Regarding memory use you should not expect huge across the board gains, the 40% figure is just relating to the “base OS” not the applications running.
Discourse consumes the vast majority of memory and mem usage is identical.
However, performance wise it is quite interesting.
btrfs means that we halve our boot time, so booting Discourse goes from 12 seconds down to 6.
Time to recompile assets (cached) rake assets:precompile
CoreOS (alpha 431.0.0)
real 0m38.563s
user 0m36.052s
sys 0m1.246s
Ubuntu (14.04.01 + Docker 1.2.0)
real 0m46.913s
user 0m40.888s
sys 0m5.576s
Time to start up rails time rake routes
CoreOS
real 0m5.790s
user 0m4.807s
sys 0m0.756s
Ubuntu
real 0m10.267s
user 0m5.945s
sys 0m4.179s
Time to recompile assets (uncached) redis-cli flushall && rake assets:precompile
###CoreOS
real 3m40.645s
user 3m26.428s
sys 0m2.381s
###Ubuntu
real 4m45.688s
user 4m38.128s
sys 0m8.702s
This doubling of boot time speed is most likely due to the usage of btrfs.