Trying to understand how aio(4), mount(8) async (and gjournal) relate

[Mel Flynn]

Hello list,

in an effort to try and understand the various async systems available, it's 
unclear to me how (if at all) the above relate.

If one mounts a disk with async, does this internally use aio system calls, or 
is there a subsystem available which does largely the same? If so, why are 
there 2 subsystems?

When using aio, for example with squid, does this mean the underlying provider 
needs to be mounted async or is this totally unrelated? Similarly if said disk 
is on a gjournalled partition, is the async mount redundant or is using aio 
redundant or neither?

The systems themselves are properly documented - I just couldn't find any info 
on how they relate, or why they are unrelated.
-- 
Mel