What are the limitations of ip_hash ?

[Mark Swanson]

    upstream tomcatcluster {
        ip_hash;
        server test1:8080 max_fails=0;
        server test2:8080 max_fails=0;
    }

It seems that the size of ip_hash is too small and it can't hold enough
keys - or perhaps nginx clears out keys after a period of time?

I'm curious what the limitations of ip_hash are because it's not working
100% of the time.

Thank you.