[Cerowrt-devel] High Performance (SSH) Data Transfers using fq_codel?

Dave Taht dave.taht at gmail.com
Sat Nov 15 17:47:06 EST 2014

On Fri, Oct 17, 2014 at 4:21 PM, Frank Horowitz <frank at horow.net> wrote:
> G’Day folks,
> Long time lurker. I’ve been using Cero for my home router for quite a while now, with reasonable results (modulo bloody OSX wifi stuffola).
> I’m running into issues doing zfs send/receive over ssh across a (mostly) internet2 backbone between Cornell (where I work) and West Virginia University (where we have a collaborator on a DOE sponsored project. Both ends are linux machines running fq_codel configured like so:
>         tc qdisc
>         qdisc fq_codel 0: dev eth0 root refcnt 2 limit 10240p flows 1024 quantum 1514 target 5.0ms interval 100.0ms ecn

So, in re-reading this now, I think I now grok that that A) Frank is
using cerowrt at home, and B) he has it hooked up on two cool boxes
connected on either side of Internet2, and B) is what the question was

so my general assumption is that his boxes are x86, and hooked up at
1GigE or so to the Internet2.

So answers A) If after a big transfer If a

tc -s qdisc show dev eth0 # on both sides

shows no drops or ecn marks, his linux servers are not the bottleneck
link, and he should use mtr to find the real bottleneck link
elsewhere, during that transfer.

IF, after a big transfer drops are seen, you are (at least some of the
time) a bottleneck link. Enable ecn between both tcps. And if you are
willing to tolerate more latency on the link, feel free to increase
the target and interval to values you are more comfortable with, but
you won´t increase actual bandwidth by all that much.

Personally I suspect ¨A¨ as the problem.

And as per my original msg, it always helps to measure, and the rrul
test between the two points is the best thing we got.

> I stumbled across hpn-ssh <https://www.psc.edu/index.php/hpn-ssh> and —  of particular interest to this group — their page on tuning TCP parameters:
> <http://www.psc.edu/index.php/networking/641-tcp-tune>
> N.B. their advice to increase buffer size…
> I’m curious, what part (if any) of that advice survives with fq_codel running on both ends?

Most of that seems to apply to TCPs.

I would suspect that enabling TCP pacing between the two points might
be helpful, but without data on whatever problem(s) you are
experiencing on your path, can´t help. Amusingly, matt mathis is one
the original authors of that page, and perhaps he has new advice.

> Any advice from the experts here would be gratefully received!
> (And thanks for all of your collective and individual efforts!)
> Cheers,
>         Frank Horowitz
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Dave Täht


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