Reduce loaded range tree memory usage

This patch implements a new tree structure for ZFS, and uses it to 
store range trees more efficiently.

The new structure is approximately a B-tree, though there are some 
small differences from the usual characterizations. The tree has core 
nodes and leaf nodes; each contain data elements, which the elements 
in the core nodes acting as separators between its children. The 
difference between core and leaf nodes is that the core nodes have an 
array of children, while leaf nodes don't. Every node in the tree may 
be only partially full; in most cases, they are all at least 50% full 
(in terms of element count) except for the root node, which can be 
less full. Underfull nodes will steal from their neighbors or merge to 
remain full enough, while overfull nodes will split in two. The data 
elements are contained in tree-controlled buffers; they are copied 
into these on insertion, and overwritten on deletion. This means that 
the elements are not independently allocated, which reduces overhead, 
but also means they can't be shared between trees (and also that 
pointers to them are only valid until a side-effectful tree operation 
occurs). The overhead varies based on how dense the tree is, but is 
usually on the order of about 50% of the element size; the per-node 
overheads are very small, and so don't make a significant difference. 
The trees can accept arbitrary records; they accept a size and a 
comparator to allow them to be used for a variety of purposes.

The new trees replace the AVL trees used in the range trees today. 
Currently, the range_seg_t structure contains three 8 byte integers 
of payload and two 24 byte avl_tree_node_ts to handle its storage in 
both an offset-sorted tree and a size-sorted tree (total size: 64 
bytes). In the new model, the range seg structures are usually two 4 
byte integers, but a separate one needs to exist for the size-sorted 
and offset-sorted tree. Between the raw size, the 50% overhead, and 
the double storage, the new btrees are expected to use 8*1.5*2 = 24 
bytes per record, or 33.3% as much memory as the AVL trees (this is 
for the purposes of storing metaslab range trees; for other purposes, 
like scrubs, they use ~50% as much memory).

We reduced the size of the payload in the range segments by teaching 
range trees about starting offsets and shifts; since metaslabs have a 
fixed starting offset, and they all operate in terms of disk sectors, 
we can store the ranges using 4-byte integers as long as the size of 
the metaslab divided by the sector size is less than 2^32. For 512-byte
sectors, this is a 2^41 (or 2TB) metaslab, which with the default
settings corresponds to a 256PB disk. 4k sector disks can handle 
metaslabs up to 2^46 bytes, or 2^63 byte disks. Since we do not 
anticipate disks of this size in the near future, there should be 
almost no cases where metaslabs need 64-byte integers to store their 
ranges. We do still have the capability to store 64-byte integer ranges 
to account for cases where we are storing per-vdev (or per-dnode) trees, 
which could reasonably go above the limits discussed. We also do not 
store fill information in the compact version of the node, since it 
is only used for sorted scrub.

We also optimized the metaslab loading process in various other ways
to offset some inefficiencies in the btree model. While individual
operations (find, insert, remove_from) are faster for the btree than 
they are for the avl tree, remove usually requires a find operation, 
while in the AVL tree model the element itself suffices. Some clever 
changes actually caused an overall speedup in metaslab loading; we use 
approximately 40% less cpu to load metaslabs in our tests on Illumos.

Another memory and performance optimization was achieved by changing 
what is stored in the size-sorted trees. When a disk is heavily 
fragmented, the df algorithm used by default in ZFS will almost always 
find a number of small regions in its initial cursor-based search; it 
will usually only fall back to the size-sorted tree to find larger 
regions. If we increase the size of the cursor-based search slightly, 
and don't store segments that are smaller than a tunable size floor 
in the size-sorted tree, we can further cut memory usage down to 
below 20% of what the AVL trees store. This also results in further 
reductions in CPU time spent loading metaslabs.

The 16KiB size floor was chosen because it results in substantial memory 
usage reduction while not usually resulting in situations where we can't 
find an appropriate chunk with the cursor and are forced to use an 
oversized chunk from the size-sorted tree. In addition, even if we do 
have to use an oversized chunk from the size-sorted tree, the chunk 
would be too small to use for ZIL allocations, so it isn't as big of a 
loss as it might otherwise be. And often, more small allocations will 
follow the initial one, and the cursor search will now find the 
remainder of the chunk we didn't use all of and use it for subsequent 
allocations. Practical testing has shown little or no change in 
fragmentation as a result of this change.

If the size-sorted tree becomes empty while the offset sorted one still 
has entries, it will load all the entries from the offset sorted tree 
and disregard the size floor until it is unloaded again. This operation 
occurs rarely with the default setting, only on incredibly thoroughly 
fragmented pools.

There are some other small changes to zdb to teach it to handle btrees, 
but nothing major.
                                           
Reviewed-by: George Wilson <[email protected]>
Reviewed-by: Matt Ahrens <[email protected]>
Reviewed by: Sebastien Roy [email protected]
Reviewed-by: Igor Kozhukhov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Paul Dagnelie <[email protected]>
Closes #9181 

1 files changed, 348 insertions, 216 deletions

diff --git a/module/zfs/range_tree.c b/module/zfs/range_tree.c
index 0e1297214..0b369a438 100644
--- a/module/zfs/range_tree.c
+++ b/module/zfs/range_tree.c
@@ -74,42 +74,38 @@
  * support removing complete segments.
  */
 
