Would it be useful if I set up a linode that was an APT mirror?

Obviously this would reduce your bandwidth costs because the traffic would be local, but there would be some cost in setting up the linode and it's bandwidth.

I've not done that before, but it might be worth me learning :]

There are apt caches in dallas,newark and london (http://www.linode.com/wiki/index.php/Internal_Services) not quite the same but close.

One thing you will want to consider is disk space, I'm not sure how big the ubuntu mirrors are but to cover all supported distro's you're talking a bucket load of space.

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Cool, I wasn't aware of these.

I ready somewhere that the 8.04 mirror is 37GB.

37gb would require a Linode 1536 which is $60/month, most people I imagine are using 10.04 which will have a bigger repo.

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The apt-caches listed on the the internal services page are all running apt-cacher-ng.

Running a full mirror for every version and arch of Ubuntu/Debian would be impractical. With apt-cacher-ng, only the packages that get used get downloaded and cached locally.

If you're curious, here are the stats on my Newark cache:

Total Clients: 20
Total Transfer In: 1.89 GB
Total Transfer Out: 2.66 GB
Total Transfer Saved: 787.80 MB
Cache Size: 1.5G

And my (much newer) Dallas cache:

Total Clients: 9
Total Transfer In: 763.59 MB
Total Transfer Out: 1.01 GB
Total Transfer Saved: 273.76 MB
Cache Size: 747M

So, in the grand scheme of things, I've saved a little over a gig's worth of transfer so far. That's a very tiny drop in the bucket, but every little bit helps, right?