Avoid field storage write when field content did not change

Background information about this change

Performance and scalability improvement opt-in for existing installs, enabled for new installations.

This is a backport for Drupal 7 of a performance and scalability improvement introduced in Drupal 8.

This change introduces a check in field SQL storage when saving entities which avoids unnecessary writes when content has apparently not changed. This can deliver a significant performance improvement, especially for sites with high-frequency writes and/or those with many fields. See #2470619: Do not attempt field storage write when field content did not change for details.

The performance gains reported for some migrations are noteworthy with one report of a migration which previously took 70+ hours taking less than 1 hour, and around a 50 percent performance improvement reported by others. (See comments #70 and #77 in the aforementioned issue.)

Installation and setup instructions

• On fresh installs this will be enabled by default, for existing sites, it's an opt-in.
• See the instructions in the newly updated default.settings.php
• add this setting to your settings.php
  

  $conf['field_sql_storage_skip_writing_unchanged_fields'] = TRUE;