Disk cache on openwrt

Hi,

I recently installed openwrt (backfire) on a wrt54gl with 2Go (SD mod) 
and I would like to have web caching to save bandwidth. So I'd install 
polipo via opkg but I can not activate disk cache, I had tried various 
configuration (manually, via Luci...) but still have "Disabling disk 
cache: No such file or directory".
I have also read FAQ and polipo's manual and don't found anything about 
that (sorry if i'm wrong).
Do I have to make something special on the cache directory? (I had 
choose to create /cache dir).
Sorry if I'm not at the wright place to post my question but after 
searching some hour (day...) I don't know where can be the probleme : 
me, or polipo's config or openwrt's config...

thank you and sorry for my english.

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