module/zfs/dmu_zfetch.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21 /*
  22  * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
  23  * Use is subject to license terms.
  24  */
  25
  26 #pragma ident   "%Z%%M% %I%     %E% SMI"
  27
  28 #include <sys/zfs_context.h>
  29 #include <sys/dnode.h>
  30 #include <sys/dmu_objset.h>
  31 #include <sys/dmu_zfetch.h>
  32 #include <sys/dmu.h>
  33 #include <sys/dbuf.h>
  34
  35 /*
  36  * I'm against tune-ables, but these should probably exist as tweakable globals
  37  * until we can get this working the way we want it to.
  38  */
  39
  40 int zfs_prefetch_disable = 0;
  41
  42 /* max # of streams per zfetch */
  43 uint32_t        zfetch_max_streams = 8;
  44 /* min time before stream reclaim */
  45 uint32_t        zfetch_min_sec_reap = 2;
  46 /* max number of blocks to fetch at a time */
  47 uint32_t        zfetch_block_cap = 256;
  48 /* number of bytes in a array_read at which we stop prefetching (1Mb) */
  49 uint64_t        zfetch_array_rd_sz = 1024 * 1024;
  50
  51 /* forward decls for static routines */
  52 static int              dmu_zfetch_colinear(zfetch_t *, zstream_t *);
  53 static void             dmu_zfetch_dofetch(zfetch_t *, zstream_t *);
  54 static uint64_t         dmu_zfetch_fetch(dnode_t *, uint64_t, uint64_t);
  55 static uint64_t         dmu_zfetch_fetchsz(dnode_t *, uint64_t, uint64_t);
  56 static int              dmu_zfetch_find(zfetch_t *, zstream_t *, int);
  57 static int              dmu_zfetch_stream_insert(zfetch_t *, zstream_t *);
  58 static zstream_t        *dmu_zfetch_stream_reclaim(zfetch_t *);
  59 static void             dmu_zfetch_stream_remove(zfetch_t *, zstream_t *);
  60 static int              dmu_zfetch_streams_equal(zstream_t *, zstream_t *);
  61
  62 /*
  63  * Given a zfetch structure and a zstream structure, determine whether the
  64  * blocks to be read are part of a co-linear pair of existing prefetch
  65  * streams.  If a set is found, coalesce the streams, removing one, and
  66  * configure the prefetch so it looks for a strided access pattern.
  67  *
  68  * In other words: if we find two sequential access streams that are
  69  * the same length and distance N appart, and this read is N from the
  70  * last stream, then we are probably in a strided access pattern.  So
  71  * combine the two sequential streams into a single strided stream.
  72  *
  73  * If no co-linear streams are found, return NULL.
  74  */
  75 static int
  76 dmu_zfetch_colinear(zfetch_t *zf, zstream_t *zh)
  77 {
  78         zstream_t       *z_walk;
  79         zstream_t       *z_comp;
  80
  81         if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
  82                 return (0);
  83
  84         if (zh == NULL) {
  85                 rw_exit(&zf->zf_rwlock);
  86                 return (0);
  87         }
  88
  89         for (z_walk = list_head(&zf->zf_stream); z_walk;
  90             z_walk = list_next(&zf->zf_stream, z_walk)) {
  91                 for (z_comp = list_next(&zf->zf_stream, z_walk); z_comp;
  92                     z_comp = list_next(&zf->zf_stream, z_comp)) {
  93                         int64_t         diff;
  94
  95                         if (z_walk->zst_len != z_walk->zst_stride ||
  96                             z_comp->zst_len != z_comp->zst_stride) {
  97                                 continue;
  98                         }
  99
 100                         diff = z_comp->zst_offset - z_walk->zst_offset;
 101                         if (z_comp->zst_offset + diff == zh->zst_offset) {
 102                                 z_walk->zst_offset = zh->zst_offset;
 103                                 z_walk->zst_direction = diff < 0 ? -1 : 1;
 104                                 z_walk->zst_stride =
 105                                     diff * z_walk->zst_direction;
 106                                 z_walk->zst_ph_offset =
 107                                     zh->zst_offset + z_walk->zst_stride;
