add comments for the COPY queue size parameters

Author: lonvia

db-copy.hpp

@@ -67,7 +67,22 @@ class db_cmd_t
 
 struct db_cmd_copy_t : public db_cmd_t
 {
-    enum { Max_buf_size = 10 * 1024 * 1024, Max_buffers = 10 };
+    enum {
+        /** Size of a single buffer with COPY data for Postgresql.
+         *  This is a trade-off between memory usage and sending large chunks
+         *  to speed up processing. Currently a one-size fits all value.
+         *  Needs more testing and individual values per queue.
+         */
+        Max_buf_size = 10 * 1024 * 1024,
+        /** Maximum length of the queue with COPY data.
+         *  In the usual case, PostgreSQL should be faster processing the
+         *  data than it can be produced and there should only be one element
+         *  in the queue. If PostgreSQL is slower, then the queue will always
+         *  be full and it is better to keep the queue smaller to reduce memory
+         *  usage. Current value is just assumed to be a reasonable trade off.
+         */
+        Max_buffers = 10
+    };
     /// Name of the target table for the copy operation
     std::shared_ptr<db_target_descr_t> target;
     /// Vector with object to delete before copying