-kmem_cache_t *range_seg_cache;
-
-/* Generic ops for managing an AVL tree alongside a range tree */
-struct range_tree_ops rt_avl_ops = {
-	.rtop_create = rt_avl_create,
-	.rtop_destroy = rt_avl_destroy,
-	.rtop_add = rt_avl_add,
-	.rtop_remove = rt_avl_remove,
-	.rtop_vacate = rt_avl_vacate,
-};
-
-void
-range_tree_init(void)
-{
-	ASSERT(range_seg_cache == NULL);
-	range_seg_cache = kmem_cache_create("range_seg_cache",
-	    sizeof (range_seg_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
-}
-
-void
-range_tree_fini(void)
-{
-	kmem_cache_destroy(range_seg_cache);
-	range_seg_cache = NULL;
+static inline void
+rs_copy(range_seg_t *src, range_seg_t *dest, range_tree_t *rt)
+{
+	ASSERT3U(rt->rt_type, <=, RANGE_SEG_NUM_TYPES);
+	size_t size = 0;
+	switch (rt->rt_type) {
+	case RANGE_SEG32:
+		size = sizeof (range_seg32_t);
+		break;
+	case RANGE_SEG64:
+		size = sizeof (range_seg64_t);
+		break;
+	case RANGE_SEG_GAP:
+		size = sizeof (range_seg_gap_t);
+		break;
+	default:
+		VERIFY(0);
+	}
+	bcopy(src, dest, size);
 }
 
 void
 range_tree_stat_verify(range_tree_t *rt)
 {
 	range_seg_t *rs;
+	zfs_btree_index_t where;
 	uint64_t hist[RANGE_TREE_HISTOGRAM_SIZE] = { 0 };
 	int i;
 
-	for (rs = avl_first(&rt->rt_root); rs != NULL;
-	    rs = AVL_NEXT(&rt->rt_root, rs)) {
-		uint64_t size = rs->rs_end - rs->rs_start;
+	for (rs = zfs_btree_first(&rt->rt_root, &where); rs != NULL;
+	    rs = zfs_btree_next(&rt->rt_root, &where, &where)) {
+		uint64_t size = rs_get_end(rs, rt) - rs_get_start(rs, rt);
 		int idx	= highbit64(size) - 1;
 
 		hist[idx]++;
@@ -128,7 +124,7 @@ range_tree_stat_verify(range_tree_t *rt)
 static void
 range_tree_stat_incr(range_tree_t *rt, range_seg_t *rs)
 {
-	uint64_t size = rs->rs_end - rs->rs_start;
+	uint64_t size = rs_get_end(rs, rt) - rs_get_start(rs, rt);
 	int idx = highbit64(size) - 1;
 
 	ASSERT(size != 0);
@@ -142,7 +138,7 @@ range_tree_stat_incr(range_tree_t *rt, range_seg_t *rs)
 static void
 range_tree_stat_decr(range_tree_t *rt, range_seg_t *rs)
 {
-	uint64_t size = rs->rs_end - rs->rs_start;
+	uint64_t size = rs_get_end(rs, rt) - rs_get_start(rs, rt);
 	int idx = highbit64(size) - 1;
 
 	ASSERT(size != 0);
@@ -153,14 +149,35 @@ range_tree_stat_decr(range_tree_t *rt, range_seg_t *rs)
 	rt->rt_histogram[idx]--;
 }
 
-/*
- * NOTE: caller is responsible for all locking.
- */
 static int
-range_tree_seg_compare(const void *x1, const void *x2)
+range_tree_seg32_compare(const void *x1, const void *x2)
+{
+	const range_seg32_t *r1 = x1;
+	const range_seg32_t *r2 = x2;
+
+	ASSERT3U(r1->rs_start, <=, r1->rs_end);
+	ASSERT3U(r2->rs_start, <=, r2->rs_end);
+
+	return ((r1->rs_start >= r2->rs_end) - (r1->rs_end <= r2->rs_start));
+}
+
+static int
+range_tree_seg64_compare(const void *x1, const void *x2)
 {
-	const range_seg_t *r1 = (const range_seg_t *)x1;
-	const range_seg_t *r2 = (const range_seg_t *)x2;
+	const range_seg64_t *r1 = x1;
+	const range_seg64_t *r2 = x2;
+
+	ASSERT3U(r1->rs_start, <=, r1->rs_end);
+	ASSERT3U(r2->rs_start, <=, r2->rs_end);
+
+	return ((r1->rs_start >= r2->rs_end) - (r1->rs_end <= r2->rs_start));
+}
+
+static int
+range_tree_seg_gap_compare(const void *x1, const void *x2)
+{
+	const range_seg_gap_t *r1 = x1;
+	const range_seg_gap_t *r2 = x2;
 
 	ASSERT3U(r1->rs_start, <=, r1->rs_end);
 	ASSERT3U(r2->rs_start, <=, r2->rs_end);
@@ -169,18 +186,42 @@ range_tree_seg_compare(const void *x1, const void *x2)
 }
 
 range_tree_t *
-range_tree_create_impl(range_tree_ops_t *ops, void *arg,
-    int (*avl_compare) (const void *, const void *), uint64_t gap)
+range_tree_create_impl(range_tree_ops_t *ops, range_seg_type_t type, void *arg,
+    uint64_t start, uint64_t shift,
+    int (*zfs_btree_compare) (const void *, const void *),
+    uint64_t gap)
 {
 	range_tree_t *rt = kmem_zalloc(sizeof (range_tree_t), KM_SLEEP);
 
