Long Grace setting for huge cache

[Erik Wickstrom]

I have a web application where the objects are expensive to generate
(about 30 seconds),
yet each individual object is accessed infrequently (a few times a
week).  The dataset in total is larger than RAM.

Can Varnish be configured to cache these items for a short amount of
time (say 20 minutes), but also have a relatively long (i.e. 7 days)
grace setting so clients will always get a fast response (but at the
same time will trigger an update of the cached item).

I would warm the cache by spidering my sitemap upon starting the
Varnish server so the first visitors won't have to wait for the object
to be generated.

Basically I'm trying to keep my cache warm, without overwhelming my
app servers, and keep access quick for my visitors.  Is this doable?

Also -- since my dataset is larger than RAM, which backend is the best
choice?  I was thinking the file backend would make the most sense --
but just wanted to make sure.

Thanks!
Erik