Add write once option to toc store to avoid creating very large datasets

src/fdb5/toc/TocStore.cc

@@ -105,17 +105,30 @@ bool TocStore::exists() const {
 eckit::DataHandle* TocStore::retrieve(Field& field) const {
     return field.dataHandle();
 }
-
+#define write_once
+bool written_once = false;
+long len = 0;
+eckit::Offset position = 0;
 std::unique_ptr<FieldLocation> TocStore::archive(const Key &key, const void *data, eckit::Length length) {
     dirty_ = true;
 
     eckit::PathName dataPath = getDataPath(key);
 
     eckit::DataHandle &dh = getDataHandle(dataPath);
 
-    eckit::Offset position = dh.position();
+
 
-    long len = dh.write( data, length );
+#ifdef write_once
+    // michalis hack
+    if (written_once == false){
+        position = dh.position();
+        len = dh.write( data, length );
+        written_once = true;
+    }
+#else   
+
+    eckit::Offset position = dh.position();
+#endif
 
     ASSERT(len == length);