-	avl_create(&rt->rt_root, range_tree_seg_compare,
-	    sizeof (range_seg_t), offsetof(range_seg_t, rs_node));
+	ASSERT3U(shift, <, 64);
+	ASSERT3U(type, <=, RANGE_SEG_NUM_TYPES);
+	size_t size;
+	int (*compare) (const void *, const void *);
+	switch (type) {
+	case RANGE_SEG32:
+		size = sizeof (range_seg32_t);
+		compare = range_tree_seg32_compare;
+		break;
+	case RANGE_SEG64:
+		size = sizeof (range_seg64_t);
+		compare = range_tree_seg64_compare;
+		break;
+	case RANGE_SEG_GAP:
+		size = sizeof (range_seg_gap_t);
+		compare = range_tree_seg_gap_compare;
+		break;
+	default:
+		panic("Invalid range seg type %d", type);
+	}
+	zfs_btree_create(&rt->rt_root, compare, size);
 
 	rt->rt_ops = ops;
 	rt->rt_gap = gap;
 	rt->rt_arg = arg;
-	rt->rt_avl_compare = avl_compare;
+	rt->rt_type = type;
+	rt->rt_start = start;
+	rt->rt_shift = shift;
+	rt->rt_btree_compare = zfs_btree_compare;
 
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_create != NULL)
 		rt->rt_ops->rtop_create(rt, rt->rt_arg);
@@ -189,9 +230,10 @@ range_tree_create_impl(range_tree_ops_t *ops, void *arg,
 }
 
 range_tree_t *
-range_tree_create(range_tree_ops_t *ops, void *arg)
+range_tree_create(range_tree_ops_t *ops, range_seg_type_t type,
+    void *arg, uint64_t start, uint64_t shift)
 {
-	return (range_tree_create_impl(ops, arg, NULL, 0));
+	return (range_tree_create_impl(ops, type, arg, start, shift, NULL, 0));
 }
 
 void
@@ -202,19 +244,20 @@ range_tree_destroy(range_tree_t *rt)
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_destroy != NULL)
 		rt->rt_ops->rtop_destroy(rt, rt->rt_arg);
 
-	avl_destroy(&rt->rt_root);
+	zfs_btree_destroy(&rt->rt_root);
 	kmem_free(rt, sizeof (*rt));
 }
 
 void
 range_tree_adjust_fill(range_tree_t *rt, range_seg_t *rs, int64_t delta)
 {
-	ASSERT3U(rs->rs_fill + delta, !=, 0);
-	ASSERT3U(rs->rs_fill + delta, <=, rs->rs_end - rs->rs_start);
+	ASSERT3U(rs_get_fill(rs, rt) + delta, !=, 0);
+	ASSERT3U(rs_get_fill(rs, rt) + delta, <=, rs_get_end(rs, rt) -
+	    rs_get_start(rs, rt));
 
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
 		rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
-	rs->rs_fill += delta;
+	rs_set_fill(rs, rt, rs_get_fill(rs, rt) + delta);
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
 		rt->rt_ops->rtop_add(rt, rs, rt->rt_arg);
 }
@@ -223,28 +266,20 @@ static void
 range_tree_add_impl(void *arg, uint64_t start, uint64_t size, uint64_t fill)
 {
 	range_tree_t *rt = arg;
-	avl_index_t where;
-	range_seg_t rsearch, *rs_before, *rs_after, *rs;
+	zfs_btree_index_t where;
+	range_seg_t *rs_before, *rs_after, *rs;
+	range_seg_max_t tmp, rsearch;
 	uint64_t end = start + size, gap = rt->rt_gap;
 	uint64_t bridge_size = 0;
 	boolean_t merge_before, merge_after;
 
 	ASSERT3U(size, !=, 0);
 	ASSERT3U(fill, <=, size);
+	ASSERT3U(start + size, >, start);
 
-	rsearch.rs_start = start;
-	rsearch.rs_end = end;
-	rs = avl_find(&rt->rt_root, &rsearch, &where);
-
-	if (gap == 0 && rs != NULL &&
-	    rs->rs_start <= start && rs->rs_end >= end) {
-		zfs_panic_recover("zfs: allocating allocated segment"
-		    "(offset=%llu size=%llu) of (offset=%llu size=%llu)\n",
-		    (longlong_t)start, (longlong_t)size,
-		    (longlong_t)rs->rs_start,
-		    (longlong_t)rs->rs_end - rs->rs_start);
-		return;
-	}
+	rs_set_start(&rsearch, rt, start);
+	rs_set_end(&rsearch, rt, end);
+	rs = zfs_btree_find(&rt->rt_root, &rsearch, &where);
 
 	/*
 	 * If this is a gap-supporting range tree, it is possible that we
@@ -255,27 +290,28 @@ range_tree_add_impl(void *arg, uint64_t start, uint64_t size, uint64_t fill)
 	 * the normal code paths.
 	 */
 	if (rs != NULL) {
+		ASSERT3U(rt->rt_gap, !=, 0);
+		uint64_t rstart = rs_get_start(rs, rt);
+		uint64_t rend = rs_get_end(rs, rt);
 		ASSERT3U(gap, !=, 0);
-		if (rs->rs_start <= start && rs->rs_end >= end) {
+		if (rstart <= start && rend >= end) {
 			range_tree_adjust_fill(rt, rs, fill);
 			return;
 		}
 
-		avl_remove(&rt->rt_root, rs);
+		zfs_btree_remove(&rt->rt_root, rs);
 		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
 			rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
 
 		range_tree_stat_decr(rt, rs);
-		rt->rt_space -= rs->rs_end - rs->rs_start;
+		rt->rt_space -= rend - rstart;
 
-		fill += rs->rs_fill;
-		start = MIN(start, rs->rs_start);
-		end = MAX(end, rs->rs_end);
+		fill += rs_get_fill(rs, rt);
+		start = MIN(start, rstart);
+		end = MAX(end, rend);
 		size = end - start;
 
 		range_tree_add_impl(rt, start, size, fill);
-
-		kmem_cache_free(range_seg_cache, rs);
 		return;
 	}
 
