Issue
Your site's cache_form MySQL table has grown uncontrollably, and you need to reclaim disk space.
Resolution
Some common options like running a DELETE or TRUNCATE command in MySQL could reclaim the space, however, if your site is under load or the MySQL disk storage is running out, they could actually create other problems. Therefore we recommend a different strategy to minimize impact.
You can run this MySQL query which will quickly get rid of any data on that table and reclaim the used disk space. You MUST run these as a single chained command (or in quick succession) to avoid triggering errors on incoming web requests to Drupal.
RENAME cache_form TO cache_form2;
CREATE TABLE cache_form LIKE cache_form2;
DROP TABLE cache_form2;Cause
The most common culprit for the cache_form table growing uncontrollably is not running a regularly-scheduled job that runs Drupal cron (which cleans up this table periodically).
Other culprits include having heavily-visited pages that contain forms (like Voting API forms or similar) that are also not cached in Varnish, and/or robot-generated traffic that visits URLs on your site that contain forms. For the latter, consider using blocking some robot user agents via robots.txt, .htaccess rules, etc.