, remove the stream. */
        end_blkid = zs->zs_blkid;
        if (end_blkid >= maxblkid) {
                dmu_zfetch_stream_remove(zf, zs);
out:
                mutex_exit(&zf->zf_lock);
                if (!have_lock)
                        rw_exit(&zf->zf_dnode->dn_struct_rwlock);
                return (NULL);
        }

        /*
         * This access was to a block that we issued a prefetch for on
         * behalf of this stream.  Calculate further prefetch distances.
         *
         * Start prefetch from the demand access size (nblks).  Double the
         * distance every access up to zfetch_min_distance.  After that only
         * if needed increase the distance by 1/8 up to zfetch_max_distance.
         *
         * Don't double the distance beyond single block if we have more
         * than ~6% of ARC held by active prefetches.  It should help with
         * getting out of RAM on some badly mispredicted read patterns.
         */
        unsigned int nbytes = nblks << dbs;
        unsigned int pf_nblks;
        if (fetch_data) {
                if (unlikely(zs->zs_pf_dist < nbytes))
                        zs->zs_pf_dist = nbytes;
                else if (zs->zs_pf_dist < zfetch_min_distance &&
                    (zs->zs_pf_dist < (1 << dbs) ||
                    aggsum_compare(&zfetch_sums.zfetchstat_io_active,
                    arc_c_max >> (4 + dbs)) < 0))
                        zs->zs_pf_dist *= 2;
                else if (zs->zs_more)
                        zs->zs_pf_dist += zs->zs_pf_dist / 8;
                zs->zs_more = B_FALSE;
                if (zs->zs_pf_dist > zfetch_max_distance)
                        zs->zs_pf_dist = zfetch_max_distance;
                pf_nblks = zs->zs_pf_dist >> dbs;
        } else {
                pf_nblks = 0;
        }
        if (zs->zs_pf_start < end_blkid)
                zs->zs_pf_start = end_blkid;
        if (zs->zs_pf_end < end_blkid + pf_nblks)
                zs->zs_pf_end = end_blkid + pf_nblks;

        /*
         * Do the same for indirects, starting where we will stop reading
         * data blocks (and the indirects that point to them).
         */
        if (unlikely(zs->zs_ipf_dist < nbytes))
                zs->zs_ipf_dist = nbytes;
        else
                zs->zs_ipf_dist *= 2;
        if (zs->zs_ipf_dist > zfetch_max_idistance)
                zs->zs_ipf_dist = zfetch_max_idistance;
        pf_nblks = zs->zs_ipf_dist >> dbs;
        if (zs->zs_ipf_start < zs->zs_pf_end)
                zs->zs_ipf_start = zs->zs_pf_end;
        ipf_start = zs->zs_ipf_end;
        if (zs->zs_ipf_end < zs->zs_pf_end + pf_nblks)
                zs->zs_ipf_end = zs->zs_pf_end + pf_nblks;

        zfs_refcount_add(&zs->zs_refs, NULL);
        /* Count concurrent callers. */
        zfs_refcount_add(&zs->zs_callers, NULL);
        mutex_exit(&zf->zf_lock);

prescient:
        /*
         * Prefetch the following indirect blocks for this access to reduce
         * dbuf_hold() sync read delays in dmu_buf_hold_array_by_dnode().
         * This covers the gap during the first couple accesses when we can
         * not predict the future yet, but know what is needed right now.
         * This should be very rare for reads/writes to need more than one
         * indirect, but more useful for cloning due to much bigger accesses.
         */
        ipf_start = MAX(ipf_start, blkid + 1);
        int epbs = zf->zf_dnode->dn_indblkshift - SPA_BLKPTRSHIFT;
        ipf_start = P2ROUNDUP(ipf_start, 1 << epbs) >> epbs;
        ipf_end = P2ROUNDUP(end_blkid, 1 << epbs) >> epbs;

        int issued = 0;
        for (int64_t iblk = ipf_start; iblk < ipf_end; iblk++) {
                issued += dbuf_prefetch(zf->zf_dnode, 1, iblk,
                    ZIO_PRIORITY_SYNC_READ, ARC_FLAG_PRESCIENT_PREFETCH);
        }

        if (!have_lock)
                rw_exit(&zf->zf_dnode->dn_struct_rwlock);
        if (issued)
                ZFETCHSTAT_ADD(zfetchstat_io_issued, issued);
        return (zs);
}

void
dmu_zfetch_run(zfetch_t *zf, zstream_t *zs, boolean_t missed,
    boolean_t have_lock)
{
        int64_t pf_start, pf_end, ipf_start, ipf_end;
        int epbs, issued;

        if (missed)
                zs->zs_missed = missed;

