Cache inconsistencies when using RDB

Why are you using a persistent state for redis cache at all?

FWIW, if you'd import an old database dump you would also have a mess, if you have bad data, you get bad results, I don't really see a way around that.

We did not make this choice and we do recommend to have RDB disabled.
But some hosting providers have it enabled by default (and sometimes don't advertise it), that's how we got bit by that.

Also, I guess there can be some legitimate use cases. For example, if the Drupal site uses the same Redis server as another website that does need to store persistent data in Redis (some other CMSs store queues in Redis for example).
And according to this doc, it seems that RDB is enabled by default if the save option is not explicitly defined, so other people could be impacted by this and not know it.

We mitigated this issue by adding a custom hook_requirements() that checks if RDB is enabled:

/** @var \Drupal\redis\ClientFactory $redisFactory */
$redisFactory = \Drupal::service('redis.factory');

/** @var \Redis $client */
$client = $redisFactory::getClient();

/** @var string[] $config */
$config = $client->config('GET', 'save');

if (!empty($config['save'])) {
  // Add warning.
}

We have a status page that already has some checks and recommendations, patches welcome to check for the save setting and add a warning that recommends to disable it. That's the only actionable option that I see here.

If you have other things you do need to store persistently, I'd set up separate server for that.

The MR adds a warning when using PhpRedis. We don't use Predis so I'm not sure how to check the config with it.