[gpfsug-discuss] connected v. datagram mode

[Laurence Horrocks-Barlow]

It also depends on the adapter.

We have seen better performance using datagram with MLNX adapters 
however we see better in connected mode when using Intel truescale. 
Again as Jonathon has mentioned we have also seen better performance 
when using connected mode on active/slave bonded interface (even between 
a mixed MLNX/TS fabric).

There is also a significant difference in the MTU size you can use in 
datagram vs connected mode, with datagram being limited to 2044 (if 
memory serves) there as connected mode can use 65536 (again if memory 
serves).

I typically now run qperf and nsdperf benchmarks to find the best 
configuration.

-- Lauz

On 12/05/2017 16:05, Jonathon A Anderson wrote:
> It may be true that you should always favor connected mode; but those instructions look like they’re specifically only talking about when you have bonded interfaces.
>
> ~jonathon
>
>
> On 5/12/17, 9:03 AM, "gpfsug-discuss-bounces at spectrumscale.org on behalf of Jan-Frode Myklebust" <gpfsug-discuss-bounces at spectrumscale.org on behalf of janfrode at tanso.net> wrote:
>
>      
>      
>      
>      I also don't know much about this, but the ESS quick deployment guide is quite clear on the we should use connected mode for IPoIB:
>      
>      --------------
>      Note: If using bonded IP over IB, do the following: Ensure that the CONNECTED_MODE=yes statement exists in the corresponding slave-bond interface scripts located in /etc/sysconfig/network-scripts directory of the management server and I/O server nodes. These
>       scripts are created as part of the IP over IB bond creation. An example of the slave-bond interface with the modification is shown below.
>      ---------------
>      
>      
>      -jf
>      fre. 12. mai 2017 kl. 16.48 skrev Aaron Knister <aaron.s.knister at nasa.gov>:
>      
>      
>      For what it's worth we've seen *significantly* better performance of
>      streaming benchmarks of IPoIB with connected mode vs datagram mode on IB.
>      
>      -Aaron
>      
>      On 5/12/17 10:43 AM, Jonathon A Anderson wrote:
>      > This won’t tell you which to use; but datagram mode and connected mode in IB is roughly analogous to UDB vs TCP in IP. One is “unreliable” in that there’s no checking/retry built into the protocol; the other is “reliable” and detects whether data is received
>       completely and in the correct order.
>      >
>      > The last advice I heard for traditional IB was that the overhead of connected mode isn’t worth it, particularly if you’re using IPoIB (where you’re likely to be using TCP anyway). That said, on our OPA network we’re seeing the opposite advice; so I, to, am
>       often unsure what the most correct configuration would be for any given fabric.
>      >
>      > ~jonathon
>      >
>      >
>      > On 5/12/17, 4:42 AM, "gpfsug-discuss-bounces at spectrumscale.org on behalf of Damir Krstic" <gpfsug-discuss-bounces at spectrumscale.org
>       on behalf of damir.krstic at gmail.com> wrote:
>      >
>      >     I never fully understood the difference between connected v. datagram mode beside the obvious packet size difference. Our NSD servers (ESS GL6 nodes) are installed with RedHat 7 and are in connected mode. Our 700+ clients are running RH6 and
>      >      are in datagram mode.
>      >
>      >
>      >     In a month we are upgrading our cluster to RedHat 7 and are debating whether to leave the compute nodes in datagram mode or whether to switch them to connected mode.
>      >     What is is the right thing to do?
>      >
>      >
>      >     Thanks in advance.
>      >     Damir
>      >
>      >
>      >
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>      
>      --
>      Aaron Knister
>      NASA Center for Climate Simulation (Code 606.2)
>      Goddard Space Flight Center
>      (301) 286-2776
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