sys/cddl/contrib/opensolaris/uts/common/fs/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 SYSCTL_DECL(_vfs_zfs);
  42 TUNABLE_INT("vfs.zfs.prefetch_disable", &zfs_prefetch_disable);
  43 SYSCTL_INT(_vfs_zfs, OID_AUTO, prefetch_disable, CTLFLAG_RDTUN,
  44     &zfs_prefetch_disable, 0, "Disable prefetch");
  45
  46 /* max # of streams per zfetch */
  47 uint32_t        zfetch_max_streams = 8;
  48 /* min time before stream reclaim */
  49 uint32_t        zfetch_min_sec_reap = 2;
  50 /* max number of blocks to fetch at a time */
  51 uint32_t        zfetch_block_cap = 256;
  52 /* number of bytes in a array_read at which we stop prefetching (1Mb) */
  53 uint64_t        zfetch_array_rd_sz = 1024 * 1024;
  54
  55 /* forward decls for static routines */
  56 static int              dmu_zfetch_colinear(zfetch_t *, zstream_t *);
  57 static void             dmu_zfetch_dofetch(zfetch_t *, zstream_t *);
  58 static uint64_t         dmu_zfetch_fetch(dnode_t *, uint64_t, uint64_t);
  59 static uint64_t         dmu_zfetch_fetchsz(dnode_t *, uint64_t, uint64_t);
  60 static int              dmu_zfetch_find(zfetch_t *, zstream_t *, int);
  61 static int              dmu_zfetch_stream_insert(zfetch_t *, zstream_t *);
  62 static zstream_t        *dmu_zfetch_stream_reclaim(zfetch_t *);
  63 static void             dmu_zfetch_stream_remove(zfetch_t *, zstream_t *);
  64 static int              dmu_zfetch_streams_equal(zstream_t *, zstream_t *);
  65
  66 /*
  67  * Given a zfetch structure and a zstream structure, determine whether the
  68  * blocks to be read are part of a co-linear pair of existing prefetch
  69  * streams.  If a set is found, coalesce the streams, removing one, and
  70  * configure the prefetch so it looks for a strided access pattern.
  71  *
  72  * In other words: if we find two sequential access streams that are
  73  * the same length and distance N appart, and this read is N from the
  74  * last stream, then we are probably in a strided access pattern.  So
  75  * combine the two sequential streams into a single strided stream.
  76  *
  77  * If no co-linear streams are found, return NULL.
  78  */
  79 static int
  80 dmu_zfetch_colinear(zfetch_t *zf, zstream_t *zh)
  81 {
  82         zstream_t       *z_walk;
  83         zstream_t       *z_comp;
  84
  85         if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
  86                 return (0);
  87
  88         if (zh == NULL) {
  89                 rw_exit(&zf->zf_rwlock);
  90                 return (0);
  91         }
  92
  93         for (z_walk = list_head(&zf->zf_stream); z_walk;
  94             z_walk = list_next(&zf->zf_stream, z_walk)) {
  95                 for (z_comp = list_next(&zf->zf_stream, z_walk); z_comp;
  96                     z_comp = list_next(&zf->zf_stream, z_comp)) {
  97                         int64_t         diff;
  98
  99                         if (z_walk->zst_len != z_walk->zst_stride ||
 100                             z_comp->zst_len != z_comp->zst_stride) {
 101                                 continue;
 102                         }
 103
 104                         diff = z_comp->zst_offset - z_walk->zst_offset;
 105                         if (z_comp->zst_offset + diff == zh->zst_offset) {
 106                                 z_walk->zst_offset = zh->zst_offset;
 107                                 z_walk->zst_direction = diff < 0 ? -1 : 1;
 108                                 z_walk->zst_stride =
 109                                     diff * z_walk->zst_direction;
 110                                 z_walk->zst_ph_offset =
 111                                     zh->zst_offset + z_walk->zst_stride;
 112                                 dmu_zfetch_stream_remove(zf, z_comp);
 113                                 mutex_destroy(&z_comp->zst_lock);
