vcl_timeout and fetch

[ll]

by the VCL define , the vcl_timeout can terminal by fetch or discard
.can I defined like this in the vcl?

sub vcl_timeout {
fetch;
}

but I find this in the default.vcl

#sub vcl_timeout {
# /* XXX: Do not redefine vcl_timeout{}, it is not yet supported */
# return (discard);
#}

and actually ,I try to set the timeout to fetch. it's failed .after the
ttl I request the url again ,it will MISS.

so,is there any ways to set the varnish to fetch the cache and flash it
automatic ? I have a webserver,it has so many pages ,I want to varnish
can fetch all pages one time and keep it a long time .and after the
ttl,it can flash it automatic .Is it some ways to accomplish this ?

thanks !!