What you need is MVCC which is in the pipelines.
With O/L writers will not block readers, but there is the overhead of copying nodes into a workspace to maintain Repeatable_Read semantics.
O/L will allow concurrent writes but this is not fail fast and the 2nd write will throw an exception at commit time when optimistic validation occurs.
Reader threads don't go through the validation phase and a reader will not throw an exception if someone writes at the same time.
Thanks for the reply! :)
You're right: it looks like MVCC is exactly what I need.
So I'm looking forward to TreeCache 3.x! :)