memcache, multiple instances vs single instance

Question

I've got an website running php/mysql. The app will be 'copied' multiple times. each instance will get its own database, set of php files (wildcard)domain, you get the idea.

Now some parts of the app is cached using memcache and I am wondering if it'd be better/faster/more convient to use

• one memcache instance per app instance

• or one big memcache instance and prefix each key with some unique token per app instance

I would guess the advantage to using a single memcache instance would be a large 'index table'

where as multiple memcache instances would cause more overhead

Thanks for any help

Answer 1

Use one memcached instance and prefix your keys -- this is the easiest way to ensure optimal resource utilization. If you have a separate memcached instance per application instance, an unused, idle instance claims memory that your active instances can't use.