[OpenIndiana-discuss] ZFS remote receive

[Jim Klimov]

On 2012-11-01 01:47, Richard Elling wrote:
> Finally, a data point: using MTU of 1500 with ixgbe you can hit wire speed on a
> modern CPU.

> There is no CSMA/CD on gigabit and faster available from any vendor today.
> Everything today is switched.

Ok then, I'll stand corrected by the practice, although my networking
education speaks against this statement. We were taught that since GbE
retains (at least formally) compatibility with older ethernet, and with
halfduplex in particular, it is capable of hubbing, csma/cd, etc. These
provisions are of course suboptimal (useless and harmful to performance)
on point-to-point fullduplex links like host-to-host and host-to-switch.

It may be a matter of particular modernized OS defaults however - to
disable those obsolete beasts. But they should be there, and when Jumbo
was introduced (for GbE, none other) - these beasts were known to cause
these sorts of problems.

Likely, interrupt coalescing, NIC offloads and CPU horsepower played
their roles as well. And buffering, I've rather forgot about this.
(Also note that buffers blindly thrown at any problem at all layers
of the stack might only hide the performance degradations and disable
protocol adaptability to reduced network abilities, as often happens
with WiFi during interference, for example - draining the several
hundred packets of the buffer over 1Mbit before even noticing that
a problem exists, can take a good part of the second, if not more
than one).

My 2c,
//Jim