[9fans] fossil+venti backup question

[bituman]

From time to time i get back to plan9 and starting fiddling with it, 
now i might found a reason to make it useful where i work..
If i understand correctly, venti uses fossil as a write buffer for 
today's modifications, and if i want to access data in previous week's 
state, then files are served by venti.
Or (as i read in plan 9 intro pdf): "In some cases, a single file server 
may provide more than one file tree. For example, the file system
program used in Plan 9, fossil, makes a snapshot of the entire file tree 
each day, at 5am,
and archives it for the posterity. It archives only the changes with 
respect to the last archive, but
provides the illusion that the whole tree was archived as it was that day."
What id like to know is:
can i simply use fossil or do i need fossil+venti?
can backup period (currently daily) be set to stg. else? Like every two 
hours?