[c-nsp] PAUSE vs PFC for loss-less traffic?

[Chris Evans]

Really it comes down to the fact that you can pause all traffic except for
your iSCSI (if identified properly) and FCoE traffic.. With PAUSE frames
everything gets paused on that link, including your storage traffic.




On Sun, Mar 9, 2014 at 12:50 PM, Mathias Sundman <mathias at nilings.se> wrote:

> There is a lot of talk about how PFC makes Ethernet loss-less and suitable
> for FCoE or ISCSI. My question is, for single-type-of-traffic interfaces,
> for example ISCSI arrays connected to a Ethernet switch. Why doesn't
> traditional PAUSE based flow control give the same level of loss-less
> performance?
>
> For example Dell claims in section 5.1 in their guide "Creating a DCB
> Compliant EqualLogic iSCSI SAN with Mixed Traffic" that retransmissions
> went from 0.315% to 0% when going from PAUSE to PFC. I just don't
> understand why. Both technologies works the same way with pause for xx
> time, and resume frames. Are the thresholds for when to send the pause
> frames different by default?
>
> Reason for asking is that Dell does not support running DCB between Nexus
> switches and Equallogic arrays, and I'm trying to understand why I couldn't
> just do traditional PAUSE based flow control on the interfaces between the
> array and the switches, as those are ISCSI only interfaces and manually map
> the traffic into the correct qos-group, and then do DCB with PFC on the
> server interfaces to have flow control only on the ISCSI VLAN.
>
> Any thoughts?
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