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 https://opensource.org/licenses/CDDL-1.0. 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 2009 Sun Microsystems, Inc. All rights reserved. 23 * Use is subject to license terms. 24 */ 25 26 /* 27 * Copyright (c) 2013, 2017 by Delphix. All rights reserved. 28 */ 29 30 #include <sys/zfs_context.h> 31 #include <sys/arc_impl.h> 32 #include <sys/dnode.h> 33 #include <sys/dmu_objset.h> 34 #include <sys/dmu_zfetch.h> 35 #include <sys/dmu.h> 36 #include <sys/dbuf.h> 37 #include <sys/kstat.h> 38 #include <sys/wmsum.h> 39 40 /* 41 * This tunable disables predictive prefetch. Note that it leaves "prescient" 42 * prefetch (e.g. prefetch for zfs send) intact. Unlike predictive prefetch, 43 * prescient prefetch never issues i/os that end up not being needed, 44 * so it can't hurt performance. 45 */ 46 47 static int zfs_prefetch_disable = B_FALSE; 48 49 /* max # of streams per zfetch */ 50 static unsigned int zfetch_max_streams = 8; 51 /* min time before stream reclaim */ 52 static unsigned int zfetch_min_sec_reap = 1; 53 /* max time before stream delete */ 54 static unsigned int zfetch_max_sec_reap = 2; 55 #ifdef _ILP32 56 /* min bytes to prefetch per stream (default 2MB) */ 57 static unsigned int zfetch_min_distance = 2 * 1024 * 1024; 58 /* max bytes to prefetch per stream (default 8MB) */ 59 unsigned int zfetch_max_distance = 8 * 1024 * 1024; 60 #else 61 /* min bytes to prefetch per stream (default 4MB) */ 62 static unsigned int zfetch_min_distance = 4 * 1024 * 1024; 63 /* max bytes to prefetch per stream (default 64MB) */ 64 unsigned int zfetch_max_distance = 64 * 1024 * 1024; 65 #endif 66 /* max bytes to prefetch indirects for per stream (default 64MB) */ 67 unsigned int zfetch_max_idistance = 64 * 1024 * 1024; 68 69 typedef struct zfetch_stats { 70 kstat_named_t zfetchstat_hits; 71 kstat_named_t zfetchstat_misses; 72 kstat_named_t zfetchstat_max_streams; 73 kstat_named_t zfetchstat_io_issued; 74 kstat_named_t zfetchstat_io_active; 75 } zfetch_stats_t; 76 77 static zfetch_stats_t zfetch_stats = { 78 { "hits", KSTAT_DATA_UINT64 }, 79 { "misses", KSTAT_DATA_UINT64 }, 80 { "max_streams", KSTAT_DATA_UINT64 }, 81 { "io_issued", KSTAT_DATA_UINT64 }, 82 { "io_active", KSTAT_DATA_UINT64 }, 83 }; 84 85 struct { 86 wmsum_t zfetchstat_hits; 87 wmsum_t zfetchstat_misses; 88 wmsum_t zfetchstat_max_streams; 89 wmsum_t zfetchstat_io_issued; 90 aggsum_t zfetchstat_io_active; 91 } zfetch_sums; 92 93 #define ZFETCHSTAT_BUMP(stat) \ 94 wmsum_add(&zfetch_sums.stat, 1) 95 #define ZFETCHSTAT_ADD(stat, val) \ 96 wmsum_add(&zfetch_sums.stat, val) 97 98 99 static kstat_t *zfetch_ksp; 100 101 static int 102 zfetch_kstats_update(kstat_t *ksp, int rw) 103 { 104 zfetch_stats_t *zs = ksp->ks_data; 105 106 if (rw == KSTAT_WRITE) 107 return (EACCES); 108 zs->zfetchstat_hits.value.ui64 = 109 wmsum_value(&zfetch_sums.zfetchstat_hits); 110 zs->zfetchstat_misses.value.ui64 = 111 wmsum_value(&zfetch_sums.zfetchstat_misses); 112 zs->zfetchstat_max_streams.value.ui64 = 113 wmsum_value(&zfetch_sums.zfetchstat_max_streams); 