[zardosht]

(I work for Tokutek)

Write concurrecy: yes, TokuMX does not have a database level reader/writer lock.

Index Building: yes, fractal trees can write data much more efficiently, so if index building is a problem, I bet TokuMX solves it.

Practically reducing file size: to be honest, I am not sure because due to our great compression, this has not been a general issue for our users. Our reindex command could reduce file size, but I cannot point to examples.

One of our big goals is to address storage issues MongoDB has.