mon/OSDMonitor: Warn when expected_num_objects will have no effect

The expected_num_objects argument to ceph osd pool create is
only effective on filestore pools when merging is disabled
(filestore_merge_threshold < 0). Warn and disallow pool creation
in this situation.

Signed-off-by: Douglas Fuller <dfuller@redhat.com>

src/mon/OSDMonitor.cc

@@ -11119,6 +11119,14 @@ bool OSDMonitor::prepare_command_impl(MonOpRequestRef op,
       goto reply;
     }
 
+    if (expected_num_objects > 0 &&
+	cct->_conf->osd_objectstore == "filestore" &&
+	cct->_conf->filestore_merge_threshold > 0) {
+      ss << "'expected_num_objects' requires 'filestore_merge_threshold < 0'";
+      err = -EINVAL;
+      goto reply;
+    }
+
     int64_t fast_read_param;
     cmd_getval(cct, cmdmap, "fast_read", fast_read_param, int64_t(-1));
     FastReadType fast_read = FAST_READ_DEFAULT;