由Memcached处理的PHP会话仍在磁盘上

We have PHP5 FPM set up under Nginx. We use Memcached as our session handler.

session.save_handler=memcached

My expectation is that, without fail (notwithstanding some fatal error like the death of our Memcached server) that all sessions should make it to Memcached and explicitly NOT disk.

However, upon inspecting our application, I've found sessions on Memcached AND in /var/lib/php5/fpm/.

Some troubleshooting:

  1. We are definitely getting new sessions set on Memcached. However, some sessions that I found on disk, don't appear on Memcached
  2. The timestamps on the file based sessions are definitely recent - there are files in the current minute.
  3. Permissions on the files are for the installation user - not root.

Despite having said point 3 above, there are SOME files that have the root user and group ownership. This I find weird. Why would there be sessions owned by root? That would mean that anyone trying to check the file (that has 0600 permissions btw) would fail.

So, I guess my questions amount to:

  1. Is there any scenario in which it is valid that new session files are created on disk despite the fact that we use Memcached?
  2. Any idea why we'd have session files that have a root ownership?

For context: I'm researching very sporadic session expiry issues. After having increased Memcached memory limits and concurrent connections (and that ultimately fixing a large number of the instances) we're still experiencing a small amount of the session expiries. Anyway, that is simply context - might not be important.

The session files were created by php-cli started by cron. cli config differs from fpm one and uses default file session handler.

Edit

Importantly, the cronjob must either be hitting a piece of code that manually starts the session
OR
the configuration directive session.auto_start for PHP5-cli must be set to true