Here is a 2006 article about it - there is no doubt more stuff on LWN if you dig around https://lwn.net/Articles/179361/ https://lwn.net/Articles/179361/ The g

Here is a 2006 article about it - there is no doubt more stuff on LWN if you dig around https://lwn.net/Articles/179361/ https://lwn.net/Articles/179361/ The gist is that there were a lot of out of tree solutions (openvz is still used eg by Travis CI) but they had conflicting requirements and a lot of the suggestions were just "we need BSD jails" which is a very monolithic single purpose solution, like having a web server in the kernel (yes there was one) rather than having the kernel APIs in place so that was not needed. What we have ended up with (mainly cgroups and namespacing basically) is much more flexible in that each piece is for a separate thing, eg you can just namespace networks without full isolation (which is really useful for eg testing network code without messing with the real config). It did take a very very long time though.