@@ -286,19 +322,21 @@ range_tree_add_impl(void *arg, uint64_t start, uint64_t size, uint64_t fill)
 	 * If gap != 0, we might need to merge with our neighbors even if we
 	 * aren't directly touching.
 	 */
-	rs_before = avl_nearest(&rt->rt_root, where, AVL_BEFORE);
-	rs_after = avl_nearest(&rt->rt_root, where, AVL_AFTER);
+	zfs_btree_index_t where_before, where_after;
+	rs_before = zfs_btree_prev(&rt->rt_root, &where, &where_before);
+	rs_after = zfs_btree_next(&rt->rt_root, &where, &where_after);
 
-	merge_before = (rs_before != NULL && rs_before->rs_end >= start - gap);
-	merge_after = (rs_after != NULL && rs_after->rs_start <= end + gap);
+	merge_before = (rs_before != NULL && rs_get_end(rs_before, rt) >=
+	    start - gap);
+	merge_after = (rs_after != NULL && rs_get_start(rs_after, rt) <= end +
+	    gap);
 
 	if (merge_before && gap != 0)
-		bridge_size += start - rs_before->rs_end;
+		bridge_size += start - rs_get_end(rs_before, rt);
 	if (merge_after && gap != 0)
-		bridge_size += rs_after->rs_start - end;
+		bridge_size += rs_get_start(rs_after, rt) - end;
 
 	if (merge_before && merge_after) {
-		avl_remove(&rt->rt_root, rs_before);
 		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL) {
 			rt->rt_ops->rtop_remove(rt, rs_before, rt->rt_arg);
 			rt->rt_ops->rtop_remove(rt, rs_after, rt->rt_arg);
@@ -307,9 +345,19 @@ range_tree_add_impl(void *arg, uint64_t start, uint64_t size, uint64_t fill)
 		range_tree_stat_decr(rt, rs_before);
 		range_tree_stat_decr(rt, rs_after);
 
-		rs_after->rs_fill += rs_before->rs_fill + fill;
-		rs_after->rs_start = rs_before->rs_start;
-		kmem_cache_free(range_seg_cache, rs_before);
+		rs_copy(rs_after, &tmp, rt);
+		uint64_t before_start = rs_get_start_raw(rs_before, rt);
+		uint64_t before_fill = rs_get_fill(rs_before, rt);
+		uint64_t after_fill = rs_get_fill(rs_after, rt);
+		zfs_btree_remove_from(&rt->rt_root, &where_before);
+
+		/*
+		 * We have to re-find the node because our old reference is
+		 * invalid as soon as we do any mutating btree operations.
+		 */
+		rs_after = zfs_btree_find(&rt->rt_root, &tmp, &where_after);
+		rs_set_start_raw(rs_after, rt, before_start);
+		rs_set_fill(rs_after, rt, after_fill + before_fill + fill);
 		rs = rs_after;
 	} else if (merge_before) {
 		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
@@ -317,8 +365,9 @@ range_tree_add_impl(void *arg, uint64_t start, uint64_t size, uint64_t fill)
 
 		range_tree_stat_decr(rt, rs_before);
 
-		rs_before->rs_fill += fill;
-		rs_before->rs_end = end;
+		uint64_t before_fill = rs_get_fill(rs_before, rt);
+		rs_set_end(rs_before, rt, end);
+		rs_set_fill(rs_before, rt, before_fill + fill);
 		rs = rs_before;
 	} else if (merge_after) {
 		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
@@ -326,22 +375,26 @@ range_tree_add_impl(void *arg, uint64_t start, uint64_t size, uint64_t fill)
 
 		range_tree_stat_decr(rt, rs_after);
 
-		rs_after->rs_fill += fill;
-		rs_after->rs_start = start;
+		uint64_t after_fill = rs_get_fill(rs_after, rt);
+		rs_set_start(rs_after, rt, start);
+		rs_set_fill(rs_after, rt, after_fill + fill);
 		rs = rs_after;
 	} else {
-		rs = kmem_cache_alloc(range_seg_cache, KM_SLEEP);
+		rs = &tmp;
 
-		rs->rs_fill = fill;
-		rs->rs_start = start;
-		rs->rs_end = end;
-		avl_insert(&rt->rt_root, rs, where);
+		rs_set_start(rs, rt, start);
+		rs_set_end(rs, rt, end);
+		rs_set_fill(rs, rt, fill);
+		zfs_btree_insert(&rt->rt_root, rs, &where);
 	}
 
-	if (gap != 0)
-		ASSERT3U(rs->rs_fill, <=, rs->rs_end - rs->rs_start);
-	else
-		ASSERT3U(rs->rs_fill, ==, rs->rs_end - rs->rs_start);
+	if (gap != 0) {
+		ASSERT3U(rs_get_fill(rs, rt), <=, rs_get_end(rs, rt) -
+		    rs_get_start(rs, rt));
+	} else {
+		ASSERT3U(rs_get_fill(rs, rt), ==, rs_get_end(rs, rt) -
+		    rs_get_start(rs, rt));
+	}
 
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
 		rt->rt_ops->rtop_add(rt, rs, rt->rt_arg);
@@ -360,22 +413,25 @@ static void
 range_tree_remove_impl(range_tree_t *rt, uint64_t start, uint64_t size,
     boolean_t do_fill)
 {
-	avl_index_t where;
-	range_seg_t rsearch, *rs, *newseg;
+	zfs_btree_index_t where;
+	range_seg_t *rs;
+	range_seg_max_t rsearch, rs_tmp;
 	uint64_t end = start + size;
 	boolean_t left_over, right_over;
 
