55a64ab8_PHP__Statistics___Manual_mysqlnd_statistics__Result_Set_Description
[Statistic] copy_on_write_saved, copy_on_write_performed [Scope] Process [Description] 
With mysqlnd, variables returned by the extensions point into mysqlnd internal network result buffers. If you do not change the variables, fetched data will be kept only once in memory. If you change the variables, mysqlnd has to perform a copy-on-write to protect the internal network result buffers from being changed. With the MySQL Client Library you always hold fetched data twice in memory. Once in the internal MySQL Client Library buffers and once in the variables returned by the extensions. In theory mysqlnd can save up to 40% memory. However, note that the memory saving cannot be measured using memory_get_usage().
PHP: Statistics - Manual
Description