 108                                 dmu_zfetch_stream_remove(zf, z_comp);
 109                                 mutex_destroy(&z_comp->zst_lock);
 110                                 kmem_free(z_comp, sizeof (zstream_t));
 111
 112                                 dmu_zfetch_dofetch(zf, z_walk);
 113
 114                                 rw_exit(&zf->zf_rwlock);
 115                                 return (1);
 116                         }
 117
 118                         diff = z_walk->zst_offset - z_comp->zst_offset;
 119                         if (z_walk->zst_offset + diff == zh->zst_offset) {
 120                                 z_walk->zst_offset = zh->zst_offset;
 121                                 z_walk->zst_direction = diff < 0 ? -1 : 1;
 122                                 z_walk->zst_stride =
 123                                     diff * z_walk->zst_direction;
 124                                 z_walk->zst_ph_offset =
 125                                     zh->zst_offset + z_walk->zst_stride;
 126                                 dmu_zfetch_stream_remove(zf, z_comp);
 127                                 mutex_destroy(&z_comp->zst_lock);
 128                                 kmem_free(z_comp, sizeof (zstream_t));
 129
 130                                 dmu_zfetch_dofetch(zf, z_walk);
 131
 132                                 rw_exit(&zf->zf_rwlock);
 133                                 return (1);
 134                         }
 135                 }
 136         }
 137
 138         rw_exit(&zf->zf_rwlock);
 139         return (0);
 140 }
 141
 142 /*
 143  * Given a zstream_t, determine the bounds of the prefetch.  Then call the
 144  * routine that actually prefetches the individual blocks.
 145  */
 146 static void
 147 dmu_zfetch_dofetch(zfetch_t *zf, zstream_t *zs)
 148 {
 149         uint64_t        prefetch_tail;
 150         uint64_t        prefetch_limit;
 151         uint64_t        prefetch_ofst;
 152         uint64_t        prefetch_len;
 153         uint64_t        blocks_fetched;
 154
 155         zs->zst_stride = MAX((int64_t)zs->zst_stride, zs->zst_len);
 156         zs->zst_cap = MIN(zfetch_block_cap, 2 * zs->zst_cap);
 157
 158         prefetch_tail = MAX((int64_t)zs->zst_ph_offset,
 159             (int64_t)(zs->zst_offset + zs->zst_stride));
 160         /*
 161          * XXX: use a faster division method?
 162          */
 163         prefetch_limit = zs->zst_offset + zs->zst_len +
 164             (zs->zst_cap * zs->zst_stride) / zs->zst_len;
 165
 166         while (prefetch_tail < prefetch_limit) {
 167                 prefetch_ofst = zs->zst_offset + zs->zst_direction *
 168                     (prefetch_tail - zs->zst_offset);
 169
 170                 prefetch_len = zs->zst_len;
 171
 172                 /*
 173                  * Don't prefetch beyond the end of the file, if working
 174                  * backwards.
 175                  */
 176                 if ((zs->zst_direction == ZFETCH_BACKWARD) &&
 177                     (prefetch_ofst > prefetch_tail)) {
 178                         prefetch_len += prefetch_ofst;
 179                         prefetch_ofst = 0;
 180                 }
 181
 182                 /* don't prefetch more than we're supposed to */
 183                 if (prefetch_len > zs->zst_len)
 184                         break;
 185
 186                 blocks_fetched = dmu_zfetch_fetch(zf->zf_dnode,
 187                     prefetch_ofst, zs->zst_len);
 188
 189                 prefetch_tail += zs->zst_stride;
 190                 /* stop if we've run out of stuff to prefetch */
 191                 if (blocks_fetched < zs->zst_len)
 192                         break;
 193         }
 194         zs->zst_ph_offset = prefetch_tail;
 195         zs->zst_last = lbolt;
 196 }
 197
 198 /*
 199  * This takes a pointer to a zfetch structure and a dnode.  It performs the
 200  * necessary setup for the zfetch structure, grokking data from the
 201  * associated dnode.
