My Silex app has always had the session data stored on the server, but I want to move to the mysql database so that I'm not so tied to a single webserver. I'm wondering about performance, though. I plan to use the PdoSessionHandler. My question is this: currently I have about 177K stored sessions. Will the garbage collection be slow? Will I be taking a performance hit by moving to the database from the filesystem?
Are you going to have an index on the session expiry? If there is no index, then yes, it will be slow. OTOH, how fast do you think searching 177,000 files on disk is? Probably a lot slower than using a database to do the thing it is expressly designed to do.
Will you take a performance hit? Probably. Will it be significant? Depends what else the system is doing with the database, the configuration of the DB, and the server it runs on.
In short - yes, there will be an inevitable cost to use the database as a session store, but it could be worth it for the abilities it gives you.
I'd suggest using Redis, backed to disk though.