从Memcached哈希中删除特定键

Question

I am trying to cache a table t_abc using memcached, for my rails application. The table has 50,000 entries. And I finally want have 20,000 keys(which will be of the form "abc_"+id). When the 20,001st entry is to be inserted in the cache, I want the least recently used key out of these 20,000(of the above form, and not some other keys in the memcached) to be deleted from the cache. How do I achieve that?

NOTE: I am keeping an expiry = 0 for all the entries.

Answer 1

No, unfortunately you cannot efficiently do what you want to do with memcached.

It has a single LRU which works across all the data you are storing. So if you are storing data from multiple tables then the fraction of the memcached entries taken up by each table depends on the relative patterns of access of the data from the different tables.

So to control the amount of rows of the table are cached, really all you can do is adjust how big your memcached is and vary what other data gets stored.

(Also, to evaluate the memcached configuration you might consider using a different metric, such as the hit rate or the median response time, rather than simply the number of rows cached.)