+ *
+ * xbf_low is used by the allocator to activate the lowspace algorithm -
+ * when free space is running low the extent allocator may choose to
+ * allocate an extent from an AG without leaving sufficient space for
+ * a btree split when inserting the new extent. In this case the allocator
+ * will enable the lowspace algorithm which is supposed to allow further
+ * allocations (such as btree splits and newroots) to allocate from
+ * sequential AGs. In order to avoid locking AGs out of order the lowspace
+ * algorithm will start searching for free space from AG 0. If the correct
+ * transaction reservations have been made then this algorithm will eventually
+ * find all the space it needs.