 114                                 kmem_free(z_comp, sizeof (zstream_t));
 115
 116                                 dmu_zfetch_dofetch(zf, z_walk);
 117
 118                                 rw_exit(&zf->zf_rwlock);
 119                                 return (1);
 120                         }
 121
 122                         diff = z_walk->zst_offset - z_comp->zst_offset;
 123                         if (z_walk->zst_offset + diff == zh->zst_offset) {
 124                                 z_walk->zst_offset = zh->zst_offset;
 125                                 z_walk->zst_direction = diff < 0 ? -1 : 1;
 126                                 z_walk->zst_stride =
 127                                     diff * z_walk->zst_direction;
 128                                 z_walk->zst_ph_offset =
 129                                     zh->zst_offset + z_walk->zst_stride;
 130                                 dmu_zfetch_stream_remove(zf, z_comp);
 131                                 mutex_destroy(&z_comp->zst_lock);
 132                                 kmem_free(z_comp, sizeof (zstream_t));
 133
 134                                 dmu_zfetch_dofetch(zf, z_walk);
 135
 136                                 rw_exit(&zf->zf_rwlock);
 137                                 return (1);
 138                         }
 139                 }
 140         }
 141
 142         rw_exit(&zf->zf_rwlock);
 143         return (0);
 144 }
 145
 146 /*
 147  * Given a zstream_t, determine the bounds of the prefetch.  Then call the
 148  * routine that actually prefetches the individual blocks.
 149  */
 150 static void
 151 dmu_zfetch_dofetch(zfetch_t *zf, zstream_t *zs)
 152 {
 153         uint64_t        prefetch_tail;
 154         uint64_t        prefetch_limit;
 155         uint64_t        prefetch_ofst;
 156         uint64_t        prefetch_len;
 157         uint64_t        blocks_fetched;
 158
 159         zs->zst_stride = MAX((int64_t)zs->zst_stride, zs->zst_len);
 160         zs->zst_cap = MIN(zfetch_block_cap, 2 * zs->zst_cap);
 161
 162         prefetch_tail = MAX((int64_t)zs->zst_ph_offset,
 163             (int64_t)(zs->zst_offset + zs->zst_stride));
 164         /*
 165          * XXX: use a faster division method?
 166          */
 167         prefetch_limit = zs->zst_offset + zs->zst_len +
 168             (zs->zst_cap * zs->zst_stride) / zs->zst_len;
 169
 170         while (prefetch_tail < prefetch_limit) {
 171                 prefetch_ofst = zs->zst_offset + zs->zst_direction *
 172                     (prefetch_tail - zs->zst_offset);
 173
 174                 prefetch_len = zs->zst_len;
 175
 176                 /*
 177                  * Don't prefetch beyond the end of the file, if working
 178                  * backwards.
 179                  */
 180                 if ((zs->zst_direction == ZFETCH_BACKWARD) &&
 181                     (prefetch_ofst > prefetch_tail)) {
 182                         prefetch_len += prefetch_ofst;
 183                         prefetch_ofst = 0;
 184                 }
 185
 186                 /* don't prefetch more than we're supposed to */
 187                 if (prefetch_len > zs->zst_len)
 188                         break;
 189
 190                 blocks_fetched = dmu_zfetch_fetch(zf->zf_dnode,
 191                     prefetch_ofst, zs->zst_len);
 192
 193                 prefetch_tail += zs->zst_stride;
 194                 /* stop if we've run out of stuff to prefetch */
 195                 if (blocks_fetched < zs->zst_len)
 196                         break;
 197         }
 198         zs->zst_ph_offset = prefetch_tail;
 199         zs->zst_last = LBOLT;
 200 }
 201
 202 /*
 203  * This takes a pointer to a zfetch structure and a dnode.  It performs the
 204  * necessary setup for the zfetch structure, grokking data from the
 205  * associated dnode.
