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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.
 */

#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;

SYSCTL_DECL(_vfs_zfs);
SYSCTL_INT(_vfs_zfs, OID_AUTO, prefetch_disable, CTLFLAG_RW,
    &zfs_prefetch_disable, 0, "Disable prefetch");
SYSCTL_NODE(_vfs_zfs, OID_AUTO, zfetch, CTLFLAG_RW, 0, "ZFS ZFETCH");
TUNABLE_INT("vfs.zfs.zfetch.max_streams", &zfetch_max_streams);
SYSCTL_UINT(_vfs_zfs_zfetch, OID_AUTO, max_streams, CTLFLAG_RW,
    &zfetch_max_streams, 0, "Max # of streams per zfetch");
TUNABLE_INT("vfs.zfs.zfetch.min_sec_reap", &zfetch_min_sec_reap);
SYSCTL_UINT(_vfs_zfs_zfetch, OID_AUTO, min_sec_reap, CTLFLAG_RDTUN,
    &zfetch_min_sec_reap, 0, "Min time before stream reclaim");
TUNABLE_INT("vfs.zfs.zfetch.block_cap", &zfetch_block_cap);
SYSCTL_UINT(_vfs_zfs_zfetch, OID_AUTO, block_cap, CTLFLAG_RDTUN,
    &zfetch_block_cap, 0, "Max number of blocks to fetch at a time");
TUNABLE_QUAD("vfs.zfs.zfetch.array_rd_sz", &zfetch_array_rd_sz);
SYSCTL_QUAD(_vfs_zfs_zfetch, OID_AUTO, array_rd_sz, CTLFLAG_RDTUN,
    &zfetch_array_rd_sz, 0,
    "Number of bytes in a array_read at which we stop prefetching");

/* forward decls for static routines */
static int        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 int        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.
 *
 * If no co-linear streams are found, return NULL.
 */
static int
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 = 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, blkid + i);
      }

      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 int
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;

                  if (mutex_tryenter(&zs->zst_lock) == 0) {
                        rc = 1;
                        goto out;
                  }
                  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;

                  if (mutex_tryenter(&zs->zst_lock) == 0) {
                        rc = 1;
                        goto out;
                  }
                  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 */

                  if (mutex_tryenter(&zs->zst_lock) == 0) {
                        rc = 1;
                        goto out;
                  }
                  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 */

                  if (mutex_tryenter(&zs->zst_lock) == 0) {
                        rc = 1;
                        goto out;
                  }
                  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 (((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;
      int         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);
            if (fetched = dmu_zfetch_colinear(zf, &zst)) {
                  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 = 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));
            }
      }
}

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