        /*
         * Postpone the prefetch if there are more concurrent callers.
         * It happens when multiple requests are waiting for the same
         * indirect block.  The last one will run the prefetch for all.
         */
        if (zfs_refcount_remove(&zs->zs_callers, NULL) != 0) {
                /* Drop reference taken in dmu_zfetch_prepare(). */
                if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0)
                        dmu_zfetch_stream_fini(zs);
                return;
        }

        mutex_enter(&zf->zf_lock);
        if (zs->zs_missed) {
                pf_start = zs->zs_pf_start;
                pf_end = zs->zs_pf_start = zs->zs_pf_end;
        } else {
                pf_start = pf_end = 0;
        }
        ipf_start = zs->zs_ipf_start;
        ipf_end = zs->zs_ipf_start = zs->zs_ipf_end;
        mutex_exit(&zf->zf_lock);
        ASSERT3S(pf_start, <=, pf_end);
        ASSERT3S(ipf_start, <=, ipf_end);

        epbs = zf->zf_dnode->dn_indblkshift - SPA_BLKPTRSHIFT;
        ipf_start = P2ROUNDUP(ipf_start, 1 << epbs) >> epbs;
        ipf_end = P2ROUNDUP(ipf_end, 1 << epbs) >> epbs;
        ASSERT3S(ipf_start, <=, ipf_end);
        issued = pf_end - pf_start + ipf_end - ipf_start;
        if (issued > 1) {
                /* More references on top of taken in dmu_zfetch_prepare(). */
                zfs_refcount_add_few(&zs->zs_refs, issued - 1, NULL);
        } else if (issued == 0) {
                /* Some other thread has done our work, so drop the ref. */
                if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0)
                        dmu_zfetch_stream_fini(zs);
                return;
        }
        aggsum_add(&zfetch_sums.zfetchstat_io_active, issued);

        if (!have_lock)
                rw_enter(&zf->zf_dnode->dn_struct_rwlock, RW_READER);

        issued = 0;
        for (int64_t blk = pf_start; blk < pf_end; blk++) {
                issued += dbuf_prefetch_impl(zf->zf_dnode, 0, blk,
                    ZIO_PRIORITY_ASYNC_READ, 0, dmu_zfetch_done, zs);
        }
        for (int64_t iblk = ipf_start; iblk < ipf_end; iblk++) {
                issued += dbuf_prefetch_impl(zf->zf_dnode, 1, iblk,
                    ZIO_PRIORITY_ASYNC_READ, 0, dmu_zfetch_done, zs);
        }

        if (!have_lock)
                rw_exit(&zf->zf_dnode->dn_struct_rwlock);

        if (issued)
                ZFETCHSTAT_ADD(zfetchstat_io_issued, issued);
}

void
dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data,
    boolean_t missed, boolean_t have_lock)
{
        zstream_t *zs;

        zs = dmu_zfetch_prepare(zf, blkid, nblks, fetch_data, have_lock);
        if (zs)
                dmu_zfetch_run(zf, zs, missed, have_lock);
}

ZFS_MODULE_PARAM(zfs_prefetch, zfs_prefetch_, disable, INT, ZMOD_RW,
        "Disable all ZFS prefetching");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_streams, UINT, ZMOD_RW,
        "Max number of streams per zfetch");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, min_sec_reap, UINT, ZMOD_RW,
        "Min time before stream reclaim");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_sec_reap, UINT, ZMOD_RW,
        "Max time before stream delete");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, min_distance, UINT, ZMOD_RW,
        "Min bytes to prefetch per stream");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_distance, UINT, ZMOD_RW,
        "Max bytes to prefetch per stream");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_idistance, UINT, ZMOD_RW,
        "Max bytes to prefetch indirects for per stream");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_reorder, UINT, ZMOD_RW,
        "Max request reorder distance within a stream");

ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, hole_shift, UINT, ZMOD_RW,
        "Max log2 fraction of holes in a stream");