diff --git a/uts/common/fs/zfs/dmu_objset.c b/uts/common/fs/zfs/dmu_objset.c
index efad81dd2cdd..b6ae9680ca6e 100644
--- a/uts/common/fs/zfs/dmu_objset.c
+++ b/uts/common/fs/zfs/dmu_objset.c
@@ -787,11 +787,17 @@ dmu_objset_create_impl(spa_t *spa, dsl_dataset_t *ds, blkptr_t *bp,
 
 		/*
 		 * Determine the number of levels necessary for the meta-dnode
-		 * to contain DN_MAX_OBJECT dnodes.
+		 * to contain DN_MAX_OBJECT dnodes.  Note that in order to
+		 * ensure that we do not overflow 64 bits, there has to be
+		 * a nlevels that gives us a number of blocks > DN_MAX_OBJECT
+		 * but < 2^64.  Therefore,
+		 * (mdn->dn_indblkshift - SPA_BLKPTRSHIFT) (10) must be
+		 * less than (64 - log2(DN_MAX_OBJECT)) (16).
 		 */
-		while ((uint64_t)mdn->dn_nblkptr << (mdn->dn_datablkshift +
+		while ((uint64_t)mdn->dn_nblkptr <<
+		    (mdn->dn_datablkshift - DNODE_SHIFT +
 		    (levels - 1) * (mdn->dn_indblkshift - SPA_BLKPTRSHIFT)) <
-		    DN_MAX_OBJECT * sizeof (dnode_phys_t))
+		    DN_MAX_OBJECT)
 			levels++;
 
 		mdn->dn_next_nlevels[tx->tx_txg & TXG_MASK] =
diff --git a/uts/common/fs/zfs/spa_misc.c b/uts/common/fs/zfs/spa_misc.c
index 699b15cc58da..a674eea6b047 100644
--- a/uts/common/fs/zfs/spa_misc.c
+++ b/uts/common/fs/zfs/spa_misc.c
@@ -341,9 +341,14 @@ int spa_asize_inflation = 24;
  * it is possible to run the pool completely out of space, causing it to
  * be permanently read-only.
  *
+ * Note that on very small pools, the slop space will be larger than
+ * 3.2%, in an effort to have it be at least spa_min_slop (128MB),
+ * but we never allow it to be more than half the pool size.
+ *
  * See also the comments in zfs_space_check_t.
  */
 int spa_slop_shift = 5;
+uint64_t spa_min_slop = 128 * 1024 * 1024;
 
 /*
  * ==========================================================================
@@ -1637,14 +1642,16 @@ spa_get_asize(spa_t *spa, uint64_t lsize)
 
 /*
  * Return the amount of slop space in bytes.  It is 1/32 of the pool (3.2%),
- * or at least 32MB.
+ * or at least 128MB, unless that would cause it to be more than half the
+ * pool size.
  *
  * See the comment above spa_slop_shift for details.
  */
 uint64_t
-spa_get_slop_space(spa_t *spa) {
+spa_get_slop_space(spa_t *spa)
+{
 	uint64_t space = spa_get_dspace(spa);
-	return (MAX(space >> spa_slop_shift, SPA_MINDEVSIZE >> 1));
+	return (MAX(space >> spa_slop_shift, MIN(space >> 1, spa_min_slop)));
 }
 
 uint64_t
diff --git a/uts/common/fs/zfs/sys/dnode.h b/uts/common/fs/zfs/sys/dnode.h
index dfa3e576c58e..98780b2566af 100644
--- a/uts/common/fs/zfs/sys/dnode.h
+++ b/uts/common/fs/zfs/sys/dnode.h
@@ -58,7 +58,7 @@ extern "C" {
  */
 #define	DNODE_SHIFT		9	/* 512 bytes */
 #define	DN_MIN_INDBLKSHIFT	12	/* 4k */
-#define	DN_MAX_INDBLKSHIFT	14	/* 16k */
+#define	DN_MAX_INDBLKSHIFT	17	/* 128k */
 #define	DNODE_BLOCK_SHIFT	14	/* 16k */
 #define	DNODE_CORE_SIZE		64	/* 64 bytes for dnode sans blkptrs */
 #define	DN_MAX_OBJECT_SHIFT	48	/* 256 trillion (zfs_fid_t limit) */
@@ -88,6 +88,11 @@ extern "C" {
 
 #define	DNODES_PER_BLOCK_SHIFT	(DNODE_BLOCK_SHIFT - DNODE_SHIFT)
 #define	DNODES_PER_BLOCK	(1ULL << DNODES_PER_BLOCK_SHIFT)
+
+/*
+ * This is inaccurate if the indblkshift of the particular object is not the
+ * max.  But it's only used by userland to calculate the zvol reservation.
+ */
 #define	DNODES_PER_LEVEL_SHIFT	(DN_MAX_INDBLKSHIFT - SPA_BLKPTRSHIFT)
 #define	DNODES_PER_LEVEL	(1ULL << DNODES_PER_LEVEL_SHIFT)
 
diff --git a/uts/common/sys/fs/zfs.h b/uts/common/sys/fs/zfs.h
index ebae8beb1da5..6282712b31db 100644
--- a/uts/common/sys/fs/zfs.h
+++ b/uts/common/sys/fs/zfs.h
@@ -600,6 +600,8 @@ typedef struct zpool_rewind_policy {
 
 /*
  * This is needed in userland to report the minimum necessary device size.
+ *
+ * Note that the zfs test suite uses 64MB vdevs.
  */
 #define	SPA_MINDEVSIZE		(64ULL << 20)