 	VERIFY3U(size, !=, 0);
 	VERIFY3U(size, <=, rt->rt_space);
+	if (rt->rt_type == RANGE_SEG64)
+		ASSERT3U(start + size, >, start);
 
-	rsearch.rs_start = start;
-	rsearch.rs_end = end;
-	rs = avl_find(&rt->rt_root, &rsearch, &where);
+	rs_set_start(&rsearch, rt, start);
+	rs_set_end(&rsearch, rt, end);
+	rs = zfs_btree_find(&rt->rt_root, &rsearch, &where);
 
 	/* Make sure we completely overlap with someone */
 	if (rs == NULL) {
-		zfs_panic_recover("zfs: freeing free segment "
-		    "(offset=%llu size=%llu)",
+		zfs_panic_recover("zfs: removing nonexistent segment from "
+		    "range tree (offset=%llu size=%llu)",
 		    (longlong_t)start, (longlong_t)size);
 		return;
 	}
@@ -388,30 +444,32 @@ range_tree_remove_impl(range_tree_t *rt, uint64_t start, uint64_t size,
 	 */
 	if (rt->rt_gap != 0) {
 		if (do_fill) {
-			if (rs->rs_fill == size) {
-				start = rs->rs_start;
-				end = rs->rs_end;
+			if (rs_get_fill(rs, rt) == size) {
+				start = rs_get_start(rs, rt);
+				end = rs_get_end(rs, rt);
 				size = end - start;
 			} else {
 				range_tree_adjust_fill(rt, rs, -size);
 				return;
 			}
-		} else if (rs->rs_start != start || rs->rs_end != end) {
+		} else if (rs_get_start(rs, rt) != start ||
+		    rs_get_end(rs, rt) != end) {
 			zfs_panic_recover("zfs: freeing partial segment of "
 			    "gap tree (offset=%llu size=%llu) of "
 			    "(offset=%llu size=%llu)",
 			    (longlong_t)start, (longlong_t)size,
-			    (longlong_t)rs->rs_start,
-			    (longlong_t)rs->rs_end - rs->rs_start);
+			    (longlong_t)rs_get_start(rs, rt),
+			    (longlong_t)rs_get_end(rs, rt) - rs_get_start(rs,
+			    rt));
 			return;
 		}
 	}
 
-	VERIFY3U(rs->rs_start, <=, start);
-	VERIFY3U(rs->rs_end, >=, end);
+	VERIFY3U(rs_get_start(rs, rt), <=, start);
+	VERIFY3U(rs_get_end(rs, rt), >=, end);
 
-	left_over = (rs->rs_start != start);
-	right_over = (rs->rs_end != end);
+	left_over = (rs_get_start(rs, rt) != start);
+	right_over = (rs_get_end(rs, rt) != end);
 
 	range_tree_stat_decr(rt, rs);
 
@@ -419,24 +477,33 @@ range_tree_remove_impl(range_tree_t *rt, uint64_t start, uint64_t size,
 		rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
 
 	if (left_over && right_over) {
-		newseg = kmem_cache_alloc(range_seg_cache, KM_SLEEP);
-		newseg->rs_start = end;
-		newseg->rs_end = rs->rs_end;
-		newseg->rs_fill = newseg->rs_end - newseg->rs_start;
-		range_tree_stat_incr(rt, newseg);
+		range_seg_max_t newseg;
+		rs_set_start(&newseg, rt, end);
+		rs_set_end_raw(&newseg, rt, rs_get_end_raw(rs, rt));
+		rs_set_fill(&newseg, rt, rs_get_end(rs, rt) - end);
+		range_tree_stat_incr(rt, &newseg);
 
-		rs->rs_end = start;
+		// This modifies the buffer already inside the range tree
+		rs_set_end(rs, rt, start);
+
+		rs_copy(rs, &rs_tmp, rt);
+		if (zfs_btree_next(&rt->rt_root, &where, &where) != NULL)
+			zfs_btree_insert(&rt->rt_root, &newseg, &where);
+		else
+			zfs_btree_add(&rt->rt_root, &newseg);
 
-		avl_insert_here(&rt->rt_root, newseg, rs, AVL_AFTER);
 		if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
-			rt->rt_ops->rtop_add(rt, newseg, rt->rt_arg);
+			rt->rt_ops->rtop_add(rt, &newseg, rt->rt_arg);
 	} else if (left_over) {
-		rs->rs_end = start;
+		// This modifies the buffer already inside the range tree
+		rs_set_end(rs, rt, start);
+		rs_copy(rs, &rs_tmp, rt);
 	} else if (right_over) {
-		rs->rs_start = end;
+		// This modifies the buffer already inside the range tree
+		rs_set_start(rs, rt, end);
+		rs_copy(rs, &rs_tmp, rt);
 	} else {
-		avl_remove(&rt->rt_root, rs);
-		kmem_cache_free(range_seg_cache, rs);
+		zfs_btree_remove_from(&rt->rt_root, &where);
 		rs = NULL;
 	}
 
