Memcached limitations

[Richard Jones]

Hi,
I have a bunch of user objects memcached, accessible via key "user:123" where 
123 is the user.id  I also need to reference users by a profile.id (one user 
can have many profiles). Currently i lookup in the database the user id 
associated with a given profile id, then fetch the user object from memcached 
using the user.id from the datbase lookup

What i need is the ability to use multiple keys pointing to the same value. 
For example, user:123  and profile:789 point to the same cached value because 
user123 has a profile with id 789 - no such thing as a profile object, i just 
use profile ids to find users.

Any clever tricks i can use to simulate this in a speedy way?

I plan to store key/vals  "profile:789 --> 123" so that i can first lookup the 
user.id  using the profile.id, and then retreive the value at user:123.

Just wanted to ask if there is a better way to do this..

As an aside, how many keys can memcache cope with before performance suffers, 
or is it just a case of having enough ram?

How much overhead is there involved in storing (say) 200,000   profile:789 --> 
123  type items in memcached? should i mainly be focussing on storing bigger 
items, or doesn't it matter?

Thanks,
Richard.