114 zs->zfetchstat_io_issued.value.ui64 = 115 wmsum_value(&zfetch_sums.zfetchstat_io_issued); 116 zs->zfetchstat_io_active.value.ui64 = 117 aggsum_value(&zfetch_sums.zfetchstat_io_active); 118 return (0); 119 } 120 121 void 122 zfetch_init(void) 123 { 124 wmsum_init(&zfetch_sums.zfetchstat_hits, 0); 125 wmsum_init(&zfetch_sums.zfetchstat_misses, 0); 126 wmsum_init(&zfetch_sums.zfetchstat_max_streams, 0); 127 wmsum_init(&zfetch_sums.zfetchstat_io_issued, 0); 128 aggsum_init(&zfetch_sums.zfetchstat_io_active, 0); 129 130 zfetch_ksp = kstat_create("zfs", 0, "zfetchstats", "misc", 131 KSTAT_TYPE_NAMED, sizeof (zfetch_stats) / sizeof (kstat_named_t), 132 KSTAT_FLAG_VIRTUAL); 133 134 if (zfetch_ksp != NULL) { 135 zfetch_ksp->ks_data = &zfetch_stats; 136 zfetch_ksp->ks_update = zfetch_kstats_update; 137 kstat_install(zfetch_ksp); 138 } 139 } 140 141 void 142 zfetch_fini(void) 143 { 144 if (zfetch_ksp != NULL) { 145 kstat_delete(zfetch_ksp); 146 zfetch_ksp = NULL; 147 } 148 149 wmsum_fini(&zfetch_sums.zfetchstat_hits); 150 wmsum_fini(&zfetch_sums.zfetchstat_misses); 151 wmsum_fini(&zfetch_sums.zfetchstat_max_streams); 152 wmsum_fini(&zfetch_sums.zfetchstat_io_issued); 153 ASSERT0(aggsum_value(&zfetch_sums.zfetchstat_io_active)); 154 aggsum_fini(&zfetch_sums.zfetchstat_io_active); 155 } 156 157 /* 158 * This takes a pointer to a zfetch structure and a dnode. It performs the 159 * necessary setup for the zfetch structure, grokking data from the 160 * associated dnode. 161 */ 162 void 163 dmu_zfetch_init(zfetch_t *zf, dnode_t *dno) 164 { 165 if (zf == NULL) 166 return; 167 zf->zf_dnode = dno; 168 zf->zf_numstreams = 0; 169 170 list_create(&zf->zf_stream, sizeof (zstream_t), 171 offsetof(zstream_t, zs_node)); 172 173 mutex_init(&zf->zf_lock, NULL, MUTEX_DEFAULT, NULL); 174 } 175 176 static void 177 dmu_zfetch_stream_fini(zstream_t *zs) 178 { 179 ASSERT(!list_link_active(&zs->zs_node)); 180 zfs_refcount_destroy(&zs->zs_callers); 181 zfs_refcount_destroy(&zs->zs_refs); 182 kmem_free(zs, sizeof (*zs)); 183 } 184 185 static void 186 dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs) 187 { 188 ASSERT(MUTEX_HELD(&zf->zf_lock)); 189 list_remove(&zf->zf_stream, zs); 190 zf->zf_numstreams--; 191 membar_producer(); 192 if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0) 193 dmu_zfetch_stream_fini(zs); 194 } 195 196 /* 197 * Clean-up state associated with a zfetch structure (e.g. destroy the 198 * streams). This doesn't free the zfetch_t itself, that's left to the caller. 199 */ 200 void 201 dmu_zfetch_fini(zfetch_t *zf) 202 { 203 zstream_t *zs; 204 205 mutex_enter(&zf->zf_lock); 206 while ((zs = list_head(&zf->zf_stream)) != NULL) 207 dmu_zfetch_stream_remove(zf, zs); 208 mutex_exit(&zf->zf_lock); 209 list_destroy(&zf->zf_stream); 210 mutex_destroy(&zf->zf_lock); 211 212 zf->zf_dnode = NULL; 213 } 214 215 /* 216 * If there aren't too many active streams already, create one more. 217 * In process delete/reuse all streams without hits for zfetch_max_sec_reap. 218 * If needed, reuse oldest stream without hits for zfetch_min_sec_reap or ever. 219 * The "blkid" argument is the next block that we expect this stream to access. 