Illumos 5987 - zfs prefetch code needs work

5987 zfs prefetch code needs work
Reviewed by: Adam Leventhal <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Paul Dagnelie <[email protected]>
Approved by: Gordon Ross <[email protected]>

References:
  https://www.illumos.org/issues/5987 zfs prefetch code needs work
  illumos/illumos-gate@cf6106c 5987 zfs prefetch code needs work

Porting notes:
- [module/zfs/dbuf.c]
  - 5f6d0b6 Handle block pointers with a corrupt logical size
- [module/zfs/dmu_zfetch.c]
  - c65aa5b Fix gcc missing parenthesis warnings
  - 428870f Update core ZFS code from build 121 to build 141.
  - 79c76d5 Change KM_PUSHPAGE -> KM_SLEEP
  - b8d06fc Switch KM_SLEEP to KM_PUSHPAGE
  - Account for ISO C90 - mixed declarations and code - warnings
  - Module parameters (new/changed):
    - Replaced zfetch_block_cap with zfetch_max_distance
      (Max bytes to prefetch per stream (default 8MB; 8 * 1024 * 1024))
    - Preserved zfs_prefetch_disable as 'int' for consistency with
      existing Linux module options.
- [include/sys/trace_arc.h]
  - Added new tracepoints
    - DEFINE_ARC_BUF_HDR_EVENT(zfs_arc__sync__wait__for__async);
    - DEFINE_ARC_BUF_HDR_EVENT(zfs_arc__demand__hit__predictive__prefetch);
- [man/man5/zfs-module-parameters.5]
  - Updated man page

Ported-by: kernelOfTruth [email protected]
Signed-off-by: Brian Behlendorf <[email protected]>

1 files changed, 140 insertions, 583 deletions

diff --git a/module/zfs/dmu_zfetch.c b/module/zfs/dmu_zfetch.c
index 9d41bdc0f..b25b0e007 100644
--- a/module/zfs/dmu_zfetch.c
+++ b/module/zfs/dmu_zfetch.c
@@ -24,7 +24,7 @@
  */
 
 /*
- * Copyright (c) 2013 by Delphix. All rights reserved.
+ * Copyright (c) 2013, 2014 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -36,209 +36,43 @@
 #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.
+ * This tunable disables predictive prefetch.  Note that it leaves "prescient"
+ * prefetch (e.g. prefetch for zfs send) intact.  Unlike predictive prefetch,
+ * prescient prefetch never issues i/os that end up not being needed,
+ * so it can't hurt performance.
  */
 
-int zfs_prefetch_disable = 0;
+int zfs_prefetch_disable = B_FALSE;
 
 /* max # of streams per zfetch */
 unsigned int	zfetch_max_streams = 8;
 /* min time before stream reclaim */
 unsigned int	zfetch_min_sec_reap = 2;
-/* max number of blocks to fetch at a time */
-unsigned int	zfetch_block_cap = 256;
-/* number of bytes in a array_read at which we stop prefetching (1Mb) */
+/* max bytes to prefetch per stream (default 8MB) */
+unsigned int	zfetch_max_distance = 8 * 1024 * 1024;
+/* number of bytes in a array_read at which we stop prefetching (1MB) */
 unsigned long	zfetch_array_rd_sz = 1024 * 1024;
 
-/* forward decls for static routines */
-static boolean_t	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 boolean_t	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;
+	kstat_named_t zfetchstat_max_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 },
+	{ "max_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);
+#define	ZFETCHSTAT_BUMP(stat) \
+	atomic_inc_64(&zfetch_stats.stat.value.ui64);
 
 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.
- *
- * Returns whether co-linear streams were found.
- */
-static boolean_t
-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 ?
-				    ZFETCH_BACKWARD : ZFETCH_FORWARD;
-				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 ?
-				    ZFETCH_BACKWARD : ZFETCH_FORWARD;
-				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 = ddi_get_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);
@@ -266,273 +100,41 @@ zfetch_fini(void)
 void
 dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
 {
-	if (zf == NULL) {
+	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));
+	    offsetof(zstream_t, zs_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, 0, blkid + i, ZIO_PRIORITY_ASYNC_READ,
-		    ARC_FLAG_PREFETCH);
-	}
-
-	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 boolean_t
-dmu_zfetch_find(zfetch_t *zf, zstream_t *zh, int prefetched)
+static void
+dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
 {
-	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;
-
-			mutex_enter(&zs->zst_lock);
-
-			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;
-
-			mutex_enter(&zs->zst_lock);
-
-			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 */
-
-			mutex_enter(&zs->zst_lock);
-
-			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 */
-
-			mutex_enter(&zs->zst_lock);
-
-			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);
+	ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
+	list_remove(&zf->zf_stream, zs);
+	mutex_destroy(&zs->zs_lock);
+	kmem_free(zs, sizeof (*zs));
 }
 
