[CentOS-mirror] accuracy of mirrorlist.centos.org

Tue Mar 3 08:33:58 UTC 2015
Seamus Ryan <s.ryan at uber.com.au>

Evening all.

I was doing some poking around with some centos servers this evening and I wanted to try and understand what type of smarts are behind mirrorlist.centos.org.

I know this has been covered before (somewhat). I know there is no way (at present) to "lock" a server on our network down to only using a specific mirror  (short of hard coding the mirror URL into the yum repo file). I am aware of such capabilities when using mirrormanager, but we aren't there yet.
As a result, when using the default method of fastestmirrors, I will generally be sent to a mirror that is close to me.

However, I also know there is some level of geo-smarts involved. Usually there is a good chance I will be sent to a mirror within the same country as my server, but not always. When querying mirrorlist, 10 results are returned as expected However out of those 10 results I find that:
7 of the mirrors are in the same country as my server (Australia)
The remaining three are in three separate countries (USA, Canada, Ireland).

If I wait a short while and query mirrorlist again, I get different results:
8 of the mirrors are in the same country as my server (Australia). Interestingly out of these 8 mirrors, one is shown twice (first for HTTP then again for FTP)
The remaining two mirrors are in Canada and Japan

It appears to alternate between 7 and 8 local mirrors, at least from all the testing I have done. With 2-3 mirrors in random countries.

Here is what I found interesting. When performing the same tests from servers in other countries (UK and USA), every time I queried mirrorlist, all 10 mirrors returned were local to the server (ie USA server was always provided 10 mirrors located in USA, and UK server was always provided 10 mirrors in UK)

Soooooo what I was wondering is how the initial list is determined? Obviously there is some level of geo-smarts involved here as most of the mirrors returned at in the same country. But why are a few outside for Australia? My guess is that something may have been implemented a long time ago before Australia had 10 mirrors in total?

Anyhow, enjoy your evening folks.


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