220 */ 221 static void 222 dmu_zfetch_stream_create(zfetch_t *zf, uint64_t blkid) 223 { 224 zstream_t *zs, *zs_next, *zs_old = NULL; 225 hrtime_t now = gethrtime(), t; 226 227 ASSERT(MUTEX_HELD(&zf->zf_lock)); 228 229 /* 230 * Delete too old streams, reusing the first found one. 231 */ 232 t = now - SEC2NSEC(zfetch_max_sec_reap); 233 for (zs = list_head(&zf->zf_stream); zs != NULL; zs = zs_next) { 234 zs_next = list_next(&zf->zf_stream, zs); 235 /* 236 * Skip if still active. 1 -- zf_stream reference. 237 */ 238 if (zfs_refcount_count(&zs->zs_refs) != 1) 239 continue; 240 if (zs->zs_atime > t) 241 continue; 242 if (zs_old) 243 dmu_zfetch_stream_remove(zf, zs); 244 else 245 zs_old = zs; 246 } 247 if (zs_old) { 248 zs = zs_old; 249 goto reuse; 250 } 251 252 /* 253 * The maximum number of streams is normally zfetch_max_streams, 254 * but for small files we lower it such that it's at least possible 255 * for all the streams to be non-overlapping. 256 */ 257 uint32_t max_streams = MAX(1, MIN(zfetch_max_streams, 258 zf->zf_dnode->dn_maxblkid * zf->zf_dnode->dn_datablksz / 259 zfetch_max_distance)); 260 if (zf->zf_numstreams >= max_streams) { 261 t = now - SEC2NSEC(zfetch_min_sec_reap); 262 for (zs = list_head(&zf->zf_stream); zs != NULL; 263 zs = list_next(&zf->zf_stream, zs)) { 264 if (zfs_refcount_count(&zs->zs_refs) != 1) 265 continue; 266 if (zs->zs_atime > t) 267 continue; 268 if (zs_old == NULL || zs->zs_atime < zs_old->zs_atime) 269 zs_old = zs; 270 } 271 if (zs_old) { 272 zs = zs_old; 273 goto reuse; 274 } 275 ZFETCHSTAT_BUMP(zfetchstat_max_streams); 276 return; 277 } 278 279 zs = kmem_zalloc(sizeof (*zs), KM_SLEEP); 280 zs->zs_fetch = zf; 281 zfs_refcount_create(&zs->zs_callers); 282 zfs_refcount_create(&zs->zs_refs); 283 /* One reference for zf_stream. */ 284 zfs_refcount_add(&zs->zs_refs, NULL); 285 zf->zf_numstreams++; 286 list_insert_head(&zf->zf_stream, zs); 287 288 reuse: 289 zs->zs_blkid = blkid; 290 zs->zs_pf_dist = 0; 291 zs->zs_pf_start = blkid; 292 zs->zs_pf_end = blkid; 293 zs->zs_ipf_dist = 0; 294 zs->zs_ipf_start = blkid; 295 zs->zs_ipf_end = blkid; 296 /* Allow immediate stream reuse until first hit. */ 297 zs->zs_atime = now - SEC2NSEC(zfetch_min_sec_reap); 298 zs->zs_missed = B_FALSE; 299 zs->zs_more = B_FALSE; 300 } 301 302 static void 303 dmu_zfetch_done(void *arg, uint64_t level, uint64_t blkid, boolean_t io_issued) 304 { 305 zstream_t *zs = arg; 306 307 if (io_issued && level == 0 && blkid < zs->zs_blkid) 308 zs->zs_more = B_TRUE; 309 if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0) 310 dmu_zfetch_stream_fini(zs); 311 aggsum_add(&zfetch_sums.zfetchstat_io_active, -1); 312 } 313 314 /* 315 * This is the predictive prefetch entry point. dmu_zfetch_prepare() 316 * associates dnode access specified with blkid and nblks arguments with 317 * prefetch stream, predicts further accesses based on that stats and returns 318 * the stream pointer on success. That pointer must later be passed to 319 * dmu_zfetch_run() to initiate the speculative prefetch for the stream and 320 * release it. dmu_zfetch() is a wrapper for simple cases when window between 321 * prediction and prefetch initiation is not needed. 