@@ -446,11 +513,12 @@ range_tree_remove_impl(range_tree_t *rt, uint64_t start, uint64_t size,
 		 * the size, since we do not support removing partial segments
 		 * of range trees with gaps.
 		 */
-		rs->rs_fill = rs->rs_end - rs->rs_start;
-		range_tree_stat_incr(rt, rs);
+		rs_set_fill_raw(rs, rt, rs_get_end_raw(rs, rt) -
+		    rs_get_start_raw(rs, rt));
+		range_tree_stat_incr(rt, &rs_tmp);
 
 		if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
-			rt->rt_ops->rtop_add(rt, rs, rt->rt_arg);
+			rt->rt_ops->rtop_add(rt, &rs_tmp, rt->rt_arg);
 	}
 
 	rt->rt_space -= size;
@@ -472,14 +540,14 @@ void
 range_tree_resize_segment(range_tree_t *rt, range_seg_t *rs,
     uint64_t newstart, uint64_t newsize)
 {
-	int64_t delta = newsize - (rs->rs_end - rs->rs_start);
+	int64_t delta = newsize - (rs_get_end(rs, rt) - rs_get_start(rs, rt));
 
 	range_tree_stat_decr(rt, rs);
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
 		rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
 
-	rs->rs_start = newstart;
-	rs->rs_end = newstart + newsize;
+	rs_set_start(rs, rt, newstart);
+	rs_set_end(rs, rt, newstart + newsize);
 
 	range_tree_stat_incr(rt, rs);
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
@@ -491,22 +559,27 @@ range_tree_resize_segment(range_tree_t *rt, range_seg_t *rs,
 static range_seg_t *
 range_tree_find_impl(range_tree_t *rt, uint64_t start, uint64_t size)
 {
-	range_seg_t rsearch;
+	range_seg_max_t rsearch;
 	uint64_t end = start + size;
 
 	VERIFY(size != 0);
 
-	rsearch.rs_start = start;
-	rsearch.rs_end = end;
-	return (avl_find(&rt->rt_root, &rsearch, NULL));
+	rs_set_start(&rsearch, rt, start);
+	rs_set_end(&rsearch, rt, end);
+	return (zfs_btree_find(&rt->rt_root, &rsearch, NULL));
 }
 
 range_seg_t *
 range_tree_find(range_tree_t *rt, uint64_t start, uint64_t size)
 {
+	if (rt->rt_type == RANGE_SEG64)
+		ASSERT3U(start + size, >, start);
+
 	range_seg_t *rs = range_tree_find_impl(rt, start, size);
-	if (rs != NULL && rs->rs_start <= start && rs->rs_end >= start + size)
+	if (rs != NULL && rs_get_start(rs, rt) <= start &&
+	    rs_get_end(rs, rt) >= start + size) {
 		return (rs);
+	}
 	return (NULL);
 }
 
@@ -533,24 +606,28 @@ boolean_t
 range_tree_find_in(range_tree_t *rt, uint64_t start, uint64_t size,
     uint64_t *ostart, uint64_t *osize)
 {
-	range_seg_t rsearch;
-	rsearch.rs_start = start;
-	rsearch.rs_end = start + 1;
+	if (rt->rt_type == RANGE_SEG64)
+		ASSERT3U(start + size, >, start);
 
-	avl_index_t where;
-	range_seg_t *rs = avl_find(&rt->rt_root, &rsearch, &where);
+	range_seg_max_t rsearch;
+	rs_set_start(&rsearch, rt, start);
+	rs_set_end_raw(&rsearch, rt, rs_get_start_raw(&rsearch, rt) + 1);
+
+	zfs_btree_index_t where;
+	range_seg_t *rs = zfs_btree_find(&rt->rt_root, &rsearch, &where);
 	if (rs != NULL) {
 		*ostart = start;
-		*osize = MIN(size, rs->rs_end - start);
+		*osize = MIN(size, rs_get_end(rs, rt) - start);
 		return (B_TRUE);
 	}
 
-	rs = avl_nearest(&rt->rt_root, where, AVL_AFTER);
-	if (rs == NULL || rs->rs_start > start + size)
+	rs = zfs_btree_next(&rt->rt_root, &where, &where);
+	if (rs == NULL || rs_get_start(rs, rt) > start + size)
 		return (B_FALSE);
 
-	*ostart = rs->rs_start;
-	*osize = MIN(start + size, rs->rs_end) - rs->rs_start;
+	*ostart = rs_get_start(rs, rt);
+	*osize = MIN(start + size, rs_get_end(rs, rt)) -
+	    rs_get_start(rs, rt);
 	return (B_TRUE);
 }
 
@@ -566,9 +643,12 @@ range_tree_clear(range_tree_t *rt, uint64_t start, uint64_t size)
 	if (size == 0)
 		return;
 
+	if (rt->rt_type == RANGE_SEG64)
+		ASSERT3U(start + size, >, start);
+
 	while ((rs = range_tree_find_impl(rt, start, size)) != NULL) {
-		uint64_t free_start = MAX(rs->rs_start, start);
-		uint64_t free_end = MIN(rs->rs_end, start + size);
+		uint64_t free_start = MAX(rs_get_start(rs, rt), start);
+		uint64_t free_end = MIN(rs_get_end(rs, rt), start + size);
 		range_tree_remove(rt, free_start, free_end - free_start);
 	}
 }
@@ -579,7 +659,7 @@ range_tree_swap(range_tree_t **rtsrc, range_tree_t **rtdst)
 	range_tree_t *rt;
 
 	ASSERT0(range_tree_space(*rtdst));
-	ASSERT0(avl_numnodes(&(*rtdst)->rt_root));
+	ASSERT0(zfs_btree_numnodes(&(*rtdst)->rt_root));
 
 	rt = *rtsrc;
 	*rtsrc = *rtdst;
@@ -589,16 +669,20 @@ range_tree_swap(range_tree_t **rtsrc, range_tree_t **rtdst)
 void
 range_tree_vacate(range_tree_t *rt, range_tree_func_t *func, void *arg)
 {
-	range_seg_t *rs;
-	void *cookie = NULL;
-
 	if (rt->rt_ops != NULL && rt->rt_ops->rtop_vacate != NULL)
 		rt->rt_ops->rtop_vacate(rt, rt->rt_arg);
 