 /*
- * 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.
+ * Clean-up state associated with a zfetch structure (e.g. destroy the
+ * streams).  This doesn't free the zfetch_t itself, that's left to the caller.
  */
 void
-dmu_zfetch_rele(zfetch_t *zf)
+dmu_zfetch_fini(zfetch_t *zf)
 {
-	zstream_t	*zs;
-	zstream_t	*zs_next;
+	zstream_t *zs;
 
 	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));
-	}
+	rw_enter(&zf->zf_rwlock, RW_WRITER);
+	while ((zs = list_head(&zf->zf_stream)) != NULL)
+		dmu_zfetch_stream_remove(zf, zs);
+	rw_exit(&zf->zf_rwlock);
 	list_destroy(&zf->zf_stream);
 	rw_destroy(&zf->zf_rwlock);
 
@@ -540,101 +142,57 @@ dmu_zfetch_rele(zfetch_t *zf)
 }
 
 /*
- * 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.
+ * If there aren't too many streams already, create a new stream.
+ * The "blkid" argument is the next block that we expect this stream to access.
+ * While we're here, clean up old streams (which haven't been
+ * accessed for at least zfetch_min_sec_reap seconds).
  */
-static int
-dmu_zfetch_stream_insert(zfetch_t *zf, zstream_t *zs)
+static void
+dmu_zfetch_stream_create(zfetch_t *zf, uint64_t blkid)
 {
-	zstream_t	*zs_walk;
-	zstream_t	*zs_next;
+	zstream_t *zs;
+	zstream_t *zs_next;
+	int numstreams = 0;
+	uint32_t max_streams;
 
 	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 (((ddi_get_lbolt() - zs->zst_last)/hz) > zfetch_min_sec_reap)
-			break;
+	/*
+	 * Clean up old streams.
+	 */
+	for (zs = list_head(&zf->zf_stream);
+	    zs != NULL; zs = zs_next) {
+		zs_next = list_next(&zf->zf_stream, zs);
+		if (((gethrtime() - zs->zs_atime) / NANOSEC) >
+		    zfetch_min_sec_reap)
+			dmu_zfetch_stream_remove(zf, zs);
+		else
+			numstreams++;
 	}
 
-	if (zs) {
-		dmu_zfetch_stream_remove(zf, zs);
-		mutex_destroy(&zs->zst_lock);
-		bzero(zs, sizeof (zstream_t));
-	} else {
-		zf->zf_alloc_fail++;
+	/*
+	 * The maximum number of streams is normally zfetch_max_streams,
+	 * but for small files we lower it such that it's at least possible
+	 * for all the streams to be non-overlapping.
+	 *
+	 * If we are already at the maximum number of streams for this file,
+	 * even after removing old streams, then don't create this stream.
+	 */
+	max_streams = MAX(1, MIN(zfetch_max_streams,
+	    zf->zf_dnode->dn_maxblkid * zf->zf_dnode->dn_datablksz /
+	    zfetch_max_distance));
+	if (numstreams >= max_streams) {
+		ZFETCHSTAT_BUMP(zfetchstat_max_streams);
+		return;
 	}
-	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));
+	zs = kmem_zalloc(sizeof (*zs), KM_SLEEP);
+	zs->zs_blkid = blkid;
+	zs->zs_pf_blkid = blkid;
+	zs->zs_atime = gethrtime();
+	mutex_init(&zs->zs_lock, NULL, MUTEX_DEFAULT, NULL);
 
-	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);
+	list_insert_head(&zf->zf_stream, zs);
 }
 
 /*
@@ -642,93 +200,91 @@ dmu_zfetch_streams_equal(zstream_t *zs1, zstream_t *zs2)
  * routines to create, delete, find, or operate upon prefetch streams.
  */
 void
-dmu_zfetch(zfetch_t *zf, uint64_t offset, uint64_t size, int prefetched)
+dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks)
 {
-	zstream_t	zst;
-	zstream_t	*newstream;
-	boolean_t	fetched;
-	int		inserted;
-	unsigned int	blkshft;
-	uint64_t	blksz;
+	zstream_t *zs;
+	int64_t pf_start;
+	int pf_nblks;
+	int i;
 
 	if (zfs_prefetch_disable)
 		return;
 