322 * fetch_data argument specifies whether actual data blocks should be fetched: 323 * FALSE -- prefetch only indirect blocks for predicted data blocks; 324 * TRUE -- prefetch predicted data blocks plus following indirect blocks. 325 */ 326 zstream_t * 327 dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks, 328 boolean_t fetch_data, boolean_t have_lock) 329 { 330 zstream_t *zs; 331 spa_t *spa = zf->zf_dnode->dn_objset->os_spa; 332 zfs_prefetch_type_t os_prefetch = zf->zf_dnode->dn_objset->os_prefetch; 333 334 if (zfs_prefetch_disable || os_prefetch == ZFS_PREFETCH_NONE) 335 return (NULL); 336 337 if (os_prefetch == ZFS_PREFETCH_METADATA) 338 fetch_data = B_FALSE; 339 340 /* 341 * If we haven't yet loaded the indirect vdevs' mappings, we 342 * can only read from blocks that we carefully ensure are on 343 * concrete vdevs (or previously-loaded indirect vdevs). So we 344 * can't allow the predictive prefetcher to attempt reads of other 345 * blocks (e.g. of the MOS's dnode object). 346 */ 347 if (!spa_indirect_vdevs_loaded(spa)) 348 return (NULL); 349 350 /* 351 * As a fast path for small (single-block) files, ignore access 352 * to the first block. 353 */ 354 if (!have_lock && blkid == 0) 355 return (NULL); 356 357 if (!have_lock) 358 rw_enter(&zf->zf_dnode->dn_struct_rwlock, RW_READER); 359 360 /* 361 * A fast path for small files for which no prefetch will 362 * happen. 363 */ 364 uint64_t maxblkid = zf->zf_dnode->dn_maxblkid; 365 if (maxblkid < 2) { 366 if (!have_lock) 367 rw_exit(&zf->zf_dnode->dn_struct_rwlock); 368 return (NULL); 369 } 370 mutex_enter(&zf->zf_lock); 371 372 /* 373 * Find matching prefetch stream. Depending on whether the accesses 374 * are block-aligned, first block of the new access may either follow 375 * the last block of the previous access, or be equal to it. 376 */ 377 for (zs = list_head(&zf->zf_stream); zs != NULL; 378 zs = list_next(&zf->zf_stream, zs)) { 379 if (blkid == zs->zs_blkid) { 380 break; 381 } else if (blkid + 1 == zs->zs_blkid) { 382 blkid++; 383 nblks--; 384 break; 385 } 386 } 387 388 /* 389 * If the file is ending, remove the matching stream if found. 390 * If not found then it is too late to create a new one now. 391 */ 392 uint64_t end_of_access_blkid = blkid + nblks; 393 if (end_of_access_blkid >= maxblkid) { 394 if (zs != NULL) 395 dmu_zfetch_stream_remove(zf, zs); 396 mutex_exit(&zf->zf_lock); 397 if (!have_lock) 398 rw_exit(&zf->zf_dnode->dn_struct_rwlock); 399 return (NULL); 400 } 401 402 /* Exit if we already prefetched this block before. */ 403 if (nblks == 0) { 404 mutex_exit(&zf->zf_lock); 405 if (!have_lock) 406 rw_exit(&zf->zf_dnode->dn_struct_rwlock); 407 return (NULL); 408 } 409 410 if (zs == NULL) { 411 /* 412 * This access is not part of any existing stream. Create 413 * a new stream for it. 414 */ 415 dmu_zfetch_stream_create(zf, end_of_access_blkid); 416 mutex_exit(&zf->zf_lock); 417 if (!have_lock) 418 rw_exit(&zf->zf_dnode->dn_struct_rwlock); 419 ZFETCHSTAT_BUMP(zfetchstat_misses); 420 return (NULL); 421 } 422 423 /* 424 * This access was to a block that we issued a prefetch for on 425 * behalf of this stream. Calculate further prefetch distances. 