Magento Redis setup on AWS

Question

I have the following setup:

Magento community 1.9.2

1 x Ec2 frontend, running Litespeed Webserver, php run through Litespeed's Php implementation - LSPHP, sort of like fast-cgi used in Nginx

1 x RDS server for database

I just configured 1 x Elasticache (Redis node)

I have configured my local.xml to send session and backend cache to the Redis database.

I can see the cache store is filling up, plus the speed difference is notable (plus no files in cache and session folders)

My three questions are:

1. do I still need to install php-Redis extension on the frontend web-server for php? (Considering it's all working) I can't seem to work out what it's used for ?

2. Is there a way to know how much space I'll need ? My file store cache was less than 50mb yet my redis cache is already used 300mb

3. What happens if the store fills up? Does it delete old session caches ? Or does the site start having issues on the front end (like carts being emptied?)

Answer 1

For #2, Magento creates a session for each visitor; to get an estimate of how much space you need, just multiply the size of one session storage by the highest number of visitors (and multiply by.. let's say 4-5 or maybe even 10, to be safe :D). You can also setup alters in AWS, for when you reach like 80% memory usage. Also, you can check if garbage collection is working and play with the expires.

For #3, see this response on SO https://stackoverflow.com/a/8654367/64944

Answer 2

php-redis is a c php extension, it will perform better than native php. If php-redis is available Magento will use that for its communications with redis, if not it will fall back to a native php implementation. You don't need to use php-redis but if its installed you might get slightly better performance.