 206  */
 207 void
 208 dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
 209 {
 210         if (zf == NULL) {
 211                 return;
 212         }
 213
 214         zf->zf_dnode = dno;
 215         zf->zf_stream_cnt = 0;
 216         zf->zf_alloc_fail = 0;
 217
 218         list_create(&zf->zf_stream, sizeof (zstream_t),
 219             offsetof(zstream_t, zst_node));
 220
 221         rw_init(&zf->zf_rwlock, NULL, RW_DEFAULT, NULL);
 222 }
 223
 224 /*
 225  * This function computes the actual size, in blocks, that can be prefetched,
 226  * and fetches it.
 227  */
 228 static uint64_t
 229 dmu_zfetch_fetch(dnode_t *dn, uint64_t blkid, uint64_t nblks)
 230 {
 231         uint64_t        fetchsz;
 232         uint64_t        i;
 233
 234         fetchsz = dmu_zfetch_fetchsz(dn, blkid, nblks);
 235
 236         for (i = 0; i < fetchsz; i++) {
 237                 dbuf_prefetch(dn, blkid + i);
 238         }
 239
 240         return (fetchsz);
 241 }
 242
 243 /*
 244  * this function returns the number of blocks that would be prefetched, based
 245  * upon the supplied dnode, blockid, and nblks.  This is used so that we can
 246  * update streams in place, and then prefetch with their old value after the
 247  * fact.  This way, we can delay the prefetch, but subsequent accesses to the
 248  * stream won't result in the same data being prefetched multiple times.
 249  */
 250 static uint64_t
 251 dmu_zfetch_fetchsz(dnode_t *dn, uint64_t blkid, uint64_t nblks)
 252 {
 253         uint64_t        fetchsz;
 254
 255         if (blkid > dn->dn_maxblkid) {
 256                 return (0);
 257         }
 258
 259         /* compute fetch size */
 260         if (blkid + nblks + 1 > dn->dn_maxblkid) {
 261                 fetchsz = (dn->dn_maxblkid - blkid) + 1;
 262                 ASSERT(blkid + fetchsz - 1 <= dn->dn_maxblkid);
 263         } else {
 264                 fetchsz = nblks;
 265         }
 266
 267
 268         return (fetchsz);
 269 }
 270
 271 /*
 272  * given a zfetch and a zsearch structure, see if there is an associated zstream
 273  * for this block read.  If so, it starts a prefetch for the stream it
 274  * located and returns true, otherwise it returns false
 275  */
 276 static int
 277 dmu_zfetch_find(zfetch_t *zf, zstream_t *zh, int prefetched)
 278 {
 279         zstream_t       *zs;
 280         int64_t         diff;
 281         int             reset = !prefetched;
 282         int             rc = 0;
 283
 284         if (zh == NULL)
 285                 return (0);
 286
 287         /*
 288          * XXX: This locking strategy is a bit coarse; however, it's impact has
 289          * yet to be tested.  If this turns out to be an issue, it can be
 290          * modified in a number of different ways.
 291          */
 292
 293         rw_enter(&zf->zf_rwlock, RW_READER);
 294 top:
 295
 296         for (zs = list_head(&zf->zf_stream); zs;
 297             zs = list_next(&zf->zf_stream, zs)) {
 298
 299                 /*
 300                  * XXX - should this be an assert?
 301                  */
 302                 if (zs->zst_len == 0) {
 303                         /* bogus stream */
 304                         continue;
 305                 }
 306
 307                 /*
 308                  * We hit this case when we are in a strided prefetch stream:
 309                  * we will read "len" blocks before "striding".
 310                  */
 311                 if (zh->zst_offset >= zs->zst_offset &&
 312                     zh->zst_offset < zs->zst_offset + zs->zst_len) {
 313                         /* already fetched */
 314                         rc = 1;
 315                         goto out;
 316                 }
 317
 318                 /*
 319                  * This is the forward sequential read case: we increment
 320                  * len by one each time we hit here, so we will enter this
 321                  * case on every read.
 322                  */
 323                 if (zh->zst_offset == zs->zst_offset + zs->zst_len) {
 324
 325                         reset = !prefetched && zs->zst_len > 1;
 326
 327                         mutex_enter(&zs->zst_lock);
 328
 329                         if (zh->zst_offset != zs->zst_offset + zs->zst_len) {
 330                                 mutex_exit(&zs->zst_lock);
 331                                 goto top;
 332                         }
 333                         zs->zst_len += zh->zst_len;
 334                         diff = zs->zst_len - zfetch_block_cap;
 335                         if (diff > 0) {
 336                                 zs->zst_offset += diff;
 337                                 zs->zst_len = zs->zst_len > diff ?