-	while ((rs = avl_destroy_nodes(&rt->rt_root, &cookie)) != NULL) {
-		if (func != NULL)
-			func(arg, rs->rs_start, rs->rs_end - rs->rs_start);
-		kmem_cache_free(range_seg_cache, rs);
+	if (func != NULL) {
+		range_seg_t *rs;
+		zfs_btree_index_t *cookie = NULL;
+
+		while ((rs = zfs_btree_destroy_nodes(&rt->rt_root, &cookie)) !=
+		    NULL) {
+			func(arg, rs_get_start(rs, rt), rs_get_end(rs, rt) -
+			    rs_get_start(rs, rt));
+		}
+	} else {
+		zfs_btree_clear(&rt->rt_root);
 	}
 
 	bzero(rt->rt_histogram, sizeof (rt->rt_histogram));
@@ -608,16 +692,18 @@ range_tree_vacate(range_tree_t *rt, range_tree_func_t *func, void *arg)
 void
 range_tree_walk(range_tree_t *rt, range_tree_func_t *func, void *arg)
 {
-	for (range_seg_t *rs = avl_first(&rt->rt_root); rs;
-	    rs = AVL_NEXT(&rt->rt_root, rs)) {
-		func(arg, rs->rs_start, rs->rs_end - rs->rs_start);
+	zfs_btree_index_t where;
+	for (range_seg_t *rs = zfs_btree_first(&rt->rt_root, &where);
+	    rs != NULL; rs = zfs_btree_next(&rt->rt_root, &where, &where)) {
+		func(arg, rs_get_start(rs, rt), rs_get_end(rs, rt) -
+		    rs_get_start(rs, rt));
 	}
 }
 
 range_seg_t *
 range_tree_first(range_tree_t *rt)
 {
-	return (avl_first(&rt->rt_root));
+	return (zfs_btree_first(&rt->rt_root, NULL));
 }
 
 uint64_t
@@ -629,7 +715,7 @@ range_tree_space(range_tree_t *rt)
 uint64_t
 range_tree_numsegs(range_tree_t *rt)
 {
-	return ((rt == NULL) ? 0 : avl_numnodes(&rt->rt_root));
+	return ((rt == NULL) ? 0 : zfs_btree_numnodes(&rt->rt_root));
 }
 
 boolean_t
@@ -639,71 +725,76 @@ range_tree_is_empty(range_tree_t *rt)
 	return (range_tree_space(rt) == 0);
 }
 
-/* Generic range tree functions for maintaining segments in an AVL tree. */
+/* ARGSUSED */
 void
-rt_avl_create(range_tree_t *rt, void *arg)
-{
-	avl_tree_t *tree = arg;
-
-	avl_create(tree, rt->rt_avl_compare, sizeof (range_seg_t),
-	    offsetof(range_seg_t, rs_pp_node));
+rt_btree_create(range_tree_t *rt, void *arg)
+{
+	zfs_btree_t *size_tree = arg;
+
+	size_t size;
+	switch (rt->rt_type) {
+	case RANGE_SEG32:
+		size = sizeof (range_seg32_t);
+		break;
+	case RANGE_SEG64:
+		size = sizeof (range_seg64_t);
+		break;
+	case RANGE_SEG_GAP:
+		size = sizeof (range_seg_gap_t);
+		break;
+	default:
+		panic("Invalid range seg type %d", rt->rt_type);
+	}
+	zfs_btree_create(size_tree, rt->rt_btree_compare, size);
 }
 
+/* ARGSUSED */
 void
-rt_avl_destroy(range_tree_t *rt, void *arg)
+rt_btree_destroy(range_tree_t *rt, void *arg)
 {
-	avl_tree_t *tree = arg;
+	zfs_btree_t *size_tree = arg;
+	ASSERT0(zfs_btree_numnodes(size_tree));
 
-	ASSERT0(avl_numnodes(tree));
-	avl_destroy(tree);
+	zfs_btree_destroy(size_tree);
 }
 
+/* ARGSUSED */
 void
-rt_avl_add(range_tree_t *rt, range_seg_t *rs, void *arg)
+rt_btree_add(range_tree_t *rt, range_seg_t *rs, void *arg)
 {
-	avl_tree_t *tree = arg;
-	avl_add(tree, rs);
-}
+	zfs_btree_t *size_tree = arg;
 
-void
-rt_avl_remove(range_tree_t *rt, range_seg_t *rs, void *arg)
-{
-	avl_tree_t *tree = arg;
-	avl_remove(tree, rs);
+	zfs_btree_add(size_tree, rs);
 }
 
+/* ARGSUSED */
 void
-rt_avl_vacate(range_tree_t *rt, void *arg)
+rt_btree_remove(range_tree_t *rt, range_seg_t *rs, void *arg)
 {
-	/*
-	 * Normally one would walk the tree freeing nodes along the way.
-	 * Since the nodes are shared with the range trees we can avoid
-	 * walking all nodes and just reinitialize the avl tree. The nodes
-	 * will be freed by the range tree, so we don't want to free them here.
-	 */
-	rt_avl_create(rt, arg);
-}
+	zfs_btree_t *size_tree = arg;
 
-uint64_t
-range_tree_min(range_tree_t *rt)
-{
-	range_seg_t *rs = avl_first(&rt->rt_root);
-	return (rs != NULL ? rs->rs_start : 0);
+	zfs_btree_remove(size_tree, rs);
 }
 
