MongoDB guys outsourced the entire caching/memory management to OS VM and on my (modest) workloads it performs really well. It probably also means that OS is a bit more relevant now and the choice b/w BSD/Linux isn't just about personal preference anymore, I can imagine that their VM characteristics are quite different and "2006 style" software like mongod won't work the same.

Anyone with low-level experience with BSD/Linux VMs?