426 * 427 * Start prefetch from the demand access size (nblks). Double the 428 * distance every access up to zfetch_min_distance. After that only 429 * if needed increase the distance by 1/8 up to zfetch_max_distance. 430 * 431 * Don't double the distance beyond single block if we have more 432 * than ~6% of ARC held by active prefetches. It should help with 433 * getting out of RAM on some badly mispredicted read patterns. 434 */ 435 unsigned int dbs = zf->zf_dnode->dn_datablkshift; 436 unsigned int nbytes = nblks << dbs; 437 unsigned int pf_nblks; 438 if (fetch_data) { 439 if (unlikely(zs->zs_pf_dist < nbytes)) 440 zs->zs_pf_dist = nbytes; 441 else if (zs->zs_pf_dist < zfetch_min_distance && 442 (zs->zs_pf_dist < (1 << dbs) || 443 aggsum_compare(&zfetch_sums.zfetchstat_io_active, 444 arc_c_max >> (4 + dbs)) < 0)) 445 zs->zs_pf_dist *= 2; 446 else if (zs->zs_more) 447 zs->zs_pf_dist += zs->zs_pf_dist / 8; 448 zs->zs_more = B_FALSE; 449 if (zs->zs_pf_dist > zfetch_max_distance) 450 zs->zs_pf_dist = zfetch_max_distance; 451 pf_nblks = zs->zs_pf_dist >> dbs; 452 } else { 453 pf_nblks = 0; 454 } 455 if (zs->zs_pf_start < end_of_access_blkid) 456 zs->zs_pf_start = end_of_access_blkid; 457 if (zs->zs_pf_end < end_of_access_blkid + pf_nblks) 458 zs->zs_pf_end = end_of_access_blkid + pf_nblks; 459 460 /* 461 * Do the same for indirects, starting where we will stop reading 462 * data blocks (and the indirects that point to them). 463 */ 464 if (unlikely(zs->zs_ipf_dist < nbytes)) 465 zs->zs_ipf_dist = nbytes; 466 else 467 zs->zs_ipf_dist *= 2; 468 if (zs->zs_ipf_dist > zfetch_max_idistance) 469 zs->zs_ipf_dist = zfetch_max_idistance; 470 pf_nblks = zs->zs_ipf_dist >> dbs; 471 if (zs->zs_ipf_start < zs->zs_pf_end) 472 zs->zs_ipf_start = zs->zs_pf_end; 473 if (zs->zs_ipf_end < zs->zs_pf_end + pf_nblks) 474 zs->zs_ipf_end = zs->zs_pf_end + pf_nblks; 475 476 zs->zs_blkid = end_of_access_blkid; 477 /* Protect the stream from reclamation. */ 478 zs->zs_atime = gethrtime(); 479 zfs_refcount_add(&zs->zs_refs, NULL); 480 /* Count concurrent callers. */ 481 zfs_refcount_add(&zs->zs_callers, NULL); 482 mutex_exit(&zf->zf_lock); 483 484 if (!have_lock) 485 rw_exit(&zf->zf_dnode->dn_struct_rwlock); 486 487 ZFETCHSTAT_BUMP(zfetchstat_hits); 488 return (zs); 489 } 490 491 void 492 dmu_zfetch_run(zstream_t *zs, boolean_t missed, boolean_t have_lock) 493 { 494 zfetch_t *zf = zs->zs_fetch; 495 int64_t pf_start, pf_end, ipf_start, ipf_end; 496 int epbs, issued; 497 498 if (missed) 499 zs->zs_missed = missed; 500 501 /* 502 * Postpone the prefetch if there are more concurrent callers. 