[Unbound-users] About the prefetch switch used in the unbound.conf.

[Hongyi Zhao]

Hi all,

Currently, I use the latest release of unbound 1.5.2 compilled by myself on
the Debian wheezy. I have some confusion of the using the the "prefetch"
option in the unbound.conf file. Let me described it as follows:

1- From the manual of unbound.conf, i.e., `man unbound.conf', I can get the
following
notes on this option:

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.

2- From the following webpage: https://calomel.org/unbound_dns.html,
I can get the explanation on this option as follows:



# perform prefetching of close to expired message cache entries. If a client
# requests the dns lookup and the TTL of the cached hostname is going to
# expire in less than 10% of its TTL, unbound will (1st) return the ip of the
# host to the client and (2nd) pre-fetch the dns request from the remote dns
# server. This method has been shown to increase the amount of cached hits by
# local clients by 10% on average.
prefetch: yes

As you can see, the above two explanations on the mechanism of
prefetching in unbound are not so

well the same. Could someone please give me some hints on the this thing?

Regards

--
Hongyi Zhao <***@gmail.com>
Xinjiang Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
GnuPG DSA: 0xD108493

[Dave Warren]

Post by Hongyi Zhao
Hi all,
Currently, I use the latest release of unbound 1.5.2 compilled by
myself on the Debian wheezy. I have some confusion of the using the
the "prefetch" option in the unbound.conf file. Let me described it
1- From the manual of unbound.conf, i.e., `man unbound.conf', I can
get the following
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.
2- From the following webpage: https://calomel.org/unbound_dns.html,
# perform prefetching of close to expired message cache entries. If a client
# requests the dns lookup and the TTL of the cached hostname is going to
# expire in less than 10% of its TTL, unbound will (1st) return the ip of the
# host to the client and (2nd) pre-fetch the dns request from the remote dns
# server. This method has been shown to increase the amount of cached hits by
# local clients by 10% on average.
prefetch: yes
As you can see, the above two explanations on the mechanism of prefetching in unbound are not so
well the same. Could someone please give me some hints on the this thing?

To be completely honest, I don't see a conflict between the two
explanations.

The description in the man unbound.conf is less comprehensive as it only
mentions that this feature keeps the cache up to date, whereas the
description from the webpage quoted explains the mechanism that is used
to determine which records need to be prefetched.

If you're unclear, just read #2 above, it's more specific and therefore
probably more useful to understand how the feature actually works, but
be aware that it does increase the outbound queries slightly since, from
an external perspective, it drops all TTLs about 10%, while internally
it respects TTLs in a consistent and reliable fashion. I found this
feature made a significant difference when we relocated and were stuck
on the end of a high latency 3Mb line for some months, but it makes
little noticeable-to-end-user difference now that we're back on a high
bandwidth, low latency connection.

[Hongyi Zhao]

Thanks a lot. Got it.

Regards

Post by Dave Warren

Post by Hongyi Zhao
Hi all,
Currently, I use the latest release of unbound 1.5.2 compilled by myself
on the Debian wheezy. I have some confusion of the using the the
"prefetch" option in the unbound.conf file. Let me described it as
1- From the manual of unbound.conf, i.e., `man unbound.conf', I can get
the following
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.
2- From the following webpage: https://calomel.org/unbound_dns.html,
# perform prefetching of close to expired message cache entries. If a client
# requests the dns lookup and the TTL of the cached hostname is going to
# expire in less than 10% of its TTL, unbound will (1st) return the ip of the
# host to the client and (2nd) pre-fetch the dns request from the remote dns
# server. This method has been shown to increase the amount of cached hits by
# local clients by 10% on average.
prefetch: yes
As you can see, the above two explanations on the mechanism of
prefetching in unbound are not so
well the same. Could someone please give me some hints on the this thing?

To be completely honest, I don't see a conflict between the two
explanations.
The description in the man unbound.conf is less comprehensive as it only
mentions that this feature keeps the cache up to date, whereas the
description from the webpage quoted explains the mechanism that is used to
determine which records need to be prefetched.
If you're unclear, just read #2 above, it's more specific and therefore
probably more useful to understand how the feature actually works, but be
aware that it does increase the outbound queries slightly since, from an
external perspective, it drops all TTLs about 10%, while internally it
respects TTLs in a consistent and reliable fashion. I found this feature
made a significant difference when we relocated and were stuck on the end
of a high latency 3Mb line for some months, but it makes little
noticeable-to-end-user difference now that we're back on a high bandwidth,
low latency connection.
--
Dave Warren
http://www.hireahit.com/
http://ca.linkedin.com/in/davejwarren
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