 202  */
 203 void
 204 dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
 205 {
 206         if (zf == NULL) {
 207                 return;
 208         }
 209
 210         zf->zf_dnode = dno;
 211         zf->zf_stream_cnt = 0;
 212         zf->zf_alloc_fail = 0;
 213
 214         list_create(&zf->zf_stream, sizeof (zstream_t),
 215             offsetof(zstream_t, zst_node));
 216
 217         rw_init(&zf->zf_rwlock, NULL, RW_DEFAULT, NULL);
 218 }
 219
 220 /*
 221  * This function computes the actual size, in blocks, that can be prefetched,
 222  * and fetches it.
 223  */
 224 static uint64_t
 225 dmu_zfetch_fetch(dnode_t *dn, uint64_t blkid, uint64_t nblks)
 226 {
 227         uint64_t        fetchsz;
 228         uint64_t        i;
 229
 230         fetchsz = dmu_zfetch_fetchsz(dn, blkid, nblks);
 231
 232         for (i = 0; i < fetchsz; i++) {
 233                 dbuf_prefetch(dn, blkid + i);
 234         }
 235
 236         return (fetchsz);
 237 }
 238
 239 /*
 240  * this function returns the number of blocks that would be prefetched, based
 241  * upon the supplied dnode, blockid, and nblks.  This is used so that we can
 242  * update streams in place, and then prefetch with their old value after the
 243  * fact.  This way, we can delay the prefetch, but subsequent accesses to the
 244  * stream won't result in the same data being prefetched multiple times.
 245  */
 246 static uint64_t
 247 dmu_zfetch_fetchsz(dnode_t *dn, uint64_t blkid, uint64_t nblks)
 248 {
 249         uint64_t        fetchsz;
 250
 251         if (blkid > dn->dn_maxblkid) {
 252                 return (0);
 253         }
 254
 255         /* compute fetch size */
 256         if (blkid + nblks + 1 > dn->dn_maxblkid) {
 257                 fetchsz = (dn->dn_maxblkid - blkid) + 1;
 258                 ASSERT(blkid + fetchsz - 1 <= dn->dn_maxblkid);
 259         } else {
 260                 fetchsz = nblks;
 261         }
 262
 263
 264         return (fetchsz);
 265 }
 266
 267 /*
 268  * given a zfetch and a zsearch structure, see if there is an associated zstream
 269  * for this block read.  If so, it starts a prefetch for the stream it
 270  * located and returns true, otherwise it returns false
 271  */
 272 static int
 273 dmu_zfetch_find(zfetch_t *zf, zstream_t *zh, int prefetched)
 274 {
 275         zstream_t       *zs;
 276         int64_t         diff;
 277         int             reset = !prefetched;
 278         int             rc = 0;
 279
 280         if (zh == NULL)
 281                 return (0);
 282
 283         /*
 284          * XXX: This locking strategy is a bit coarse; however, it's impact has
 285          * yet to be tested.  If this turns out to be an issue, it can be
 286          * modified in a number of different ways.
 287          */
 288
 289         rw_enter(&zf->zf_rwlock, RW_READER);
 290 top:
 291
 292         for (zs = list_head(&zf->zf_stream); zs;
 293             zs = list_next(&zf->zf_stream, zs)) {
 294
 295                 /*
 296                  * XXX - should this be an assert?
 297                  */
 298                 if (zs->zst_len == 0) {
 299                         /* bogus stream */
 300                         continue;
 301                 }
 302
 303                 /*
 304                  * We hit this case when we are in a strided prefetch stream:
 305                  * we will read "len" blocks before "striding".
 306                  */
 307                 if (zh->zst_offset >= zs->zst_offset &&
 308                     zh->zst_offset < zs->zst_offset + zs->zst_len) {
 309                         /* already fetched */
 310                         rc = 1;
 311                         goto out;
 312                 }
 313
 314                 /*
 315                  * This is the forward sequential read case: we increment
 316                  * len by one each time we hit here, so we will enter this
 317                  * case on every read.
