Couchbase bucket and LRU (least recently used)

I'm looking into using couchbase to replace memcached and mysql.

Currently we use memcached to cache volatile cache with a 0 expiry. The cache key rolls when a website is published and old cache is eventually ejected. Sessions are stored in MySQL for a constant time (say 8 hours) which updates the last modified on each access.

When trying out couchbase I would like to move both volatile cache and sessions into the couchbase bucket. When storing items with a 0 expiry in couchbase it seems like there is no LRU policy in effect. The memory quickly fulls up as expected, but items with a 0 expiry are then ejected to disk (as per documentation). Items ejected to disk then appear to never be evicted (removed completely). The disk space grows without bounds.

I can't seem to find any documentation anywhere about configuration for max disk size or expiry for the couchbase bucket. If someone has time could they point me to where I can configure couchbase to restrict the disk storage growing out of control? Is infinite cache storage supported in couchbase or should I set up a memcached bucket for that and only use the couchbase bucket for items with a fixed expiry time?

Thanks for any help,

- dave

Conventions I have found in documentation:

* Ejection ( Data is moved out of memory and onto disk, can be recovered from disk )
* Eviction ( Old cache item is deleted )
* LRU - Algorithm in memcached to remove older Least Recently Used cache items from the cache when the cache is full.

Testing Environment:

- 3 Virtual Machines, 256M bucket size, 5G disk (fulls up and then crashes)