diff --git a/uts/common/fs/zfs/arc.c b/uts/common/fs/zfs/arc.c
index 3804888443e0..95e0bbbbfa37 100644
--- a/uts/common/fs/zfs/arc.c
+++ b/uts/common/fs/zfs/arc.c
@@ -215,6 +215,11 @@ static int arc_dead;
  */
 static boolean_t arc_warm;
 
+/*
+ * log2 fraction of the zio arena to keep free.
+ */
+int arc_zio_arena_free_shift = 2;
+
 /*
  * These tunables are for performance analysis.
  */
@@ -3235,7 +3240,7 @@ arc_available_memory(void)
 	 * heap is allocated.  (Or, in the calculation, if less than 1/4th is
 	 * free)
 	 */
-	n = vmem_size(heap_arena, VMEM_FREE) -
+	n = (int64_t)vmem_size(heap_arena, VMEM_FREE) -
 	    (vmem_size(heap_arena, VMEM_FREE | VMEM_ALLOC) >> 2);
 	if (n < lowest) {
 		lowest = n;
@@ -3246,15 +3251,16 @@ arc_available_memory(void)
 	/*
 	 * If zio data pages are being allocated out of a separate heap segment,
 	 * then enforce that the size of available vmem for this arena remains
-	 * above about 1/16th free.
+	 * above about 1/4th (1/(2^arc_zio_arena_free_shift)) free.
 	 *
-	 * Note: The 1/16th arena free requirement was put in place
-	 * to aggressively evict memory from the arc in order to avoid
-	 * memory fragmentation issues.
+	 * Note that reducing the arc_zio_arena_free_shift keeps more virtual
+	 * memory (in the zio_arena) free, which can avoid memory
+	 * fragmentation issues.
 	 */
 	if (zio_arena != NULL) {
-		n = vmem_size(zio_arena, VMEM_FREE) -
-		    (vmem_size(zio_arena, VMEM_ALLOC) >> 4);
+		n = (int64_t)vmem_size(zio_arena, VMEM_FREE) -
+		    (vmem_size(zio_arena, VMEM_ALLOC) >>
+		    arc_zio_arena_free_shift);
 		if (n < lowest) {
 			lowest = n;
 			r = FMR_ZIO_ARENA;