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My resolvers don't have enough traffic to make prefetch very
interesting, so I haven't played with it.
Nonetheless, I did think about it.

So while this isn't exactly a 'bug' report, I thought I'd share the
thoughts in case they are useful.

The 9.10 'prefetch' mechanism is currently tuned by a TTL threshold
('elegibility') and a window ("trigger").
Both are fixed global parameters.

The threshold makes intuitive sense - no point in prefetching if TTL is
very small.

The window is intended to avoid prefetching low activity zones so the
records can expire from the cache; prefetch is initiated if a request
arrives in the window at the end of TTL.

It's not obvious that a fixed window is the only sensible way to specify
when to prefetch.  Has any thought been given to alternatives?

One that comes immediately to mind is % TTL remaining.  This would adapt
to the TTL of the record, and provide some more useful opportunities for
tuning caching at the resolver. 

Some examples:
    .5   - aggressive - cuts TTL in 1/2, minimize chance of miss
    .95   - conservative - fetch near the end, maximize caching.
    .01  - 'somewhat insane' - try to cache even low use records

One might specify this using a fractional trigger value, say:
    prefetch .8 9;           # 80%

The integer part could also be a limit (e.g. 90.8 = 80%, but no sooner
than 90 seconds remaining). 

This would be consistent with the current definitions.

A 'and no later than' might also be useful - but risks making thing a
bit messier.

Speaking of messier:

the option syntax
   prefetch number number;

is not desirable, as it's not easy to remember.    Please stop adding
options of this form. (dnssec-sig-validity-interval also has this sort
of syntax.)

I understand the desire to not have a zillion top-level keywords (e.g.
prefetch-trigger and prefetch-elegibility).  As an alternative,
consider a position-independent keyword syntax such as
  prefetch trigger=5 elegibility=60

This could be retrofitted to the existing mult-value options and be
backward-compatible (no keyword => defined by order), but would make it
a lot easier to remember what's going on - whether reading a config file
or generating one.

-- 
Timothe Litt
ACM Distinguished Engineer
--------------------------
This communication may not represent the ACM or my employer's views,
if any, on the matters discussed.