Hello First, which Zope version do you have?
Sometimes it reported pool-size of 10, and sometimes it reported 7! What could this be? Maybe you're seeing warnings from few storages? but I'm not sure how this works an I have no idea why you're seeing pool sizes as 7 and 10 if you set this to 20....
Another question: My expect a load of 50 concurrent requests, so I set zserver-threads to 25 on both zeo-clients, and a pool-size of 20 (but behaving as explained above). Do this conf make any sense? Where should I read to get more info about this matter? I think it is better to use more Zeo clients than bump number of threads. There were some benchmarks (or sth like that) showing that bigger number of threads doesn't help too much to performance, and it may be even worse.
In your setup, if you have 25 threads you should have pool size bigger than 25 - every thread will possibly use ZODB so it should have connection to use. In fact connection is created as needed, even if pool_size is too small. That is why number of connections in your logs is bigger than pool_size. -- Maciej Wisniowski