-uint64_t
-range_tree_max(range_tree_t *rt)
+/* ARGSUSED */
+void
+rt_btree_vacate(range_tree_t *rt, void *arg)
 {
-	range_seg_t *rs = avl_last(&rt->rt_root);
-	return (rs != NULL ? rs->rs_end : 0);
-}
+	zfs_btree_t *size_tree = arg;
+	zfs_btree_clear(size_tree);
+	zfs_btree_destroy(size_tree);
 
-uint64_t
-range_tree_span(range_tree_t *rt)
-{
-	return (range_tree_max(rt) - range_tree_min(rt));
+	rt_btree_create(rt, arg);
 }
 
+range_tree_ops_t rt_btree_ops = {
+	.rtop_create = rt_btree_create,
+	.rtop_destroy = rt_btree_destroy,
+	.rtop_add = rt_btree_add,
+	.rtop_remove = rt_btree_remove,
+	.rtop_vacate = rt_btree_vacate
+};
+
 /*
  * Remove any overlapping ranges between the given segment [start, end)
  * from removefrom. Add non-overlapping leftovers to addto.
@@ -712,42 +803,62 @@ void
 range_tree_remove_xor_add_segment(uint64_t start, uint64_t end,
     range_tree_t *removefrom, range_tree_t *addto)
 {
-	avl_index_t where;
-	range_seg_t starting_rs = {
-		.rs_start = start,
-		.rs_end = start + 1
-	};
+	zfs_btree_index_t where;
+	range_seg_max_t starting_rs;
+	rs_set_start(&starting_rs, removefrom, start);
+	rs_set_end_raw(&starting_rs, removefrom, rs_get_start_raw(&starting_rs,
+	    removefrom) + 1);
 
-	range_seg_t *curr = avl_find(&removefrom->rt_root,
+	range_seg_t *curr = zfs_btree_find(&removefrom->rt_root,
 	    &starting_rs, &where);
 
 	if (curr == NULL)
-		curr = avl_nearest(&removefrom->rt_root, where, AVL_AFTER);
+		curr = zfs_btree_next(&removefrom->rt_root, &where, &where);
 
 	range_seg_t *next;
 	for (; curr != NULL; curr = next) {
-		next = AVL_NEXT(&removefrom->rt_root, curr);
-
 		if (start == end)
 			return;
 		VERIFY3U(start, <, end);
 
 		/* there is no overlap */
-		if (end <= curr->rs_start) {
+		if (end <= rs_get_start(curr, removefrom)) {
 			range_tree_add(addto, start, end - start);
 			return;
 		}
 
-		uint64_t overlap_start = MAX(curr->rs_start, start);
-		uint64_t overlap_end = MIN(curr->rs_end, end);
+		uint64_t overlap_start = MAX(rs_get_start(curr, removefrom),
+		    start);
+		uint64_t overlap_end = MIN(rs_get_end(curr, removefrom),
+		    end);
 		uint64_t overlap_size = overlap_end - overlap_start;
 		ASSERT3S(overlap_size, >, 0);
+		range_seg_max_t rs;
+		rs_copy(curr, &rs, removefrom);
+
 		range_tree_remove(removefrom, overlap_start, overlap_size);
 
 		if (start < overlap_start)
 			range_tree_add(addto, start, overlap_start - start);
 
 		start = overlap_end;
+		next = zfs_btree_find(&removefrom->rt_root, &rs, &where);
+		/*
+		 * If we find something here, we only removed part of the
+		 * curr segment. Either there's some left at the end
+		 * because we've reached the end of the range we're removing,
+		 * or there's some left at the start because we started
+		 * partway through the range.  Either way, we continue with
+		 * the loop. If it's the former, we'll return at the start of
+		 * the loop, and if it's the latter we'll see if there is more
+		 * area to process.
+		 */
+		if (next != NULL) {
+			ASSERT(start == end || start == rs_get_end(&rs,
+			    removefrom));
+		}
+
+		next = zfs_btree_next(&removefrom->rt_root, &where, &where);
 	}
 	VERIFY3P(curr, ==, NULL);
 
@@ -767,9 +878,30 @@ void
 range_tree_remove_xor_add(range_tree_t *rt, range_tree_t *removefrom,
     range_tree_t *addto)
 {
-	for (range_seg_t *rs = avl_first(&rt->rt_root); rs;
-	    rs = AVL_NEXT(&rt->rt_root, rs)) {
-		range_tree_remove_xor_add_segment(rs->rs_start, rs->rs_end,
-		    removefrom, addto);
+	zfs_btree_index_t where;
+	for (range_seg_t *rs = zfs_btree_first(&rt->rt_root, &where); rs;
+	    rs = zfs_btree_next(&rt->rt_root, &where, &where)) {
+		range_tree_remove_xor_add_segment(rs_get_start(rs, rt),
+		    rs_get_end(rs, rt), removefrom, addto);
 	}
 }
+
+uint64_t
+range_tree_min(range_tree_t *rt)
+{
+	range_seg_t *rs = zfs_btree_first(&rt->rt_root, NULL);
+	return (rs != NULL ? rs_get_start(rs, rt) : 0);
+}
+
+uint64_t
+range_tree_max(range_tree_t *rt)
+{
+	range_seg_t *rs = zfs_btree_last(&rt->rt_root, NULL);
+	return (rs != NULL ? rs_get_end(rs, rt) : 0);
+}
+
+uint64_t
+range_tree_span(range_tree_t *rt)
+{
+	return (range_tree_max(rt) - range_tree_min(rt));
+}