 318                  */
 319                 if (zh->zst_offset == zs->zst_offset + zs->zst_len) {
 320
 321                         reset = !prefetched && zs->zst_len > 1;
 322
 323                         mutex_enter(&zs->zst_lock);
 324
 325                         if (zh->zst_offset != zs->zst_offset + zs->zst_len) {
 326                                 mutex_exit(&zs->zst_lock);
 327                                 goto top;
 328                         }
 329                         zs->zst_len += zh->zst_len;
 330                         diff = zs->zst_len - zfetch_block_cap;
 331                         if (diff > 0) {
 332                                 zs->zst_offset += diff;
 333                                 zs->zst_len = zs->zst_len > diff ?
 334                                     zs->zst_len - diff : 0;
 335                         }
 336                         zs->zst_direction = ZFETCH_FORWARD;
 337
 338                         break;
 339
 340                 /*
 341                  * Same as above, but reading backwards through the file.
 342                  */
 343                 } else if (zh->zst_offset == zs->zst_offset - zh->zst_len) {
 344                         /* backwards sequential access */
 345
 346                         reset = !prefetched && zs->zst_len > 1;
 347
 348                         mutex_enter(&zs->zst_lock);
 349
 350                         if (zh->zst_offset != zs->zst_offset - zh->zst_len) {
 351                                 mutex_exit(&zs->zst_lock);
 352                                 goto top;
 353                         }
 354
 355                         zs->zst_offset = zs->zst_offset > zh->zst_len ?
 356                             zs->zst_offset - zh->zst_len : 0;
 357                         zs->zst_ph_offset = zs->zst_ph_offset > zh->zst_len ?
 358                             zs->zst_ph_offset - zh->zst_len : 0;
 359                         zs->zst_len += zh->zst_len;
 360
 361                         diff = zs->zst_len - zfetch_block_cap;
 362                         if (diff > 0) {
 363                                 zs->zst_ph_offset = zs->zst_ph_offset > diff ?
 364                                     zs->zst_ph_offset - diff : 0;
 365                                 zs->zst_len = zs->zst_len > diff ?
 366                                     zs->zst_len - diff : zs->zst_len;
 367                         }
 368                         zs->zst_direction = ZFETCH_BACKWARD;
 369
 370                         break;
 371
 372                 } else if ((zh->zst_offset - zs->zst_offset - zs->zst_stride <
 373                     zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
 374                         /* strided forward access */
 375
 376                         mutex_enter(&zs->zst_lock);
 377
 378                         if ((zh->zst_offset - zs->zst_offset - zs->zst_stride >=
 379                             zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
 380                                 mutex_exit(&zs->zst_lock);
 381                                 goto top;
 382                         }
 383
 384                         zs->zst_offset += zs->zst_stride;
 385                         zs->zst_direction = ZFETCH_FORWARD;
 386
 387                         break;
 388
 389                 } else if ((zh->zst_offset - zs->zst_offset + zs->zst_stride <
 390                     zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
 391                         /* strided reverse access */
 392
 393                         mutex_enter(&zs->zst_lock);
 394
 395                         if ((zh->zst_offset - zs->zst_offset + zs->zst_stride >=
 396                             zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
 397                                 mutex_exit(&zs->zst_lock);
 398                                 goto top;
 399                         }
 400
 401                         zs->zst_offset = zs->zst_offset > zs->zst_stride ?
 402                             zs->zst_offset - zs->zst_stride : 0;
 403                         zs->zst_ph_offset = (zs->zst_ph_offset >
 404                             (2 * zs->zst_stride)) ?
 405                             (zs->zst_ph_offset - (2 * zs->zst_stride)) : 0;
 406                         zs->zst_direction = ZFETCH_BACKWARD;
 407
 408                         break;
 409                 }
 410         }
 411
 412         if (zs) {
 413                 if (reset) {
 414                         zstream_t *remove = zs;
 415
 416                         rc = 0;
 417                         mutex_exit(&zs->zst_lock);
 418                         rw_exit(&zf->zf_rwlock);
 419                         rw_enter(&zf->zf_rwlock, RW_WRITER);
 420                         /*
 421                          * Relocate the stream, in case someone removes
 422                          * it while we were acquiring the WRITER lock.
