> I'd be interested if applying this sort of tuning to worker nodes will have any effect at the other sites that are having transfer problems - Brunel, Liverpool, Sheffield and Bristol.
We tried it at Liverpool last Thursday, and sure enough, the quality of the
transfers went into the dark green.
However, I can't quite see how the optimisations would explain that particularly
given the seeming correlation with NAT. Why would NAT cause problems just
because the transfers are taking longer? Wouldn't larger transfers, or increased
contention, also cause problems if that was the case?
So looking at the changes, aside from the changes to buffer sizes, there are two
TCP options turned off, timestamps and selective acknowledgements (along with
duplicate selective acknowledgements).
I tried turning timestamps back on, that made no difference, transfers stayed green.
I then tried turning SACK and DSACK back on and we saw a drop in quality over
the next few hours down into the 30-40% orange/yellow range. Disabling SACK and
DSACK saw the quality go back to 100%.
I then restored all settings to defaults apart from SACK and DSACK being
disabled, and all transfers since then have been 100%. However, there haven't
been that many transfers since then, so I don't think I can really say with
certainty that SACK/DSACK are the issue, but the evidence so far would appear to
indicate that may be the case, at Liverpool at least.
--
Robert Fay [log in to unmask]
System Administrator office: 220
High Energy Physics Division tel (int): 43396
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