 338                                     zs->zst_len - diff : 0;
 339                         }
 340                         zs->zst_direction = ZFETCH_FORWARD;
 341
 342                         break;
 343
 344                 /*
 345                  * Same as above, but reading backwards through the file.
 346                  */
 347                 } else if (zh->zst_offset == zs->zst_offset - zh->zst_len) {
 348                         /* backwards sequential access */
 349
 350                         reset = !prefetched && zs->zst_len > 1;
 351
 352                         mutex_enter(&zs->zst_lock);
 353
 354                         if (zh->zst_offset != zs->zst_offset - zh->zst_len) {
 355                                 mutex_exit(&zs->zst_lock);
 356                                 goto top;
 357                         }
 358
 359                         zs->zst_offset = zs->zst_offset > zh->zst_len ?
 360                             zs->zst_offset - zh->zst_len : 0;
 361                         zs->zst_ph_offset = zs->zst_ph_offset > zh->zst_len ?
 362                             zs->zst_ph_offset - zh->zst_len : 0;
 363                         zs->zst_len += zh->zst_len;
 364
 365                         diff = zs->zst_len - zfetch_block_cap;
 366                         if (diff > 0) {
 367                                 zs->zst_ph_offset = zs->zst_ph_offset > diff ?
 368                                     zs->zst_ph_offset - diff : 0;
 369                                 zs->zst_len = zs->zst_len > diff ?
 370                                     zs->zst_len - diff : zs->zst_len;
 371                         }
 372                         zs->zst_direction = ZFETCH_BACKWARD;
 373
 374                         break;
 375
 376                 } else if ((zh->zst_offset - zs->zst_offset - zs->zst_stride <
 377                     zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
 378                         /* strided forward access */
 379
 380                         mutex_enter(&zs->zst_lock);
 381
 382                         if ((zh->zst_offset - zs->zst_offset - zs->zst_stride >=
 383                             zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
 384                                 mutex_exit(&zs->zst_lock);
 385                                 goto top;
 386                         }
 387
 388                         zs->zst_offset += zs->zst_stride;
 389                         zs->zst_direction = ZFETCH_FORWARD;
 390
 391                         break;
 392
 393                 } else if ((zh->zst_offset - zs->zst_offset + zs->zst_stride <
 394                     zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
 395                         /* strided reverse access */
 396
 397                         mutex_enter(&zs->zst_lock);
 398
 399                         if ((zh->zst_offset - zs->zst_offset + zs->zst_stride >=
 400                             zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
 401                                 mutex_exit(&zs->zst_lock);
 402                                 goto top;
 403                         }
 404
 405                         zs->zst_offset = zs->zst_offset > zs->zst_stride ?
 406                             zs->zst_offset - zs->zst_stride : 0;
 407                         zs->zst_ph_offset = (zs->zst_ph_offset >
 408                             (2 * zs->zst_stride)) ?
 409                             (zs->zst_ph_offset - (2 * zs->zst_stride)) : 0;
 410                         zs->zst_direction = ZFETCH_BACKWARD;
 411
 412                         break;
 413                 }
 414         }
 415
 416         if (zs) {
 417                 if (reset) {
 418                         zstream_t *remove = zs;
 419
 420                         rc = 0;
 421                         mutex_exit(&zs->zst_lock);
 422                         rw_exit(&zf->zf_rwlock);
 423                         rw_enter(&zf->zf_rwlock, RW_WRITER);
 424                         /*
 425                          * Relocate the stream, in case someone removes
 426                          * it while we were acquiring the WRITER lock.
