We have 600+ active CentOS mirrors[1] and sometimes there are critical
updates that would need to get published to the mirror network as
quickly as possible. As of now, it takes around five hours to go from 0
up-to-date mirrors to 80% up-to-date mirrors, with a longer tail for the
remaining 20%.
Current guidelines for setting up a mirror[2] specify that mirrors
should sync 2-4 times per day. Instead of telling mirrors to sync hourly
I thought we could come up with something smarter.
One option that I have considered would be something similar to what the
ClamAV guys use to signal end users that there is new content. They use
a DNS TXT record for that purpose. For example, as of this writing "dig
txt +short current.cvd.clamav.net" produces
"0.99.2:57:23148:1488371340:1:63:45637:290", which shows the version
numbers for ClamAV itself, main virus database, daily virus database,
timestamp and other version numbers.
We could have something similar, showing the timestamps when the content
for CentOS, CentOS AltArch and CentOS Vault was last modified, like
"1488372781:1487767981:1488113581". "Last modified" in the sense that
new packages got added at that time. The idea is that mirrors could set
up scripts to check that timestamp from DNS more frequently (such as
hourly) without causing load issues to msync nodes by rsyncing hourly.
The TTL for the TXT record could be relatively small, like 10 minutes.
As you're all aware, DNS is a prime example of a very scalable system,
and that's why I'm fond of this solution. Another option would be to
publish the same data in a central location and served over http(s), if
relaying the timestamp data via DNS is not desired for some reason.
The basic principle would be "if timestamp in TXT record > my current
timestamp (TIME file), synchronize the mirror". With more frequent
syncs, mirror admins would need to take care that no two rsync runs
would happen at the same time. Using lockfile in the scripts would help
with this. I hope that many of the mirror admins already use lockfiles,
but providing an example script might help for the newer mirror admins.
The timestamps should be updated only after it has been verified that
all (or at least the majority) of msync nodes actually have the content.
It takes a while for the data to reach all the msync nodes from the master.
On the other hand, this may cause some traffic peaks for the msync
nodes. I don't know how well they would handle the peaks. One obvious
way to alleviate the peaks would be to instruct mirror admins to pick a
random minute when to check for new content. echo $[ $RANDOM % 60 ]
works nicely for this. I don't have statistics, but I believe there
might be mirrors that sync at "0 */6" ie. at the top of the hour.
If the traffic peaks to msync nodes is deemed to be a problem, there
might be ways to reduce the load to msync nodes. The following idea
could be implemented separately from the above timestamp idea, if needed.
There could be some sort of a "web service" which instructs mirrors
where to sync from. The core idea in this is that the source might not
always be a msync.centos.org server, but it could also be a nearby
public mirror that offers rsync and has been verified to have the new
content. If requested from Finland, that service could say "ok, you're
from .fi, go sync from ftp.funet.fi as it has the new content already"
or "uh oh, no nearby external mirrors have the new content, please rsync
from eu-msync.centos.org". It could simply return a list of rsync
servers in descending priority, with some msync.centos.org addresses at
the bottom as fallback. Once the mirror has rsynced, the mirror could
ping back and say "I have the content now, please check, and if OK, add
me to the list of mirrors that have the new content".
One concern is that the list of rsync sources would need to be
protected, so that mirrors could not be tricked into syncing from a
malicious source (think DNS poisoning). Ways to protect from this
include DNSSEC, TLS and PGP signed data.
Any thoughts about this?
[1] http://mirror-status.centos.org/
[2] https://wiki.centos.org/HowTos/CreatePublicMirrors
