- * If we previously allocated a block out beyond eof and we are
- * now coming back to use it then we will need to flag it as new
- * even if it has a disk address.
+ * If we previously allocated a block out beyond eof and we are now
+ * coming back to use it then we will need to flag it as new even if it
+ * has a disk address.
+ *
+ * With sub-block writes into unwritten extents we also need to mark
+ * the buffer as new so that the unwritten parts of the buffer gets
+ * correctly zeroed.