 427                          */
 428                         for (zs = list_head(&zf->zf_stream); zs;
 429                             zs = list_next(&zf->zf_stream, zs)) {
 430                                 if (zs == remove) {
 431                                         dmu_zfetch_stream_remove(zf, zs);
 432                                         mutex_destroy(&zs->zst_lock);
 433                                         kmem_free(zs, sizeof (zstream_t));
 434                                         break;
 435                                 }
 436                         }
 437                 } else {
 438                         rc = 1;
 439                         dmu_zfetch_dofetch(zf, zs);
 440                         mutex_exit(&zs->zst_lock);
 441                 }
 442         }
 443 out:
 444         rw_exit(&zf->zf_rwlock);
 445         return (rc);
 446 }
 447
 448 /*
 449  * Clean-up state associated with a zfetch structure.  This frees allocated
 450  * structure members, empties the zf_stream tree, and generally makes things
 451  * nice.  This doesn't free the zfetch_t itself, that's left to the caller.
 452  */
 453 void
 454 dmu_zfetch_rele(zfetch_t *zf)
 455 {
 456         zstream_t       *zs;
 457         zstream_t       *zs_next;
 458
 459         ASSERT(!RW_LOCK_HELD(&zf->zf_rwlock));
 460
 461         for (zs = list_head(&zf->zf_stream); zs; zs = zs_next) {
 462                 zs_next = list_next(&zf->zf_stream, zs);
 463
 464                 list_remove(&zf->zf_stream, zs);
 465                 mutex_destroy(&zs->zst_lock);
 466                 kmem_free(zs, sizeof (zstream_t));
 467         }
 468         list_destroy(&zf->zf_stream);
 469         rw_destroy(&zf->zf_rwlock);
 470
 471         zf->zf_dnode = NULL;
 472 }
 473
 474 /*
 475  * Given a zfetch and zstream structure, insert the zstream structure into the
 476  * AVL tree contained within the zfetch structure.  Peform the appropriate
 477  * book-keeping.  It is possible that another thread has inserted a stream which
 478  * matches one that we are about to insert, so we must be sure to check for this
 479  * case.  If one is found, return failure, and let the caller cleanup the
 480  * duplicates.
 481  */
 482 static int
 483 dmu_zfetch_stream_insert(zfetch_t *zf, zstream_t *zs)
 484 {
 485         zstream_t       *zs_walk;
 486         zstream_t       *zs_next;
 487
 488         ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
 489
 490         for (zs_walk = list_head(&zf->zf_stream); zs_walk; zs_walk = zs_next) {
 491                 zs_next = list_next(&zf->zf_stream, zs_walk);
 492
 493                 if (dmu_zfetch_streams_equal(zs_walk, zs)) {
 494                     return (0);
 495                 }
 496         }
 497
 498         list_insert_head(&zf->zf_stream, zs);
 499         zf->zf_stream_cnt++;
 500
 501         return (1);
 502 }
 503
 504
 505 /*
 506  * Walk the list of zstreams in the given zfetch, find an old one (by time), and
 507  * reclaim it for use by the caller.
 508  */
 509 static zstream_t *
 510 dmu_zfetch_stream_reclaim(zfetch_t *zf)
 511 {
 512         zstream_t       *zs;
 513
 514         if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
 515                 return (0);
 516
 517         for (zs = list_head(&zf->zf_stream); zs;
 518             zs = list_next(&zf->zf_stream, zs)) {
 519
 520                 if (((LBOLT - zs->zst_last) / hz) > zfetch_min_sec_reap)
 521                         break;
 522         }
 523
 524         if (zs) {
 525                 dmu_zfetch_stream_remove(zf, zs);
 526                 mutex_destroy(&zs->zst_lock);
 527                 bzero(zs, sizeof (zstream_t));
 528         } else {
 529                 zf->zf_alloc_fail++;
 530         }
 531         rw_exit(&zf->zf_rwlock);
 532
 533         return (zs);
 534 }
 535
 536 /*
 537  * Given a zfetch and zstream structure, remove the zstream structure from its
 538  * container in the zfetch structure.  Perform the appropriate book-keeping.
