Hi Ian,
On 09/08/2024 03:00, Ian Z via Unbound-users wrote:
Hello. I'd like to understand correctly and precisely the meaning of
the _prefetch_ server config option. The only documentation I've been
able to find is the unbound.conf manpage, which has this to say:
prefetch: <yes or no>
If yes, message cache elements are prefetched before they expire to
keep the cache up to date. Default is no. Turning it on gives
about 10 percent more traffic and load on the machine, but popular
items do not expire from the cache.
This is clear but not precise enough. Under what circumstances are
cache entries refreshed? Do they have to be close to expiration? How
close? Or is there some other condition that triggers a refresh?
The prefetch happens when there is a cache hit and the record is on its
last 10% of the TTL.
And then there is the _minimal-responses_ option. I have a good idea
what it does, by itself, but the documentation helpfully drags in
prefetching again:
minimal-responses: <yes or no>
If yes, Unbound does not insert authority/additional sections into
response messages when those sections are not required. This
reduces response size significantly, and may avoid TCP fallback for
some responses. This may cause a slight speedup. The default is
yes, even though the DNS protocol RFCs mandate these sections, and
the additional content could be of use and save roundtrips for
clients. Because they are not used, and the saved roundtrips are
easier saved with prefetch, whilst this is faster.
That last sentence sounds like Google Translate before LLM era :-(
What's "they" and "this" there? Why is prefetch "easier", given the
10 percent figure in the quote above?
It tries to say that providing the additional content in the reply can
save roundtrips for clients that need them. However almost no client
reads that content. And for those that do, enabling prefetch will help
keep that data in the cache.
I do hope that I solved both text issues with
https://github.com/NLnetLabs/unbound/commit/24ebca7df6039ddcff52884c19db846024d162a4
The online documentation will be updated when a new release is out.
Best regards,
-- Yorgos
