Fix metaslab group fragmentation math

Since we are calculating a free space fragmentation, we should
weight metaslabs by the amount of their free space, not a full
size.  Fragmentation of full metaslabs may not matter in presence
empty ones.  The old algorithm did not differentiate metaslabs
having only one one empty 4KB block from metaslabs having 50% of
space free in 4KB blocks, reporting higher fragmentation.

While there, move metaslab_group_alloc_update() call after setting
mg_fragmentation, otherwise the effect may be delayed by one TXG.

Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.

module/zfs/metaslab.c

@@ -1175,9 +1175,8 @@ metaslab_group_sort(metaslab_group_t *mg, metaslab_t *msp, uint64_t weight)
 }
 
 /*
- * Calculate the fragmentation for a given metaslab group. We can use
- * a simple average here since all metaslabs within the group must have
- * the same size. The return value will be a value between 0 and 100
+ * Calculate the fragmentation for a given metaslab group.  Weight metaslabs
+ * on the amount of free space.  The return value will be between 0 and 100
  * (inclusive), or ZFS_FRAG_INVALID if less than half of the metaslab in this
  * group have a fragmentation metric.
  */
@@ -1186,24 +1185,29 @@ metaslab_group_fragmentation(metaslab_group_t *mg)
 {
 	vdev_t *vd = mg->mg_vd;
 	uint64_t fragmentation = 0;
-	uint64_t valid_ms = 0;
+	uint64_t valid_ms = 0, total_ms = 0;
+	uint64_t free, total_free = 0;
 
 	for (int m = 0; m < vd->vdev_ms_count; m++) {
 		metaslab_t *msp = vd->vdev_ms[m];
 
-		if (msp->ms_fragmentation == ZFS_FRAG_INVALID)
-			continue;
 		if (msp->ms_group != mg)
 			continue;
+		total_ms++;
+		if (msp->ms_fragmentation == ZFS_FRAG_INVALID)
+			continue;
 
 		valid_ms++;
-		fragmentation += msp->ms_fragmentation;
+		free = (msp->ms_size - metaslab_allocated_space(msp)) /
+		    SPA_MINBLOCKSIZE;  /* To prevent overflows. */
+		total_free += free;
+		fragmentation += msp->ms_fragmentation * free;
 	}
 
-	if (valid_ms <= mg->mg_vd->vdev_ms_count / 2)
+	if (total_ms == 0 || valid_ms < (total_ms + 1) / 2 || total_free == 0)
 		return (ZFS_FRAG_INVALID);
 
-	fragmentation /= valid_ms;
+	fragmentation /= total_free;
 	ASSERT3U(fragmentation, <=, 100);
 	return (fragmentation);
 }
@@ -4457,8 +4461,8 @@ metaslab_sync_reassess(metaslab_group_t *mg)
 	spa_t *spa = mg->mg_class->mc_spa;
 
 	spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);
-	metaslab_group_alloc_update(mg);
 	mg->mg_fragmentation = metaslab_group_fragmentation(mg);
+	metaslab_group_alloc_update(mg);
 
 	/*
 	 * Preload the next potential metaslabs but only on active