 423                          */
 424                         for (zs = list_head(&zf->zf_stream); zs;
 425                             zs = list_next(&zf->zf_stream, zs)) {
 426                                 if (zs == remove) {
 427                                         dmu_zfetch_stream_remove(zf, zs);
 428                                         mutex_destroy(&zs->zst_lock);
 429                                         kmem_free(zs, sizeof (zstream_t));
 430                                         break;
 431                                 }
 432                         }
 433                 } else {
 434                         rc = 1;
 435                         dmu_zfetch_dofetch(zf, zs);
 436                         mutex_exit(&zs->zst_lock);
 437                 }
 438         }
 439 out:
 440         rw_exit(&zf->zf_rwlock);
 441         return (rc);
 442 }
 443
 444 /*
 445  * Clean-up state associated with a zfetch structure.  This frees allocated
 446  * structure members, empties the zf_stream tree, and generally makes things
 447  * nice.  This doesn't free the zfetch_t itself, that's left to the caller.
 448  */
 449 void
 450 dmu_zfetch_rele(zfetch_t *zf)
 451 {
 452         zstream_t       *zs;
 453         zstream_t       *zs_next;
 454
 455         ASSERT(!RW_LOCK_HELD(&zf->zf_rwlock));
 456
 457         for (zs = list_head(&zf->zf_stream); zs; zs = zs_next) {
 458                 zs_next = list_next(&zf->zf_stream, zs);
 459
 460                 list_remove(&zf->zf_stream, zs);
 461                 mutex_destroy(&zs->zst_lock);
 462                 kmem_free(zs, sizeof (zstream_t));
 463         }
 464         list_destroy(&zf->zf_stream);
 465         rw_destroy(&zf->zf_rwlock);
 466
 467         zf->zf_dnode = NULL;
 468 }
 469
 470 /*
 471  * Given a zfetch and zstream structure, insert the zstream structure into the
 472  * AVL tree contained within the zfetch structure.  Peform the appropriate
 473  * book-keeping.  It is possible that another thread has inserted a stream which
 474  * matches one that we are about to insert, so we must be sure to check for this
 475  * case.  If one is found, return failure, and let the caller cleanup the
 476  * duplicates.
 477  */
 478 static int
 479 dmu_zfetch_stream_insert(zfetch_t *zf, zstream_t *zs)
 480 {
 481         zstream_t       *zs_walk;
 482         zstream_t       *zs_next;
 483
 484         ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
 485
 486         for (zs_walk = list_head(&zf->zf_stream); zs_walk; zs_walk = zs_next) {
 487                 zs_next = list_next(&zf->zf_stream, zs_walk);
 488
 489                 if (dmu_zfetch_streams_equal(zs_walk, zs)) {
 490                     return (0);
 491                 }
 492         }
 493
 494         list_insert_head(&zf->zf_stream, zs);
 495         zf->zf_stream_cnt++;
 496
 497         return (1);
 498 }
 499
 500
 501 /*
 502  * Walk the list of zstreams in the given zfetch, find an old one (by time), and
 503  * reclaim it for use by the caller.
 504  */
 505 static zstream_t *
 506 dmu_zfetch_stream_reclaim(zfetch_t *zf)
 507 {
 508         zstream_t       *zs;
 509
 510         if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
 511                 return (0);
 512
 513         for (zs = list_head(&zf->zf_stream); zs;
 514             zs = list_next(&zf->zf_stream, zs)) {
 515
 516                 if (((lbolt - zs->zst_last) / hz) > zfetch_min_sec_reap)
 517                         break;
 518         }
 519
 520         if (zs) {
 521                 dmu_zfetch_stream_remove(zf, zs);
 522                 mutex_destroy(&zs->zst_lock);
 523                 bzero(zs, sizeof (zstream_t));
 524         } else {
 525                 zf->zf_alloc_fail++;
 526         }
 527         rw_exit(&zf->zf_rwlock);
 528
 529         return (zs);
 530 }
 531
 532 /*
 533  * Given a zfetch and zstream structure, remove the zstream structure from its
 534  * container in the zfetch structure.  Perform the appropriate book-keeping.