503 * It happens when multiple requests are waiting for the same 504 * indirect block. The last one will run the prefetch for all. 505 */ 506 if (zfs_refcount_remove(&zs->zs_callers, NULL) != 0) { 507 /* Drop reference taken in dmu_zfetch_prepare(). */ 508 if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0) 509 dmu_zfetch_stream_fini(zs); 510 return; 511 } 512 513 mutex_enter(&zf->zf_lock); 514 if (zs->zs_missed) { 515 pf_start = zs->zs_pf_start; 516 pf_end = zs->zs_pf_start = zs->zs_pf_end; 517 } else { 518 pf_start = pf_end = 0; 519 } 520 ipf_start = zs->zs_ipf_start; 521 ipf_end = zs->zs_ipf_start = zs->zs_ipf_end; 522 mutex_exit(&zf->zf_lock); 523 ASSERT3S(pf_start, <=, pf_end); 524 ASSERT3S(ipf_start, <=, ipf_end); 525 526 epbs = zf->zf_dnode->dn_indblkshift - SPA_BLKPTRSHIFT; 527 ipf_start = P2ROUNDUP(ipf_start, 1 << epbs) >> epbs; 528 ipf_end = P2ROUNDUP(ipf_end, 1 << epbs) >> epbs; 529 ASSERT3S(ipf_start, <=, ipf_end); 530 issued = pf_end - pf_start + ipf_end - ipf_start; 531 if (issued > 1) { 532 /* More references on top of taken in dmu_zfetch_prepare(). */ 533 zfs_refcount_add_few(&zs->zs_refs, issued - 1, NULL); 534 } else if (issued == 0) { 535 /* Some other thread has done our work, so drop the ref. */ 536 if (zfs_refcount_remove(&zs->zs_refs, NULL) == 0) 537 dmu_zfetch_stream_fini(zs); 538 return; 539 } 540 aggsum_add(&zfetch_sums.zfetchstat_io_active, issued); 541 542 if (!have_lock) 543 rw_enter(&zf->zf_dnode->dn_struct_rwlock, RW_READER); 544 545 issued = 0; 546 for (int64_t blk = pf_start; blk < pf_end; blk++) { 547 issued += dbuf_prefetch_impl(zf->zf_dnode, 0, blk, 548 ZIO_PRIORITY_ASYNC_READ, 0, dmu_zfetch_done, zs); 549 } 550 for (int64_t iblk = ipf_start; iblk < ipf_end; iblk++) { 551 issued += dbuf_prefetch_impl(zf->zf_dnode, 1, iblk, 552 ZIO_PRIORITY_ASYNC_READ, 0, dmu_zfetch_done, zs); 553 } 554 555 if (!have_lock) 556 rw_exit(&zf->zf_dnode->dn_struct_rwlock); 557 558 if (issued) 559 ZFETCHSTAT_ADD(zfetchstat_io_issued, issued); 560 } 561 562 void 563 dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data, 564 boolean_t missed, boolean_t have_lock) 565 { 566 zstream_t *zs; 567 568 zs = dmu_zfetch_prepare(zf, blkid, nblks, fetch_data, have_lock); 569 if (zs) 570 dmu_zfetch_run(zs, missed, have_lock); 571 } 572 573 ZFS_MODULE_PARAM(zfs_prefetch, zfs_prefetch_, disable, INT, ZMOD_RW, 574 "Disable all ZFS prefetching"); 575 576 ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_streams, UINT, ZMOD_RW, 577 "Max number of streams per zfetch"); 578 579 ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, min_sec_reap, UINT, ZMOD_RW, 580 "Min time before stream reclaim"); 581 582 ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_sec_reap, UINT, ZMOD_RW, 583 "Max time before stream delete"); 584 585 ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, min_distance, UINT, ZMOD_RW, 586 "Min bytes to prefetch per stream"); 587 588 ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_distance, UINT, ZMOD_RW, 589 "Max bytes to prefetch per stream"); 590 591 ZFS_MODULE_PARAM(zfs_prefetch, zfetch_, max_idistance, UINT, ZMOD_RW, 592 "Max bytes to prefetch indirects for per stream"); 593