-	/* files that aren't ln2 blocksz are only one block -- nothing to do */
-	if (!zf->zf_dnode->dn_datablkshift)
+	/*
+	 * As a fast path for small (single-block) files, ignore access
+	 * to the first block.
+	 */
+	if (blkid == 0)
 		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;
+	rw_enter(&zf->zf_rwlock, RW_READER);
 
-	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);
+	for (zs = list_head(&zf->zf_stream); zs != NULL;
+	    zs = list_next(&zf->zf_stream, zs)) {
+		if (blkid == zs->zs_blkid) {
+			mutex_enter(&zs->zs_lock);
+			/*
+			 * zs_blkid could have changed before we
+			 * acquired zs_lock; re-check them here.
+			 */
+			if (blkid != zs->zs_blkid) {
+				mutex_exit(&zs->zs_lock);
+				continue;
+			}
+			break;
 		}
 	}
 
-	if (!fetched) {
-		newstream = dmu_zfetch_stream_reclaim(zf);
-
+	if (zs == NULL) {
 		/*
-		 * we still couldn't find a stream, drop the lock, and allocate
-		 * one if possible.  Otherwise, give up and go home.
+		 * This access is not part of any existing stream.  Create
+		 * a new stream for it.
 		 */
-		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);
-		}
+		ZFETCHSTAT_BUMP(zfetchstat_misses);
+		if (rw_tryupgrade(&zf->zf_rwlock))
+			dmu_zfetch_stream_create(zf, blkid + nblks);
+		rw_exit(&zf->zf_rwlock);
+		return;
+	}
 
-		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 = ddi_get_lbolt();
+	/*
+	 * This access was to a block that we issued a prefetch for on
+	 * behalf of this stream. Issue further prefetches for this stream.
+	 *
+	 * Normally, we start prefetching where we stopped
+	 * prefetching last (zs_pf_blkid).  But when we get our first
+	 * hit on this stream, zs_pf_blkid == zs_blkid, we don't
+	 * want to prefetch to block we just accessed.  In this case,
+	 * start just after the block we just accessed.
+	 */
+	pf_start = MAX(zs->zs_pf_blkid, blkid + nblks);
 
-		mutex_init(&newstream->zst_lock, NULL, MUTEX_DEFAULT, NULL);
+	/*
+	 * Double our amount of prefetched data, but don't let the
+	 * prefetch get further ahead than zfetch_max_distance.
+	 */
+	pf_nblks =
+	    MIN((int64_t)zs->zs_pf_blkid - zs->zs_blkid + nblks,
+	    zs->zs_blkid + nblks +
+	    (zfetch_max_distance >> zf->zf_dnode->dn_datablkshift) - pf_start);
 
-		rw_enter(&zf->zf_rwlock, RW_WRITER);
-		inserted = dmu_zfetch_stream_insert(zf, newstream);
-		rw_exit(&zf->zf_rwlock);
+	zs->zs_pf_blkid = pf_start + pf_nblks;
+	zs->zs_atime = gethrtime();
+	zs->zs_blkid = blkid + nblks;
 
-		if (!inserted) {
-			mutex_destroy(&newstream->zst_lock);
-			kmem_free(newstream, sizeof (zstream_t));
-		}
+	/*
+	 * dbuf_prefetch() issues the prefetch i/o
+	 * asynchronously, but it may need to wait for an
+	 * indirect block to be read from disk.  Therefore
+	 * we do not want to hold any locks while we call it.
+	 */
+	mutex_exit(&zs->zs_lock);
+	rw_exit(&zf->zf_rwlock);
+	for (i = 0; i < pf_nblks; i++) {
+		dbuf_prefetch(zf->zf_dnode, 0, pf_start + i,
+		    ZIO_PRIORITY_ASYNC_READ, ARC_FLAG_PREDICTIVE_PREFETCH);
 	}
+	ZFETCHSTAT_BUMP(zfetchstat_hits);
 }
 
 #if defined(_KERNEL) && defined(HAVE_SPL)
@@ -741,8 +297,9 @@ MODULE_PARM_DESC(zfetch_max_streams, "Max number of streams per zfetch");
 module_param(zfetch_min_sec_reap, uint, 0644);
 MODULE_PARM_DESC(zfetch_min_sec_reap, "Min time before stream reclaim");
 
-module_param(zfetch_block_cap, uint, 0644);
-MODULE_PARM_DESC(zfetch_block_cap, "Max number of blocks to fetch at a time");
+module_param(zfetch_max_distance, uint, 0644);
+MODULE_PARM_DESC(zfetch_max_distance,
+	"Max bytes to prefetch per stream (default 8MB)");
 
 module_param(zfetch_array_rd_sz, ulong, 0644);
 MODULE_PARM_DESC(zfetch_array_rd_sz, "Number of bytes in a array_read");