Cache flush interval

I had a download at 54% of around 700 Meg.
In this case there was a crash.
After a reboot the % to go was very low.( 5%)
I also deliberately exited another time and the same
thing happened, quite a substatacial loss of data.
The deliberate exit was very unclean, by the way.
Hanging processes blue screen ...
A clean deliberate shutdown option would be nice.

Anyway I was interested in the frequency of flushing to disk.
Also is if you are downloading a 200Meg file and you have
512 Meg how likely is it to buffer the majority of the file
in RAM?

It at least appeared to work this way for me.