civicrm_cache table is nearly 3GB... does it matter if we empty it, if not can we just not back it up ?
If you TRUNCATE the table, be sure you do so at a time when there are no in-process contributions or other users actively filling out a form or their submission will be lost. Otherwise, it's safe to do so.
Your backups can just skip that table, no need to back it up. Although you do need to get the structure of the table in order to be able to restore a complete db from your backup.
Best to exclude its content from your backups. You can dump a table's structure only with
As Coleman points out, form data is persisted there while a multistep form is being used, so using TRUNCATE will invalidate any forms currently being processed.
You might want to review why your
civicrm_cache has grown to this size - check the range of values in
expired_date there. Odds are your CiviCRM isn't "cleaning up" old cache data, and if this was (not) happening you might experience other effects (eg performance degradation).