Ursidinoj/The Bjornsdottirs
ellenor2000@mastodon.top@charlotte you'll need a VPN that does not impose a minimum limit on the outer MTU #lang_en
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Ursidinoj/The Bjornsdottirs
ellenor2000@mastodon.top@charlotte I'm aware. you're going to need a VPN that can somehow reassemble packets #lang_en
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Charlotte 
/Cinny 
charlotte
@ellenor2000 i am not going to switch the entire raccnet to a different VPN just because ČD can’t even support technologies older than me
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Ursidinoj/The Bjornsdottirs
ellenor2000@mastodon.top@charlotte can you set a noncompliantly low MTU or does the OS not let you #lang_en
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Charlotte /Cinny
charlotte
@ellenor2000 systemd-networkd clamps it to an MTU of 1280 and if i try to set it via the ip command it just Deletes the ipv6 connection
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Ursidinoj/The Bjornsdottirs
ellenor2000@mastodon.top@charlotte oh. fuck. I'm sorry. so the OS doesn't let you
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@charlotte somewhat related: we have a bunch of servers in the same rack at Hetzner, and we're paying extra for them to be connected together in a separate 10G local network.
Guess what is the MTU of that 10G datacenter-grade switch for local networking.
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Charlotte /Cinny
charlotte
@IngaLovinde 1500 because even though it can probably do 9000 or more…
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Inga stands with 🇺🇦 🇵🇸
IngaLovinde@embracing.space@charlotte IIRC it's more like 1460.
Also not documented anywhere; I only realized when transfers of any files that were not just four bytes saying "test" were getting stuck, and then had to do binary search with ping manually in order to determine the MTU.
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Charlotte /Cinny
charlotte
mtu sizes in theory: 1500 except via vpn. every device could do 9000 or more tho
mtu sizes in practice: here is an unusable mtu. fuck you
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@charlotte imagine how hard must this little datacenter-grade switch be working, switching a million of tiny packets per second... when it could have only switched 100K... if only...
(We use this network primarily to transfer build artifacts over NFS, with file sizes typically measured in gigabytes)
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Charlotte /Cinny
charlotte
the lowest wired mtu of my devices i measured was my vf2 which could not send more than 9000 bytes in one frame
forgot the exact value but everything, running through a cheap gigabit switch, could receive something like 9120 bytes
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@charlotte exactly.
Most of the stuff is mirrored across nodes, but even though mirroring is not the bottleneck, mirroring 17GB from one node into the remaining seven (so 120GB of data transfer total) surely does take a lot of time even over 10G network, especially when non-negligible part of network transfer is the packet overhead.
And test bundles aren't mirrored (because they're specific to commit / pipeline), so test workers each have to fetch 2-3GB bundles over network at the same time, which also adds noticeable latency to the CI pipeline.
Although of course things would be much worse (and unbearably slow) on 1G network.
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Botch Frivarg
deetwenty@todon.nl
@charlotte should reinvestigate but highest workable MTU for internet usage has been nearly universally be around 1400 for me (at 1500 things tend to go wonky with some stuff working and some stuff just broken intermittently)
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