Do this in terminal, if you use Drush and the delete all module:
drush delete-all [content-type-machine-name] Examples: drush delete-all article Delect all article nodes. drush delete-all all Delete nodes of all types. drush delete-all --reset Delete nodes of all types, and reset node, revision and comment counters. drush delete-all users Delete users. Options: --reset Reset counter for node, revision and comment tables. --roles pick roles 
