The vislab mirror is in China, and I've personally never ever been able to access it

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(Even when @chrox was still active, I could only extremely rarely access it, and with downright terrible transfer rates (i.e., sub kB)).
I've vaguely considered commenting it out, but it may still be kept up to date, in which case it's useful for Chinese users. But I have no way to check that

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TL;DR: Don't use that one

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http://ota.koreader.rocks is the master, but it's behind a CloudFlare CDN, which has been known to screw with zsync in some circumstances.
http://koreader.ak-team.com is my own mirror, it's a physical dedicated server, hosted in France, but with somewhat limited bandwidth. Provided you have decent peering to it from your current location, it should behave okay w/ zsync.
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I've almost never seen refresh glitches on the OTA popups, but I did saw a couple recently in a very specific range of versions, for some mysterious reasons. Haven't seen any since.
As usual, it's pretty sensitive to timing and ambient temperature, so, eh, who knows ;p.
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I also keep a couple ObjectStorage-backed mirrors in sync:
http://koreader-fr.ak-team.com in France
http://koreader-pl.ak-team.com in Poland
http://koreader-na.ak-team.com in Canada
But those are commented out, because zsync can't handle the quirks of an OpenStack HTTP frontend.