I am using Magento ver. 18.104.22.168. Site is having an error in admin side "cache_dir is not writable". Site frontend is working fine and in admin side only dashboard and configuration first page is working. other pages in admin side is giving this error "cache_dir is not writable". I checked the cache and var folder permission ,permission is set to 0777. I deleted the cache folder too. error is still not gone.
You'll get this error if your disk is full or partition has run outof inodes. Also check if
/tmp/magento exists. Usually it's that partition that's the real cause. In these cases, suphp or mod_ruid is usually in play, denying access to the var/ directory because it's owner is not the right user and Magento will fall back to
/tmp/magento. Then add the default
tmp_dir for php sessions and MySQL temporary tables and you have your scenario for randomly seeing this error.