 535  */
 536 static void
 537 dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
 538 {
 539         ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
 540
 541         list_remove(&zf->zf_stream, zs);
 542         zf->zf_stream_cnt--;
 543 }
 544
 545 static int
 546 dmu_zfetch_streams_equal(zstream_t *zs1, zstream_t *zs2)
 547 {
 548         if (zs1->zst_offset != zs2->zst_offset)
 549                 return (0);
 550
 551         if (zs1->zst_len != zs2->zst_len)
 552                 return (0);
 553
 554         if (zs1->zst_stride != zs2->zst_stride)
 555                 return (0);
 556
 557         if (zs1->zst_ph_offset != zs2->zst_ph_offset)
 558                 return (0);
 559
 560         if (zs1->zst_cap != zs2->zst_cap)
 561                 return (0);
 562
 563         if (zs1->zst_direction != zs2->zst_direction)
 564                 return (0);
 565
 566         return (1);
 567 }
 568
 569 /*
 570  * This is the prefetch entry point.  It calls all of the other dmu_zfetch
 571  * routines to create, delete, find, or operate upon prefetch streams.
 572  */
 573 void
 574 dmu_zfetch(zfetch_t *zf, uint64_t offset, uint64_t size, int prefetched)
 575 {
 576         zstream_t       zst;
 577         zstream_t       *newstream;
 578         int             fetched;
 579         int             inserted;
 580         unsigned int    blkshft;
 581         uint64_t        blksz;
 582
 583         if (zfs_prefetch_disable)
 584                 return;
 585
 586         /* files that aren't ln2 blocksz are only one block -- nothing to do */
 587         if (!zf->zf_dnode->dn_datablkshift)
 588                 return;
 589
 590         /* convert offset and size, into blockid and nblocks */
 591         blkshft = zf->zf_dnode->dn_datablkshift;
 592         blksz = (1 << blkshft);
 593
 594         bzero(&zst, sizeof (zstream_t));
 595         zst.zst_offset = offset >> blkshft;
 596         zst.zst_len = (P2ROUNDUP(offset + size, blksz) -
 597             P2ALIGN(offset, blksz)) >> blkshft;
 598
 599         fetched = dmu_zfetch_find(zf, &zst, prefetched);
 600         if (!fetched) {
 601                 fetched = dmu_zfetch_colinear(zf, &zst);
 602         }
 603
 604         if (!fetched) {
 605                 newstream = dmu_zfetch_stream_reclaim(zf);
 606
 607                 /*
 608                  * we still couldn't find a stream, drop the lock, and allocate
 609                  * one if possible.  Otherwise, give up and go home.
 610                  */
 611                 if (newstream == NULL) {
 612                         uint64_t        maxblocks;
 613                         uint32_t        max_streams;
 614                         uint32_t        cur_streams;
 615
 616                         cur_streams = zf->zf_stream_cnt;
 617                         maxblocks = zf->zf_dnode->dn_maxblkid;
 618
 619                         max_streams = MIN(zfetch_max_streams,
 620                             (maxblocks / zfetch_block_cap));
 621                         if (max_streams == 0) {
 622                                 max_streams++;
 623                         }
 624
 625                         if (cur_streams >= max_streams) {
 626                                 return;
 627                         }
 628
 629                         newstream = kmem_zalloc(sizeof (zstream_t), KM_SLEEP);
 630                 }
 631
 632                 newstream->zst_offset = zst.zst_offset;
 633                 newstream->zst_len = zst.zst_len;
 634                 newstream->zst_stride = zst.zst_len;
 635                 newstream->zst_ph_offset = zst.zst_len + zst.zst_offset;
 636                 newstream->zst_cap = zst.zst_len;
 637                 newstream->zst_direction = ZFETCH_FORWARD;
 638                 newstream->zst_last = lbolt;
 639
 640                 mutex_init(&newstream->zst_lock, NULL, MUTEX_DEFAULT, NULL);
 641
 642                 rw_enter(&zf->zf_rwlock, RW_WRITER);
 643                 inserted = dmu_zfetch_stream_insert(zf, newstream);
 644                 rw_exit(&zf->zf_rwlock);
 645
 646                 if (!inserted) {
 647                         mutex_destroy(&newstream->zst_lock);
 648                         kmem_free(newstream, sizeof (zstream_t));
 649                 }
 650         }
 651 }