 539  */
 540 static void
 541 dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
 542 {
 543         ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
 544
 545         list_remove(&zf->zf_stream, zs);
 546         zf->zf_stream_cnt--;
 547 }
 548
 549 static int
 550 dmu_zfetch_streams_equal(zstream_t *zs1, zstream_t *zs2)
 551 {
 552         if (zs1->zst_offset != zs2->zst_offset)
 553                 return (0);
 554
 555         if (zs1->zst_len != zs2->zst_len)
 556                 return (0);
 557
 558         if (zs1->zst_stride != zs2->zst_stride)
 559                 return (0);
 560
 561         if (zs1->zst_ph_offset != zs2->zst_ph_offset)
 562                 return (0);
 563
 564         if (zs1->zst_cap != zs2->zst_cap)
 565                 return (0);
 566
 567         if (zs1->zst_direction != zs2->zst_direction)
 568                 return (0);
 569
 570         return (1);
 571 }
 572
 573 /*
 574  * This is the prefetch entry point.  It calls all of the other dmu_zfetch
 575  * routines to create, delete, find, or operate upon prefetch streams.
 576  */
 577 void
 578 dmu_zfetch(zfetch_t *zf, uint64_t offset, uint64_t size, int prefetched)
 579 {
 580         zstream_t       zst;
 581         zstream_t       *newstream;
 582         int             fetched;
 583         int             inserted;
 584         unsigned int    blkshft;
 585         uint64_t        blksz;
 586
 587         if (zfs_prefetch_disable)
 588                 return;
 589
 590         /* files that aren't ln2 blocksz are only one block -- nothing to do */
 591         if (!zf->zf_dnode->dn_datablkshift)
 592                 return;
 593
 594         /* convert offset and size, into blockid and nblocks */
 595         blkshft = zf->zf_dnode->dn_datablkshift;
 596         blksz = (1 << blkshft);
 597
 598         bzero(&zst, sizeof (zstream_t));
 599         zst.zst_offset = offset >> blkshft;
 600         zst.zst_len = (P2ROUNDUP(offset + size, blksz) -
 601             P2ALIGN(offset, blksz)) >> blkshft;
 602
 603         fetched = dmu_zfetch_find(zf, &zst, prefetched);
 604         if (!fetched) {
 605                 fetched = dmu_zfetch_colinear(zf, &zst);
 606         }
 607
 608         if (!fetched) {
 609                 newstream = dmu_zfetch_stream_reclaim(zf);
 610
 611                 /*
 612                  * we still couldn't find a stream, drop the lock, and allocate
 613                  * one if possible.  Otherwise, give up and go home.
 614                  */
 615                 if (newstream == NULL) {
 616                         uint64_t        maxblocks;
 617                         uint32_t        max_streams;
 618                         uint32_t        cur_streams;
 619
 620                         cur_streams = zf->zf_stream_cnt;
 621                         maxblocks = zf->zf_dnode->dn_maxblkid;
 622
 623                         max_streams = MIN(zfetch_max_streams,
 624                             (maxblocks / zfetch_block_cap));
 625                         if (max_streams == 0) {
 626                                 max_streams++;
 627                         }
 628
 629                         if (cur_streams >= max_streams) {
 630                                 return;
 631                         }
 632
 633                         newstream = kmem_zalloc(sizeof (zstream_t), KM_SLEEP);
 634                 }
 635
 636                 newstream->zst_offset = zst.zst_offset;
 637                 newstream->zst_len = zst.zst_len;
 638                 newstream->zst_stride = zst.zst_len;
 639                 newstream->zst_ph_offset = zst.zst_len + zst.zst_offset;
 640                 newstream->zst_cap = zst.zst_len;
 641                 newstream->zst_direction = ZFETCH_FORWARD;
 642                 newstream->zst_last = LBOLT;
 643
 644                 mutex_init(&newstream->zst_lock, NULL, MUTEX_DEFAULT, NULL);
 645
 646                 rw_enter(&zf->zf_rwlock, RW_WRITER);
 647                 inserted = dmu_zfetch_stream_insert(zf, newstream);
 648                 rw_exit(&zf->zf_rwlock);
 649
 650                 if (!inserted) {
 651                         mutex_destroy(&newstream->zst_lock);
 652                         kmem_free(newstream, sizeof (zstream_t));
 653                 }
 654         }
 655 }